  Modem-HOWTO
  David S.Lawyer
   <mailto:dave@lafn.org>
  v0.29, July 2003

  Help with selecting, connecting, configuring, trouble-shooting, and
  understanding analog modems for a PC.  Limited coverage of V.90 digi­
  tal modems.
  ______________________________________________________________________

  Table of Contents



  1. Introduction

     1.1 DSL, Cable, and ISDN Modems in other HOWTOs
     1.2 Also not covered: PCMCIA Modems, PPP
     1.3 Copyright, Disclaimer, Trademarks, & Credits
        1.3.1 Copyright
        1.3.2 Disclaimer
        1.3.3 Trademarks.
        1.3.4 Credits
     1.4 Contacting the Author
     1.5 New Versions of this HOWTO
     1.6 New in Recent Versions
     1.7 What is a Modem ?
     1.8 Does My Computer Contain an Internal Modem ?
     1.9 Quick Install
        1.9.1 External Modem Install
        1.9.2 Internal Modems (ISA, PCI and AMR)
        1.9.3 Internal Modems: Manual configuration
           1.9.3.1 ISA Modems: What IOs and IRQs may be used?
           1.9.3.2 Both PCI and ISA: Use setserial to tell the serial driver
           1.9.3.3 Use MS Windows to set the BIOS (A last resort method)
        1.9.4 All Modems

  2. Modems for a Linux PC

     2.1 External vs. Internal
     2.2 Is a Driver Needed ?
     2.3 External Modems
        2.3.1 PnP External Modems
        2.3.2 Cabling & Installation
        2.3.3 What the Lights (LED's) Mean (for some external modems)
     2.4 Internal Modems
     2.5 Software-based Modems (winmodems, linmodems)
        2.5.1 Introduction to software modems (winmodems)
        2.5.2 Linmodems
        2.5.3 Linmodem sites and documentation
        2.5.4 Software-based modem types
        2.5.5 Is this modem a software modem?
        2.5.6 Should I get a software modem?
     2.6 PCI Modems
     2.7 AMR Modems
     2.8 USB Modems
     2.9 Which Internal Modems might not work with Linux
        2.9.1 MWave and some DSP Modems
        2.9.2 Rockwell (RPI) Drivers

  3. Modem Pools

     3.1 Introduction
     3.2 Analog Modem Pools, Multi-modem Cards
     3.3 Digital Modems, RAS

  4. Serial Port and Modem Basics

     4.1 Modem Converts Digital to Analog (and conversely)
     4.2 What is a Serial Port ?
        4.2.1 Intro to Serial
        4.2.2 Pins and Wires
        4.2.3 Internal Modem Contains Serial Port
     4.3 IO Address & IRQ
     4.4 Names: ttyS0, ttyS1, etc.
     4.5 Interrupts
     4.6 Data Compression (by the Modem)
     4.7 Error Correction
     4.8 Data Flow (Speeds)
     4.9 Flow Control
        4.9.1 Example of Flow Control
        4.9.2 Hardware vs. Software Flow Control
        4.9.3 Symptoms of No Flow Control
        4.9.4 Modem-to-Modem Flow Control
     4.10 Data Flow Path; Buffers
     4.11 Modem Commands
     4.12 Serial Driver Module

  5. Configuring Overview

  6. Locating the Serial Port: IO address, IRQs

     6.1 IO & IRQ Overview
     6.2 PCI Bus Support
        6.2.1 Introduction
        6.2.2 More info on PCI
     6.3 Common mistakes made re low-level configuring
     6.4 IRQ & IO Address Must be Correct
     6.5 What is the IO Address and IRQ per the driver ?
        6.5.1 Introduction
        6.5.2 I/O Address & IRQ: Boot-time messages
        6.5.3 The /proc directory and setserial
     6.6 What is the IO Address & IRQ of my Serial Port Hardware?
        6.6.1 Introduction
        6.6.2 PCI: What IOs and IRQs have been set?
        6.6.3 PCI: Enabling a disabled port
        6.6.4 ISA PnP ports
        6.6.5 Finding a port that is not disabled (ISA, PCI, PnP, non-PnP)
        6.6.6 Exploring via MS Windows (a last resort)
     6.7 Choosing Serial IRQs
        6.7.1 IRQ 0 is not an IRQ
        6.7.2 Interrupt sharing,  Kernels 2.2+
        6.7.3 What IRQs to choose?
     6.8 Choosing Addresses --Video card conflict with ttyS3
     6.9 Set IO Address & IRQ in the hardware (mostly for PnP)
        6.9.1 Using a PnP BIOS to I0-IRQ Configure
     6.10 Giving the IRQ and IO Address to Setserial

  7. Configuring the Serial Driver (high-level) "stty"

     7.1 Introduction
     7.2 Hardware flow control (RTS/CTS)
     7.3 Speed Settings
     7.4 Ignore CD Setting: clocal
     7.5 What is stty ?

  8. Modem Configuration (excluding serial port)

     8.1 Finding Your Modem
     8.2 AT Commands
     8.3 Init Strings: Saving and Recalling
        8.3.1 Where is my "init string" so I can modify it ?
     8.4 Other AT Modem Commands
     8.5 Blacklisting
     8.6 What AT Commands are Now Set in my Modem?
     8.7 Modem States (or Modes)

  9. Serial Port Devices /dev/ttyS2, (or /dev/ttys/2) etc.

     9.1 Devfs (The new Device File System)
     9.2 Serial Port Device Names & Numbers
     9.3 USB (Universal Serial Bus) Ports
     9.4 Link ttySN to /dev/modem
     9.5 cua Device Obsolete

  10. Interesting Programs You Should Know About
     10.1 What is setserial ?
        10.1.1 Introduction
        10.1.2 Probing
        10.1.3 Boot-time Configuration
        10.1.4 Configuration Scripts/Files
        10.1.5 Edit a script (required prior to version 2.15)
        10.1.6 New configuration method using /etc/serial.conf
        10.1.7 IRQs
        10.1.8 Laptops: PCMCIA
     10.2 What is isapnp ?
     10.3 What is wvdialconf ?

  11. Trying Out Your Modem (Dialing Out)

     11.1 Are You Ready to Dial Out ?
     11.2 Dialing Out with wvdial
     11.3 Dialing Out with Minicom
     11.4 Dialing Out with Kermit

  12. Dial-In

     12.1 Dial-In Overview
     12.2 What Happens when Someone Dials In ?
     12.3 56k Doesn't Work for Dialin
     12.4 Getty
        12.4.1 Introduction to Getty
        12.4.2 How getty respawns
        12.4.3 About mgetty
        12.4.4 About uugetty
        12.4.5 About getty_em
        12.4.6 About agetty
        12.4.7 About mingetty, and fbgetty
     12.5 Why "Manual" Answer is Best
     12.6 Dialing Out while Waiting for an Incoming Call
     12.7 Ending a Dial-in Call
        12.7.1 Caller logs out
        12.7.2 When DTR drops (is negated)
        12.7.3 Caller hangs up
     12.8 Dial-in Modem Configuration
     12.9 Callback
     12.10 Voice Mail
     12.11 Simple Manual Dial-In
     12.12 Complex GUI Dial-In, VNC
     12.13 Interoperability with MS Windows

  13. Uugetty for Dial-In (from the old Serial-HOWTO)

     13.1 Installing getty_ps
     13.2 Setting up uugetty
        13.2.1 Modern Modems
        13.2.2 Old slow modems
        13.2.3 Login Banner
     13.3 Customizing uugetty

  14. What Speed Should I Use with My Modem?

     14.1 Speed and Data Compression
     14.2 Where do I Set Speed ?
     14.3 Can't Set a High Enough Speed
        14.3.1 Speeds over 115.2k
        14.3.2 How speed is set in hardware: the divisor and baud_base
        14.3.3 Setting the divisor, speed accounting
        14.3.4 Crystal frequency is higher than baud_base
     14.4 Speed Table

  15. Communications Programs And Utilities
     15.1 Minicom vs. Kermit
     15.2 List of Communication Software
        15.2.1 Least Popular Dialout
        15.2.2 Most Popular Dialout
        15.2.3 Fax
        15.2.4 Voicemail Software
        15.2.5 Dial-in (uses getty)
        15.2.6 Other
     15.3 SLiRP and term
     15.4 MS Windows

  16. Two Modems (Modem Doubling)

     16.1 Introduction
     16.2 Modem Bonding
        16.2.1 EQL
        16.2.2 Multilink

  17. Troubleshooting

     17.1 My Modem is Physically There but Can't be Found
        17.1.1 No response to AT
     17.2 "Modem is busy"
     17.3 "You are already online!  Hang up first." (from minicom)
     17.4 I can't get near 56k on my 56k modem
     17.5 Uploading (downloading) files is broken/slow
     17.6 For Dial-in I Keep Getting "line NNN of inittab invalid"
     17.7 I Keep Getting: ``Id "S3" respawning too fast: disabled for 5 minutes''
     17.8 My Modem is Hosed after Someone Hangs Up, or uugetty doesn't respawn
     17.9 NO DIALTONE
     17.10 NO CARRIER
     17.11 uugetty Still Doesn't Work
     17.12 (The following subsections are in both the Serial and Modem HOWTOs)
     17.13 My Serial Port is Physically There but Can't be Found
     17.14 Extremely Slow: Text appears on the screen slowly after long delays
     17.15 Somewhat Slow: I expected it to be a few times faster
     17.16 The Startup Screen Show Wrong IRQs for the Serial Ports.
     17.17 "Cannot open /dev/ttyS?: Permission denied"
     17.18 "Operation not supported by device" for ttyS?
     17.19 "Cannot create lockfile. Sorry"
     17.20 "Device /dev/ttyS? is locked."
     17.21 "/dev/tty? Device or resource busy"
     17.22 "Input/output error" from setserial, stty, pppd, etc.
     17.23 "LSR safety check engaged"
     17.24 Overrun errors on serial port
     17.25 Modem doesn't pick up incoming calls
     17.26 Port get characters only sporadically
     17.27 Troubleshooting Tools

  18. Flash Upgrades

  19. Other Sources of Information

     19.1 Misc
     19.2 Books
     19.3 HOWTOs
     19.4 Usenet newsgroups
     19.5 Web Sites

  20. Appendix A:  How Analog Modems Work (technical) (unfinished)

     20.1 Modulation Details
        20.1.1 Intro to Modulation
        20.1.2 Frequency Modulation
        20.1.3 Amplitude Modulation
        20.1.4 Phase Modulation
        20.1.5 Combination Modulation
     20.2 56k Modems (V.90, V.92)
     20.3 Full Duplex on One Circuit
     20.4 Echo Cancellation

  21. Appendix B: Analog Voice Infeasible Over Non-Voice Modem

  22. Appendix C: "baud" vs. "bps"

     22.1 A simple example
     22.2 Real examples

  23. Appendix D: Terminal Server Connection

  24. Appendix E: Digital Modems: ISDN, DSL, RAS

     24.1 Introduction
     24.2 ISDN "Modems"
     24.3 Digital Subscriber Line (DSL)
     24.4 56k Digital-Modems

  25. Appendix F: Leased Line Modems

  26. Appendix G: Fax pixels (dots)

  27. Appendix H: Antique Modems

     27.1 Introduction
     27.2 Old CCITT U) and Bell Protocols
     27.3 Historical Overview
        27.3.1 Teletypes and dumb terminals
        27.3.2 PCs and BBSs
        27.3.3 The Internet
        27.3.4 Speeds
     27.4 Proprietary protocols, etc.
     27.5 Autobauding
     27.6 Modem-to-modem Speed
     27.7 Modem-to-serial_port Speed
        27.7.1 Same speed required
        27.7.2 Equalizing speed
        27.7.3 Use "CONNECT" message to set speed
        27.7.4 Setting modem-to-modem speeds by the serial speed
        27.7.5 Manual bauding
        27.7.6 Unsupported speeds
        27.7.7 Modern modems, speed buffering
     27.8 Before AT Commands
     27.9 Acoustic-Coupling
     27.10 Data Compression and Error Correction


  ______________________________________________________________________

  1.  Introduction

  1.1.  DSL, Cable, and ISDN Modems in other HOWTOs

  This HOWTO covers conventional analog modems for PCs on the ISA, PCI,
  or USB buses.  USB coverage is weak.  For other types of modems see:


  ·  DSL-HOWTO

  ·  ADSL-Bandwidth-Management-HOWTO

  ·  Cable-Modems-HOWTO (same as Cable Modem Providers HOWTO)

  ·  SuSE ISDN Howto (not a LDP Howto)
     <http://brenner.chemietechnik.uni-
     dortmund.de/doc/sdb/en/html/isdn.html>

  ·  <http://public.swbell.net/ISDN/overview.html> tutorial on ISDN

  ·  ISDN docs in the kernel documentation subdirectory: "isdn".

  ·  <http://www.isdn4linux.de> (1998, old)

  ·  ``Appendix D:  Other Types of  Modems''


  1.2.  Also not covered: PCMCIA Modems, PPP

  For modems on the PCMCIA bus see the PCMCIA-HOWTO: PCMCIA serial and
  modem devices.  This HOWTO also doesn't cover PPP (used to connect to
  the Internet via a modem) or communication programs.  Except it does
  show how to use communication programs to test that your modem works
  OK and can make phone calls.  If you want to use a modem to connect to
  the Internet then you need to set up PPP.  There's a lot of
  documentation for PPP (including a PPP-HOWTO).  More documentation
  should be found in /usr/doc/ppp, /usr/share/doc/ppp or the like.


  1.3.  Copyright, Disclaimer, Trademarks, & Credits

  1.3.1.  Copyright

  Copyright (c) 1998-2003 by David S. Lawyer  <mailto:dave@lafn.org>

  Please freely copy and distribute (sell or give away) this document in
  any format.  Send any corrections and comments to the document
  maintainer.  You may create a derivative work and distribute it
  provided that you:


  1. If it's not a translation: Email a copy of your derivative work (in
     a format LDP accepts) to the author(s) and maintainer (could be the
     same person).  If you don't get a response then email the LDP
     (Linux Documentation Project): submit@en.tldp.org.

  2. License the derivative work in the spirit of this license or use
     GPL.  Include a copyright notice and at least a pointer to the
     license used.

  3. Give due credit to previous authors and major contributors.

  If you're considering making a derived work other than a translation,
  it's requested that you discuss your plans with the current
  maintainer.


  1.3.2.  Disclaimer

  While I haven't intentionally tried to mislead you, there are likely a
  number of errors in this document.  Please let me know about them.
  Since this is free documentation, it should be obvious that I cannot
  be held legally responsible for any errors.


  1.3.3.  Trademarks.

  Any brand names (starts with a capital letter such as MS Windows)
  should be assumed to be a trademark).  Such trademarks belong to their
  respective owners.
  "Hayes" is a trademark of Microcomputer Products Inc.  I use
  "winmodem" to mean any modem which requires MS-Windows and not in the
  trademark sense.  All other trademarks belong to their respective
  owners.


  1.3.4.  Credits

  The following is only a rough approximation of how this this document
  (as of 2000) was created:  About 1/4 of the material here was lifted
  directly from Serial-HOWTO v. 1.11 (1997) by Greg Hankins.
  <mailto:gregh@twoguys.org> (with his permission).  About another 1/4
  was taken from that Serial-HOWTO and revised.  The remaining 1/2 is
  newly created by the new author: David S. Lawyer
   <mailto:dave@lafn.org>.


  1.4.  Contacting the Author

  Since I don't follow the many different brands/models of modems please
  don't email me with questions about them (or suggestions of which one
  to buy).  If you are interested in a certain model (to find out if it
  works under Linux, etc.) see the huge list at ``Web Sites''.  Also,
  please don't ask me how to configure a modem unless you've looked over
  this HOWTO and still can't do it.  I've no personal experience with
  software-based modems.

  Please let me know of any errors in facts, opinions, logic, spelling,
  grammar, clarity, links, etc.  But first, if the date is over a month
  or two old, check to see that you have the latest version.  Please
  send me any other info that you think belongs in this document.


  1.5.  New Versions of this HOWTO

  New versions of this Modem-HOWTO should come out every few months.
  Your problem might be solved in the latest version.  It will be
  available to browse and/or download at LDP mirror sites.  For a list
  of such sites see:  <http://www.tldp.org/mirrors.html> If you only
  want to quickly compare the date of this the version v0.29, July 2003
  with the date of the latest version go to:
  <http://www.tldp.org/HOWTO/Modem-HOWTO.html>


  1.6.  New in Recent Versions

  For a full revision history going back to the first version see the
  source file (in linuxdoc format) at
  <http://www.ibiblio.org/pub/linux/docs/HOWTO/other-formats/sgml/Modem-
  HOWTO.sgml.gz>.

  ·  v0.29 July 2003 New gromitkc url, but many links in it are broken

  ·  v0.28 June 2003: Parallel port modems, lockfile permissions for
     wvdial

  ·  v0.27 May 2003: "Flow control" improved

  ·  v0.26 March 2003: USB clarity improved, v.92 modem "on hold"
     supported?, 3Com AT codes

  ·  v0.25 September 2002: Must restart minicom after configuring it
     unless you used the -s option.  HCF is not an all-software modem as
     was incorrectly claimed.  Better clarity for "Quick Install" and
     56k modems.  Does my PC have a modem?

  ·  v0.24 June 2002: new callback link, "You are already online" error,
     fixed several typos reported by Francesco Ronconi


  1.7.  What is a Modem ?

  A modem is a device that lets one send digital signals over an
  ordinary telephone line not designed for digital signals.  If
  telephone lines were all digital then you wouldn't need a modem.  It
  permits your computer to connect to and communicate with the rest of
  the world.  When you use a modem, you normally use a communication
  program or web browser to utilize the modem and dial-out on a
  telephone line.  Advanced modem users can set things up so that others
  may phone in to them and use their computer.  This is called "dial-
  in".

  There are four basic types of modems for a PC: external, USB, internal
  and built-in.  The external and USB set on your desk outside the PC
  while the other two types are not visible since they're inside the PC.
  Sometimes the USB type is called "USB external".  The external serial
  modem plugs into a connector on the back of the PC known as a "serial
  port".  The USB modem plugs into the USB bus cable.  See ``USB
  Modems''.  The internal modem is a card that is inserted inside the
  computer.  The built-in modem is part of the motherboard and is thus
  built into the computer.  It's is just like an internal modem except
  it can't be removed or replaced.  As of 2001, built-in modems are
  primarily for laptops.  What is said in this HOWTO regarding internal
  modems will generally apply also to built-in modems.

  For a more detailed comparison see ``External vs.  Internal''.   When
  you get an internal or, built-in, modem, you also get a dedicated
  serial port (which can only be used with the modem and not with
  anything else such as another modem or a printer).  In Linux, the
  serial ports are named ttyS0, ttyS1, etc. (usually corresponding
  respectively to COM1, COM2, etc. in Dos/Windows).  For the new devfs
  they are all in the /dev/ttys/ directory and named 0, 1, etc.  See
  ``Modem & Serial Port Basics'' for more details on modems and serial
  ports.  With a USB modem, the driver simulates a serial port at say
  /dev/usb/asm/0.

  Modems usually include the ability to send Faxes (Fax Modems).  See
  ``Fax'' for a list of fax software.  "Voice" modems can work like an
  automatic answering machine and handle voicemail.  See ``Voicemail
  Software''.

  The v.92 protocol can put the modem "on hold" when someone makes an
  ordinary voice call to your telephone, provided that you have "call
  waiting" from your telephone company.  Thus you can get a phone call
  while online.  As of Jan. 2003 Linux doesn't seem to support it.  If
  this is the latest version of this HOWTO, let me know about any Linux
  support for it.  Some linmodem drivers may support it (but what if you
  have a hardware modem ?).


  1.8.  Does My Computer Contain an Internal Modem ?

  Internal modems usually have a pair of modular telephone jacks on the
  back of the computer.  They should be right next to each other and
  look like a jack on the wall of a house where a telephone plugs in.
  Network cards also have modular jacks, but they are seldom in pairs
  and are slightly wider since they normally have 8 pins.  Internal DSL
  "modems" exist and also have telephone jacks, but I think they are not
  very common (most are external) as of 2002.



  1.9.  Quick Install

  1.9.1.  External Modem Install

  With a straight-thru or modem cable, connect the modem to an unused
  serial port on the PC.  Make sure you know the name of the serial
  port: in most cases COM1 is ttyS0, COM2 is ttyS1, etc.  You may need
  to check the BIOS setup menu to determine this.  Plug in the power
  cord to provide power to the modem.  See ``All Modems'' for further
  instructions.


  1.9.2.  Internal Modems (ISA, PCI and AMR)

  The first thing to do is to make sure that the modem will work under
  Linux since (as of 2002) many modems don't.  See modem list
  <http://personal.myvine.com/~gromitkc/winmodem.html>.  If the modem is
  both PnP and directly supported by the serial driver (kernel 2.4 +)
  then there is no configuring for you to do since the Linux serial
  driver will configure it.

  To physically install a modem card, remove the cover of the PC by
  /removing some screws (perhaps screw size 6-32 in the U.S.).  Find a
  matching vacant slot for it next to the other adapter cards.  Before
  inserting the card in the slot, remove a small cover plate on the back
  of the PC so that the telephone jacks on the card will be accessible
  from the rear of the PC.  Then carefully align the card with the slot
  and push the card all the way down into the slot.  Attach the card
  with a mounting screw (usually 3mm, .5mm pitch --don't use the wrong
  size).

  If you have a modem that is not a winmodem (see ``Software-based
  Modems (winmodems)'') the serial driver may configure it for you and
  you have nothing to do.  This should be noted in the boot-time
  messages (use dmesg to see them or shift-page-up after they have
  flashed by).


  1.9.3.  Internal Modems: Manual configuration

  If it didn't get automatically assigned a port ttySx and an IRQ (or
  you need to change these values) then you need to configure it
  yourself.  You first need to decide which ttySx (or ttys/x) to assign
  it to.  Pick a ttySx that is not already in use by other serial ports.
  Then you have the problem of setting an IRQ number and IO address.
  For PnP modems: If the BIOS has already set these in the physical
  device (which a PnP BIOS will do if it thinks you don't have a PnP OS)
  then you need to determine the IRQ and IO address and then tell this
  to "setserial".

  In other cases you may have some choice of IRQs and IO addresses
  (including the case where you are able to change what the BIOS has
  set).  See ``Choosing Serial IRQs'' and ``Choosing Addresses''.  For
  ISA modems there are standard IO addresses to use (corresponding to
  the ttySx).  For example you may find it feasible to use /dev/ttyS2 at
  IO address 0x3e8 and IRQ 11.  PCI modems seem to use different IO
  addresses so as not to conflict with ISA modems.


  1.9.3.1.  ISA Modems: What IOs and IRQs may be used?

  For old modems with jumpers look at the manual (or jumpers if they
  say).  If the BIOS has already configured the ISA modem then "pnpdump
  --dumpregs" should show it.  If you need to set or change them use
  "isapnp".  Use the "pnpdump" to see what changes are possible.

  1.9.3.2.  Both PCI and ISA: Use setserial to tell the serial driver

  You must find the file where "setserial" is run at boot-time and add a
  line something like: "setserial /dev/ttyS2 irq 5 port 0x0b8".  For
  setserial v2.15 and later the results of running "setserial" on the
  command line may (or may not) be saved to /etc/serial.conf so that it
  runs each time you boot.  See ``What is Setserial'' for more info.
  See the next subsection ``All Modems'' for further instructions on
  quick installation.


  1.9.3.3.  Use MS Windows to set the BIOS (A last resort method)

  If you are using the BIOS to configure you may attempt to use MS
  Windows9x to "force" the BIOS to set a certain IRQ and/or IO.  It can
  set them into the PnP BIOS's flash memory where they will be used to
  configure for Linux as well as Windows.  See "Plug-and-Play-HOWTO and
  search for "forced" (occurs in several places).  For Windows3.x you
  can do the same thing using the ICU under Windows 3.x.  A few modems
  have a way to disable PnP in the modem hardware using software (under
  Windows) that came with the modem.


  1.9.4.  All Modems

  Plug the modem into a telephone line.  Then  configure a dialing
  program.  If you have an  Internet Service Provider (ISP) you might
  configure one of these : wvdial, gnome-ppp, modem lights (Gnome) or
  kppp.  They not only dial out but start PPP, which you need to connect
  to the Internet.  Otherwise, you might try configuring the minicom
  dialer which isn't designed for connecting to the Internet, but is
  good for testing.  Whether it's minicom or a dialer that starts PPP,
  set the serial port speed to a baud rate a few times higher than the
  bit rate of your modem.  See ``Speed Table'' for more details on the
  "best" speeds to use.  Tell it the full name of your serial port such
  as /dev/ttyS1 (or /dev/ttys/1).


  Minicom is one way to set up and test your modem.  Set hardware flow
  control (RTS/CTS).  With minicom you may check to see if your modem is
  there (and ready to dial).  Once you've set up minicom, type the
  command: AT, hit enter and you should see an "OK" response which comes
  directly from the modem.  See ``Dialing Out with Minicom''.


  2.  Modems for a Linux PC

  2.1.  External vs. Internal

  A modem for a PC may be either internal or external.  The internal one
  is installed inside of your PC (you must remove screws, etc. to
  install it) and the external one just plugs into a serial port
  connector on a PC.  Internal modems are less expensive, are less
  likely to to suffer data loss due to buffer overrun, usually use less
  electricity, and use up no space on your desk.

  External modems are usually easier to install and usually require less
  configuration.  They have lights which may give you a clue as to what
  is happening and aid in troubleshooting.  The fact that the serial
  port and modem can be physically separated also aids in
  troubleshooting.  External modems are easy to move to another
  computer.  If you need to turn the power off to reset your modem (this
  is seldom necessary) then with an external you don't have to power
  down the entire PC.


  Unfortunately most external modems have no switch to turn off the
  power supply when not in use and thus are likely to consume a little
  electricity even when turned off (unless you unplug the power supply
  from the wall).  Each watt they draw usually costs you over $1/yr.
  Another possible disadvantage of an external is that you will be
  forced to use an existing serial port which may not support a speed of
  over 115,200 bps (although as of late 2000 most new internal modems
  don't either --but some do).  For details ``Can't Set a High Enough
  Speed''

  Internal modems present a special problem for Linux, but will work
  just as well as external modems provided you avoid the ones that will
  work only for MS Windows.   Configuring them ranges from very easy
  (automatically) to difficult depending on both the modem, your skills,
  and how easy it is to find info about your modem --info that is not
  all in this HOWTO.  Some of the modems will work only under MS Windows
  due to lack of Linux modem drivers (for software modems).  If you buy
  a new one that you're not sure will work under Linux, try to get an
  agreement that you can return it for a refund if it doesn't work out.

  While most new modems are plug-and-play you have various ways to deal
  with the PnP configuring:

  ·  The serial driver does it all for you (more likely for a PCI modem)

  ·  Use the "isapnp" program

  ·  Let a PnP BIOS do the configuring

     The last 2 items of the above have shortcomings.  Isapnp
     documentation is difficult to understand although reading the Plug-
     and-Play-HOWTO (long) will aid in understanding it.  If you want
     the PnP BIOS to do the configuring, all you need to do is to make
     sure that it knows that you don't have a PnP operating system.  But
     it may not do it correctly and you may need to find out what it's
     done see ``What is set in my serial port hardware?''.

  Many Linux users still say that it's a lot simpler just to get an
  external modem and plug it in.  But if you get the right internal
  modem it may be just as easy.


  2.2.  Is a Driver Needed ?

  Hardware modems (including all serial external modems) don't really
  need any modem driver.  But any software modem (winmodem, linmodem)
  must have a modem driver (if it exists for Linux).  The serial port
  the modem resides on does need a driver and it's supplied either as a
  Linux serial module or compiled into the kernel.  Any serial driver
  for a PCI Modem should install automatically since they are detected
  by system software.

  Software modems require software to run them and obviously do need a
  driver.  The drivers for MS Windows are *.exe programs which will not
  run under Linux.  So you must use a Linux driver (if it exists).  See
  ``Software-based Modems (winmodems, linmodems)''


  2.3.  External Modems

  2.3.1.  PnP External Modems

  Many external modems are labeled "Plug and Play" (PnP) but they should
  all work fine as non-PnP modems.  While the serial port itself may
  need to be configured (IRQ number and IO address) unless the default
  configuration is OK, an external modem uses no such IRQ/IO
  configuration.  You just plug the modem into the serial port.

  How can an external modem be called PnP since it can't be configured
  by PnP?   Well, it has a special PnP identification built into it that
  can be read (thru the serial port) by a PnP operating system.  Such an
  operating system would then know that you have a modem on a certain
  port and would also know the id number.  If it's a software modem, it
  could try to locate a driver for it.  It could also tell application
  programs what port your modem is on.  (such as /dev/ttyS2 or COM3).
  But since you don't have such a PnP operating system you may need to
  configure your application program manually by giving it the /dev id
  (such as /dev/ttyS2).  Some programs can probe for a modem on various
  ports.


  2.3.2.  Cabling & Installation

  Connecting an external modem is simple compared to connecting most
  other devices to a serial port that require various types of "null
  modem" cables (which will not work for modems).  Modems use straight
  through cable, with no pins crossed over.  Most computer stores should
  have this.  Make sure you get the correct gender and number of pins.
  Hook up your modem to one of your serial ports.  If you are willing to
  accept the default IRQ and IO address of the port you connect it to,
  then you are ready to start your communication program and configure
  the modem itself.


  2.3.3.  What the Lights (LED's) Mean (for some external modems)


  ·  TM Test Modem

  ·  AA Auto Answer (If on, your modem will answer an incoming call)

  ·  RD Receive Data line = RxD

  ·  SD Send Data line = TxD

  ·  TR data Terminal Ready = DTR (set by your PC)

  ·  RI Ring Indicator (If on, someone is "ringing" your modem)

  ·  OH Off Hook (If off, your modem has hung up the phone line)

  ·  MR Modem Ready = DSR ??

  ·  EC Error Correction

  ·  DC Data Compression

  ·  HS High Speed (for this modem)


  2.4.  Internal Modems

  An internal modem is installed in a PC by taking off the cover of the
  PC and inserting the modem card into a vacant slot on the motherboard.
  There are modems for PCI slots,  other modems for the older ISA slots,
  and ARM software "modems" for the new small AMR slot.  Some new PCs
  don't have any ISA slots.   Only some newer PCs will have ARM slots.
  While external modems plug into the serial port (via a short cable)
  the internal modems have the serial port built into the modem.  In
  other words, the modem card is both a serial port and a modem.


  Setting the IO address and IRQ for a serial port was formerly done by
  jumpers on the card.  These are little black rectangular "cubes" about
  5x4x2 mm in size which push in over pins on the card.  Plug-and-Play
  modems (actually the serial port part of the modems) don't use jumpers
  for setting these but instead are configured by sending configuration
  commands to them over the bus inside the computer.  Such configuration
  commands can be sent by a PnP BIOS, by the isapnp program (for the ISA
  bus only), by setpci (for the PCI bus), or by newer serial device
  drivers for certain modems.  Under Linux you may have a choice of how
  to configure the ones that don't get io-irq configured by the serial
  driver.


  1. ISA bus: Use "isapnp" which may be run automatically at every boot-
     time

  2. Let a PnP BIOS do it, and then maybe tell setserial the IO and IRQ

  3. PCI bus: Use lspci -vv to look at it and setpci to configure.

  See ``Quick Install'' for more details, especially for the PCI bus.


  2.5.

  Software-based Modems (winmodems, linmodems)

  2.5.1.  Introduction to software modems (winmodems)

  Software modems turn over some (or even almost all) of the work of the
  modem to the main processor (CPU) chip of your computer (such as a
  Pentium chip).  This requires special software (a modem driver) to do
  the job.  Until late 1999, such software was released only for MS
  Windows and wouldn't work with Linux.  Even worse was that the maker
  of the modem kept the interface to the modem secret so that no one
  could write a Linux driver for it (even though a few volunteers were
  willing to write Linux drivers).

  But things have improved some since then so that today (late 2001)
  many such modems do have a linux driver.  There is no standard
  interface so that different brands/models of software-modems need
  different drivers (unless the different brands/models happen to use
  the same chipset internally).

  Another name for a software modem (used by MS) is "driver-based
  modem".  The conventional hardware-based modem (that works with Linux)
  doesn't need a modem driver (but does use the Linux serial driver)
  After about mid-1998 most new internal modems were software modems.

  Software modems fall into 2 categories: linmodems and winmodems.
  Winmodems will only work under MS Windows.  Linmodems will work under
  Linux (but formerly were mostly winmodems so some still call them
  "winmodems").  The term "Winmodem" is also a trademark for a certain
  model of "winmodem" but that's not the meaning of it in this document.


  2.5.2.  Linmodems

  In late 1999, two software-based modems appeared that could work under
  Linux and were thus called "linmodems".  Lucent Technologies (LT)
  unofficially released a Linux binary-only code to support most of its
  PCI modems.  PC-TEL (includes "Zoltrix") introduced a new software-
  based modem for Linux.  After that, interest increased for getting
  winmodems to work under linux.  There is a GPL'ed driver for Intel's
  (Modem Silicon Operations) MD563x HaM chipset (nee Ambient division of
  Cirrus Logic).  As of mid-2001 there are also drivers for: Conexant
  HSF and HCF, Motorola SM56, ESS (ISA only), and IBM's Mwave for
  Thinkpads 600+.

  What percent of software modems now (2001) work under Linux?  Well,
  there's a number of modem chips not supported: Lucent/Agere ARM
  (Scorpio), 3COM/US Robotics, some SmartLink (3 different chipsets),
  Ambient HSP, and possibly others.  But there might be support for some
  of these by the time you read this.  So it seems that over half the
  software modem chips are now supported (as of late 2001).

  Be warned in advance that determining if your modem is a linmodem may
  or may not be easy.  You may need to first find out what chipset you
  have and who makes it.  Just knowing the brand and model number of
  your modem may not be sufficient.  There are complex ways to find this
  out using say "lspci" (more than once) and then looking up the chip
  maker using the long modem number.  This requires checking a database
  or searching the Internet.  It's not always simple.  It could happen
  that you will put a fair amount of effort into this only to get the
  bad news that your modem isn't supported.  See Linmodem-HOWTO for more
  details.


  2.5.3.  Linmodem sites and documentation


  ·  Linmodem-HOWTO

  ·  Winmodems-and-Linux-HOWTO (not as well written as Linmodem-HOWTO)

  ·  <http://linmodems.org> is a project to turn winmodems into
     linmodems.  Has a mailing list.

  ·  Conexant+Rockwell-modem-HOWTO

  ·  modem list <http://personal.myvine.com/~gromitkc/winmodem.html>.
     Has links to linmodem info.

  ·  PCTel-HSP-MicroModem-Configuration-mini-HOWTO


  2.5.4.  Software-based modem types

  There are two basic types of software modems.  In one type the
  software does almost all of the work.  The other is where the software
  only does the "control" operations (which is everything except
  processing the digital waveshapes --to be explained later).  Since the
  hardware doesn't do the control it's called a "controllerless" modem.
  The first type is an all-software modem (sometimes just called a
  software modem).

  For both of these types there must be analog hardware in the modem to
  generate an electrical waveshape to send out the phone line.  It's
  generated from a digital signal (which is sort of a "digital
  waveshape").  It's something like the digital electronics creates a
  lot of discrete points on graph paper and then the modem draws a
  smooth curve thru them.  There must also be hardware to convert the
  incoming waveshape to digital.  This is just analog-to-digital
  conversion (and conversely).  It's done by a codec (coder-decoder).

  Then this digital waveshape must be converted to a data byte stream.
  This is known as demodulation, while turning data bytes into a digital
  waveshape is known as modulation.  The modem can't just send an
  incoming data byte stream to the PC but must first do decompression,
  error correction, and convert from serial to the parallel bus of the
  computer.  Likewise for an outgoing data byte stream.

  The difference between the two types of software-based modems is where
  the digital modulation takes place.  In the all-software modem this
  modulation is done in the CPU using a Host Signal Processor (HSP).  In
  the controllerless modem it's done in the modem but all other digital
  work is done by the CPU.  This other digital work consists of dealing
  with AT-commands, data compression, error correction, and simulating a
  serial port.  In the all-software modem, there are still two items
  handled by hardware: the A/D conversion of waveshapes by the codec and
  echo cancellation.


  2.5.5.  Is this modem a software modem?

  How do you determine if an internal modem is a software modem?  First
  see if the name, description of it, or even the name of the MS Windows
  driver for it indicates it's a software modem: HSP (Host Signal
  Processor) , HCF (Host Controlled Family), HSF (Host Signal Family),
  controllerless, host-controlled, host-based, and soft-...  modem.  If
  it's one of these modem it will only work for the cases where a Linux
  driver is available.  Since software modems cost less, a low price is
  a clue that it's a software modem.

  If you don't know the model of the modem and you also have Windows on
  your Linux PC, click on the "Modem" icon in the "Control Panel".  Then
  check out the modem list (see ``Web Sites''.  If the above doesn't
  work (or isn't feasible), you can look at the package it came in (or a
  manual).   Read the section on the package that says something like
  "Minimum System Requirements" or just "System Requirements".

  A hardware modem will work fine on old CPUs (such as the 386 or
  better).  So if it requires a modern CPU (such as a Pentium or other
  "high speed" CPU of say over 150 MHz) this is a clue that it's a all-
  software modem.  If it only requires a 486 CPU (or better) then it's
  likely a host-controlled software modem.   Saying that it only works
  with Windows is also bad news.  However, even in this case there may
  be a Linux driver for it.

  Otherwise, it may be a hardware modem if it fails to state explicitly
  that you must have Windows.  By saying it's "designed for Windows" it
  may only mean that it fully supports Microsoft's plug-and-play which
  is OK since Linux uses the same plug-and-play specs (but it's harder
  to configure under Linux).  Being "designed for Windows" thus gives no
  clue as to whether or not it will work under Linux.  You might check
  the Website of the manufacturer or inquire via email.  Some
  manufacturers are specifically stating that certain models work under
  Linux.  Sometimes they are linmodems that require you to obtain and
  install a certain linmodem driver.


  2.5.6.  Should I get a software modem?

  Only if you know there is a Linux driver for it that works OK.
  Besides the problems of getting a driver, what are the pros and cons
  of software modems?  Since the software modem uses the CPU to do some
  (or all) of its work, the software modem requires less on-board
  electronics and thus costs less.  At the same time, the CPU work load
  is increased by the modem which may result in slower operation.

  The percentage of loading of the CPU by the modem depends on both what
  CPU you have and whether or not it's an all-software modem.  For a
  modern CPU and a modem that only uses the CPU as a controller, there's
  little loss of performance.  Even if it's an all-software modem, you
  will not suffer a loss of performance if there are no other CPU-
  intensive tasks are running at the same time.  Of course, when you're
  not using the software modem there is no degradation in performance at
  all.
  Is the modem cost savings worth it?  In many cases yes, especially if
  you don't use the modem much and/or are not running any other CPU
  intensive tasks when the modem is in use.  The savings in modem cost
  could be used for a better CPU which would speed things up a little.
  But the on-board electronics of a modem can do the job more
  efficiently than a general purpose CPU (except that it's not efficient
  at all when it's not in use).  So if you use the modem a lot it's
  probably better to avoid all-software modems (and then you can use a
  less powerful CPU :-).


  2.6.  PCI Modems

  A PCI modem card is one which inserts into a PCI-bus slot on the
  motherboard of a PC.  While many PCI winmodems will not work under
  Linux (no driver available) other PCI modems will work under Linux.
  The Linux serial driver has been modified to support certain PCI
  hardware modem cards (but not winmodems/linmodems).  If it's a
  linmodem, it will work only if you install a certain linmodem driver.
  If the Linux serial driver supports your hardware modem then the
  driver will set up the PnP configuration for you.  See ``PCI Bus
  Support Underway''.  If no special support for your PCI hardware modem
  is in the Linux serial driver it may still work OK but you have to do
  some work to configure it.


  2.7.  AMR Modems

  These are all winmodems that insert into a special AMR (Audio Modem
  Riser) slot on the motherboard.  Audio cards are sometimes used in
  this slot.  The slot's main use is for HSF type modems where the CPU
  does almost all of the work.  This results in a small modem card and
  thus a short AMR slot.  Such a "modem" is actually little more than a
  codec which transforms digital signals representing an analog voltage
  wave into the analog wave itself (and conversely).  Linux supports at
  least one of them.


  2.8.  USB Modems

  USB = Universal Serial Bus.  Some USB modems work with Linux and some
  don't.  Linux has support for modems that conform to the USB
  Communication Device Class Abstract Control Model (= USB CDC ACM).
  There's a module for ACM named acm.o.  See the /usb/acm.txt document
  in the kernel documentation directory (/usr/share/doc/kernel-doc-2.4.x
  in Debian, perhaps /usr/doc/kernel... in some distributions).  The ACM
  "serial port" for the first (0th) such modem is: /dev/usb/acm/0 or
  possibly /dev/usb/ttyACM0.  This should be the case regardless of
  whether or not you use the new "device file system".  It's not really
  a serial port, but the driver makes it look like a serial port to
  software which uses the modem.


  2.9.  Which Internal Modems might not work with Linux


  ·  ``Software-based Modems (winmodems,  linmodems)'' Only about half
     have a Linux driver available.

  ·  ``MWave and DSP Modems'' might work, but only if you first start
     Windows/Dos each time you power on your PC

  ·  Modems with ``RPI (Rockwell)'' drivers work but with reduced
     performance


  2.9.1.  MWave and some DSP Modems

  Note that there's now a Linux driver for the ACP (Mwave) modem used in
  IBM Thinkpads 600+.  See the mini-HOWTO: ACP-Modem.

  While hardware modems used use DSPs (Digital Signal Processors) some
  of these DSPs are programmed by a driver which must be downloaded from
  the hard disk to the DSPs memory just before using the modem.
  Unfortunately, such downloading is normally done by Dos/Windows
  programs (which doesn't work for Linux).  But there has been
  substantial success in getting some of these modems to work with
  Linux.  For example, there is a Linux driver available to run a Lucent
  (DSP) modem.

  Ordinary modems that work fine with Linux (without needing a driver
  for the modem) often have a DSP too (and may mention this on the
  packaging), but the program that runs the DSP is stored inside the
  modem.  These work fine under Linux.  An example of a DSP modem that
  has problems working under Linux is the old IBM's Aptiva MWAVE.

  One way to get some DSP modems to work with Linux is to boot from DOS
  (if you have it on your Linux PC).  You first install the driver under
  DOS (using DOS and not Window drivers).  Then start Dos/Windows and
  start the driver for the modem so as to program the DSP.  Then without
  turning off the computer, start Linux.

  One may write a "batch" file (actually a script) to do this.  Here is
  an example but you must modify it to suit your situation.


       rem mwave is a batch file supplied by the modem maker
       call c:\mww\dll\mwave start
       rem loadlin.exe is a DOS program that will boot Linux from DOS (See
       rem Config-HOWTO).
       c:\linux\loadlin f:\vmlinuz root=/dev/hda3 ro



  One may create an icon for the Window's desktop which points to such a
  batch file and set the icon properties to "Run in MSDOS Mode".  Then
  by clicking on this icon one sets up the modem and goes to Linux.
  Another possible way to boot Linux from DOS is to press CTRL-ALT-DEL
  and tell it to reboot (assuming that you have set things up so that
  you can boot directly into Linux).  The modem remains on the same com
  port (same IO address) that it used under DOS.

  The Newcom ifx modem needs a small kernel patch to work correctly
  since its simulation of a serial port is non-standard.  The patch and
  other info for using this modem with Linux is at
  <http://quinine.pharmacy.ohio-state.edu/~ejolson/linux/newcom.html>.


  2.9.2.  Rockwell (RPI) Drivers

  Some older Rockwell chips need Rockwell RPI (Rockwell Protocol
  Interface) drivers for compression and error correction.  They can
  still be used with Linux even though the driver software works only
  under MS Windows.  This is because the MS Windows software (which you
  don't have) does only compression and error correction.  If you are
  willing to operate the modem without compression and error correction
  then it's feasible to use it with Linux.  To do this you will need to
  disable RPI by sending the modem (via the initialization string) a
  "RPI disable" command each time you power on your modem.  On my old
  modem this command was +H0.  Not having data compression available
  makes it slower to get webpages but is just as fast when downloading
  files that are already compressed.


  3.  Modem Pools

  3.1.  Introduction

  These are multiple modems which might be used by an ISP or by an
  organization that has a number of phone lines for dialing in and out.
  There are two types of modem pools: analog and digital.  An ISP will
  use digital so that they can support 56k (V.90, V.92) modems at near
  maximum speed.

  A modem pool could be a number of modems on the same card (such as an
  analog multi-port modem card) or many modems in an external chassis
  (something like an external modem).  The modems may be analog modems
  similar to modems used for home/office PCs (can't send above 33.6k
  even if they are "56k modems").  They also could be "digital modems"
  which can send at nearly 56k (if you have a good line).  The "digital
  modems" require a digital connection to the telephone line and don't
  use any serial ports at all.  All of these modem pools will require
  that you install special drivers for them.


  3.2.  Analog Modem Pools, Multi-modem Cards

  A "multimodem card" is short for "multiport modem card".  Some put a
  hyphen after "multi": multi-modem or multi-port.  An analog modem pool
  is just many analog modems (the common home/office modem) provided
  either on an internal plug-in card or in an external chassis.  Each
  modem comes with a built-in serial port.  There is usually a system of
  sharing interrupts or of handling interrupts by their own electronics,
  thus removing much of this burden from the CPU.  Note that these
  modems are not "digital modems" and will thus not be able to use 56k
  for people who dial-in.

  Here is a list of some companies that make analog multiport modem
  cards which plug into slots in a PC.  8 modems/card is common.  The
  cards listed claim to work with Linux and the websites should point
  you to a driver for them.

  Multi-modem Cards (analog, not digital):

  ·  Equinox SST Multi-modem. PCI, 56k, 4 or 8 ports
     <http://www.equinox.com/product/multi-modem.htm>

  ·  MultiModemISI by Multi-Tech Systems.  56k or 33.6k, PCI, 4 or 8
     ports.  ISDN/56k hybrids.
     <http://www.multitech.com/PRODUCTS/MultiModemISI/>

  ·  PCI-RAS cards by Perle. 56k, 4 or 8 ports.
     <http://www.perle.com/solutions/app_notes/multi_modem/pci_ras_fax.html>

  ·  RocketModem by Comtrol.  ISA 33.6k, 4 or 8 port.
     <http://www.comtrol.com/sales/specs/rm.htm>

  ·  RocketModem II by Comtrol.  PCI 56k, 4 or 6 port
     <http://www.comtrol.com/sales/specs/rmii.htm>

  ·  RockForce. 56k, 2 or 4 port Two port V.92/V.44
     <http://www.mainpine.com/> #RockForce+ Two port V.90
     (www.mainpine.com/prodrockplus.html) #RockForceDUO Two port
     V.92/V.44 (www.mainpine.com/prodduo.html) #RockForceQUATRO Four
     port V.92/V.44 (www.mainpine.com/prodquatro.html) #RockForceDUO+
     Two port V.92/V.44/V.34 SuperG3 Fax =
     #(www.mainpine.com/prodduoplus.html) #RockForceQUATRO+ Four port
     V.92/V.44/V.34 SuperG3 Fax =
     #(www.mainpine.com/prodquatroplus.html) #

  ·  Multi-modem communication adapters by Digi.
     <http:/www.dgii.com/solutions/mmcommadapters/index.html>


  3.3.  Digital Modems, RAS

  "digital modems" are much different than the analog modems that most
  people use in their PCs.  They require a digital connection to the
  telephone line and don't use serial ports for the interface to the
  computer.  Instead, they interface directly to a computer bus via a
  special card(s) (which may also contain the "digital modems").  They
  are able to send at near 56k, something no analog modem can do.  They
  are often a component of "remote access servers" (RASs) or "digital
  modem pools"

  The cables from the phone company that carry digital signals have been
  designed for high bandwidth so that the same cable carries multiple
  telephone calls.  It's done by "time-division multiplexing".  A single
  phone call in a cable is carried on two different channels, one for
  each direction.  So the RAS must connect each such channel-pair to the
  appropriate "digital modem" that services that phone call.  Such tasks
  are done by what is sometimes called a "...  concentrator".

  Now the digital signal received by a "digital modem" may really
  represent an analog signal which has been sent to it by an analog
  modem.  One way to deal with it would be to convert it to an analog
  signal and then put that thru an analog modem to get the digital data
  sent by the analog modem.  But why do all this work?  Since the signal
  is already in digital form, why not process it digitally?   That's how
  it's done.  The digital signal is processed and converted to another
  digital stream of bytes which represents data bytes sent by the analog
  modem.  A "digital signal processor" (DSP) is commonly used for this
  task.  A CPU could also handle it but it would be heavily loaded.

  Likewise, a "digital modem" must handle sending digital signals in the
  opposite direction from a RAS to a digital telephone line.  Thus it
  only makes digital-to-digital conversions and doesn't deal in analog
  at all.  It thus is not really a modem at all since it doesn't
  modulate any analog carrier.  So the name "digital modem" is a
  misnomer but it does do the job formerly done by modems.  Thus some
  "digital modems" call themselves "digital signal processors", or
  "remote access servers", etc. and may not even mention the word
  "modem".

  Such a RAS system may be a stand-alone proprietary server, a chassis
  containing digital modems that connects to a PC via a special
  interface card, or just a card itself.  Digi calls one such card a
  "remote access server concentrator adapter".   One incomplete
  description of what is needed to become an ISP is: See What do I need
  to be an ISP?
  </http://www.cyclades.com/solutions/techtalk/techtalk0l.php>.
  Cyclades promotes their own products here so please do comparison
  shopping before buying anything.


  4.  Serial Port and Modem Basics

  You don't have to understand the basics to use and install a modem.
  But understanding it may help to determine what is wrong if you run
  into problems.  After reading this section, if you want to understand
  it even better you may want to see ``How Modems Work'' in this
  document (not yet complete).  More details on the serial port
  (including much of this section) will be found in Serial-HOWTO.
  4.1.  Modem Converts Digital to Analog (and conversely)

  Most all telephone main lines are digital already but the lines
  leading to your house (or business) are usually analog which means
  that they were designed to transmit a voltage wave which is an exact
  replica of the sound wave coming out of your mouth.  Such a voltage
  wave is called "analog".  If viewed on an oscilloscope it looks like a
  sine wave of varying frequency and amplitude.  A digital signal is
  like a square wave.  For example 3 v (volts) might be a 1-bit and 0 v
  could be a 0-bit.  For most serial ports (used by external modems) +12
  v is a 0-bit and -12 v is a 1-bit (some are + or - 5 v).

  To send data from your computer over the phone line, the modem takes
  the digital signal from your computer and converts it to "analog".  It
  does this by both creating an analog sine wave and then "MODulating"
  it.  Since the result still represents digital data, it could also be
  called a digital signal instead of analog.  But it looks something
  like an analog signal and almost everyone calls it analog.  At the
  other end of the phone line another modem "DEModulates" this signal
  and the pure digital signal is recovered.  Put together the "mod" and
  "dem" parts of the two words above and you get "modem" (if you drop
  one of the two d's).  A "modem" is thus a MODulator-DEModulator.  Just
  what modulation is may be found in the section ``Modulation Details''.



  4.2.  What is a Serial Port ?

  4.2.1.  Intro to Serial

  The UART serial port (or just "serial port for short" is an I/O
  (Input/Output) device.  Since modems have a serial port between them
  and the computer, it's necessary to understand the serial port as well
  as the modem.

  Most PC's have one or two serial ports.  Each has a 9-pin connector
  (sometimes 25-pin) on the back of the computer.  Computer programs can
  send data (bytes) to the transmit pin (output) and receive bytes from
  the receive pin (input).  The other pins are for control purposes and
  ground.

  The serial port is much more than just a connector.  It converts the
  data from parallel to serial and changes the electrical representation
  of the data.  Inside the computer, data bits flow in parallel (using
  many wires at the same time).  Serial flow is a stream of bits over a
  single wire (such as on the transmit or receive pin of the serial
  connector).  For the serial port to create such a flow, it must
  convert data from parallel (inside the computer) to serial on the
  transmit pin (and conversely).

  Most of the electronics of the serial port is found in a computer chip
  (or a part of a chip) known as a UART.  For more details on UARTs see
  the section "What are UARTS" in the Serial-HOWTO.

  But you may want to finish this section first so that you will
  hopefully understand how the UART fits into the overall scheme of
  things.


  4.2.2.  Pins and Wires

  Old PC's used 25 pin connectors but only about 9 pins were actually
  used so today most connectors are only 9-pin.  Each of the 9 pins
  usually connects to a wire.  Besides the two wires used for
  transmitting and receiving data, another pin (wire) is signal ground.
  The voltage on any wire is measured with respect to this ground.  Thus
  the minimum number of wires to use for 2-way transmission of data is
  3.  Except that it has been known to work with no signal ground wire
  but with degraded performance and sometimes with errors.

  There are still more wires which are for control purposes (signalling)
  only and not for sending bytes.  All of these signals could have been
  shared on a single wire, but instead, there is a separate dedicated
  wire for every type of signal.  Some (or all) of these control wires
  are called "modem control lines".  Modem control wires are either in
  the asserted state (on) of +12 volts or in the negated state (off) of
  -12 volts.  One of these wires is to signal the computer to stop
  sending bytes out the serial port cable.  Conversely, another wire
  signals the device attached to the serial port to stop sending bytes
  to the computer.  If the attached device is a modem, other wires may
  tell the modem to hang up the telephone line or tell the computer that
  a connection has been made or that the telephone line is ringing
  (someone is attempting to call in).  See the Serial-HOWTO: Pinout and
  Signals for more details.


  4.2.3.  Internal Modem Contains Serial Port

  For an internal modem there is no 9-pin connector but the behavior is
  almost exactly as if the above mentioned cable wires existed.  Instead
  of a a 12 volt signal in a wire giving the state of a modem control
  line, the internal modem may just use a status bit in its own memory
  (a register) to determine the state of this non-existent "wire".  The
  internal modem's serial port looks just like a real serial port to the
  computer.  It even includes the speed limits that one may set at
  ordinary serial ports such as 115200 bits/sec.  Unfortunately for
  Linux, many internal modems today don't work exactly this way but
  instead use software (running on the CPU) to do much of the modem's
  work.  Unfortunately, such software is often only available for the MS
  Windows OS (it hasn't been ported to Linux).  Thus you can't use most
  of these modems with Linux See ``Software-based Modems (winmodems)''.



  4.3.  IO Address & IRQ

  Since the computer needs to communicate with each serial port, the
  operating system must know that each serial port exists and where it
  is (its I/O address).  It also needs to know which wire (IRQ number)
  the serial port must use to request service from the computer's CPU.
  It requests service by sending an interrupt on this wire.  Thus every
  serial port device must store in its non-volatile memory both its I/O
  address and its Interrupt ReQuest number: IRQ.  See ``Interrupts''.
  For the PCI bus it doesn't work exactly this way since the PCI bus has
  its own system of interrupts.  But since the PCI-aware BIOS sets up
  chips to map these PCI interrupts to IRQs, it seemingly behaves just
  as described above except that sharing of interrupts is allowed (2 or
  more devices may use the same IRQ number).

  I/O addresses are not the same as memory addresses.  When an I/O
  addresses is put onto the computer's address bus, another wire is
  energized.  This both tells main memory to ignore the address and
  tells all devices which have I/O addresses (such as the serial port)
  to listen to the address to see if it matches the device's.  If the
  address matches, then the I/O device reads the data on the data bus.



  4.4.  Names: ttyS0, ttyS1, etc.

  The serial ports are named ttyS0, ttyS1, etc. (and usually correspond
  respectively to COM1, COM2, etc. in DOS/Windows).  The /dev directory
  has a special file for each port.  Type "ls /dev/ttyS*" to see them.
  Just because there may be (for example) a ttyS3 file, doesn't
  necessarily mean that there exists a physical serial port there.

  Which one of these names (ttyS0, ttyS1, etc.) refers to which physical
  serial port is determined as follows.  The serial driver (software)
  maintains a table showing which I/O address corresponds to which ttyS.
  This mapping of names (such as ttyS1) to I/O addresses (and IRQ's) may
  be both set and viewed by the "setserial" command.  See ``What is
  Setserial''.  This does not set the I/O address and IRQ in the
  hardware itself (which is set by jumpers or by plug-and-play
  software).  Thus which physical port corresponds to say ttyS1 depends
  both on what the serial driver thinks (per setserial) and what is set
  in the hardware.  If a mistake has been made, the physical port may
  not correspond to any name (such as ttyS2) and thus it can't be used.
  See ``Serial Port Devices /dev/ttyS2, etc.'' for more details>


  4.5.  Interrupts

  Bytes come in over the phone line to the modem, are converted from
  analog to digital by the modem and passed along to the serial port on
  their way to their destination inside your computer.  When the serial
  port receives a number of bytes (may be set to 1, 4, 8, or 14) into
  its FIFO buffer, it signals the CPU to fetch them by sending an
  electrical signal known as an interrupt on a certain wire normally
  used only by that port.  Thus the FIFO waits until it has received a
  number of bytes and then issues an interrupt.

  However, this interrupt will also be sent if there is an unexpected
  delay while waiting for the next byte to arrive (known as a timeout).
  Thus if the bytes are being received slowly (such as from someone
  typing on a terminal keyboard) there may be an interrupt issued for
  every byte received.  For some UART chips the rule is like this: If 4
  bytes in a row could have been received in an interval of time, but
  none of these 4 show up, then the port gives up waiting for more bytes
  and issues an interrupt to fetch the bytes currently in the FIFO.  Of
  course, if the FIFO is empty, no interrupt will be issued.

  Each interrupt conductor (inside the computer) has a number (IRQ) and
  the serial port must know which conductor to use to signal on.  For
  example, ttyS0 normally uses IRQ number 4 known as IRQ4 (or IRQ 4).  A
  list of them and more will be found in "man setserial" (search for
  "Configuring Serial Ports").  Interrupts are issued whenever the
  serial port needs to get the CPU's attention.  It's important to do
  this in a timely manner since the buffer inside the serial port can
  hold only 16 incoming bytes.  If the CPU fails to remove such received
  bytes promptly, then there will not be any space left for any more
  incoming bytes and the small buffer may overflow (overrun) resulting
  in a loss of data bytes.

  For an external modem, there may be no way (such as flow control) to
  stop the flow rapidly enough to prevent such an overrun.  For an
  internal modem, the 16-byte FIFO buffer is on the same card and a good
  modem will not write to it if it's full.  Thus a good internal modem
  will not overrun the 16-byte buffers but it may need to use ``Modem-
  to-Modem Flow Control'' to prevent the modem itself from being
  overrun.  This is one advantage of an internal modem over an external.

  Interrupts are also issued when the serial port has just sent out all
  of its bytes from its small transmit FIFO buffer out the external
  cable.  It then has space for 16 more outgoing bytes.  The interrupt
  is to notify the CPU of that fact so that it may put more bytes in the
  small transmit buffer to be transmitted.  Also, when a modem control
  line changes state, an interrupt is issued.

  The buffers mentioned above are all hardware buffers.  The serial port
  also has large buffers in main memory.  This will be explained later

  Interrupts convey a lot of information but only indirectly.  The
  interrupt itself just tells a chip called the interrupt controller
  that a certain serial port needs attention.  The interrupt controller
  then signals the CPU.  The CPU then runs a special program to service
  the serial port.  That program is called an interrupt service routine
  (part of the serial driver software).  It tries to find out what has
  happened at the serial port and then deals with the problem such a
  transferring bytes from (or to) the serial port's hardware buffer.
  This program can easily find out what has happened since the serial
  port has registers at IO addresses known to the the serial driver
  software.  These registers contain status information about the serial
  port.  The software reads these registers and by inspecting the
  contents, finds out what has happened and takes appropriate action.


  4.6.  Data Compression (by the Modem)

  Before continuing with the basics of the serial port, one needs to
  understand about something done by the modem: data compression.  In
  some cases this task is actually done by software run on the
  computer's CPU but unfortunately at present, such software only works
  for MS Windows.  The discussion here will be for the case where the
  modem itself does the compression since this is what must happen in
  order for the modem to work under Linux.

  In order to send data faster over the phone line, one may compress
  (encode it) using a custom encoding scheme which itself depends on the
  data.  The encoded data is smaller than the original (less bytes) and
  can be sent over the Internet in less time.  This process is called
  "data compression".

  If you download files from the Internet, they are likely already
  compressed and it is not feasible for the modem to try to compress
  them further.  Your modem may sense that what is passing thru has
  already been compressed and refrain from trying a compress it any
  more.  If you are receiving data which has been compressed by the
  other modem, your modem will decompress it and create many more bytes
  than were sent over the phone line.  Thus the flow of data from your
  modem into your computer will be higher than the flow over the phone
  line to you.  The ratio of this flow is called the compression ratio.
  Compression ratios as high as 4 are possible, but not very likely.


  4.7.  Error Correction

  Similar to data compression, modems may be set to do error correction.
  While there is some overhead cost involved which slows down the
  byte/sec flow rate, the fact that error correction strips off start
  and stop bits actually increases the data byte/sec flow rate.

  For the serial port's interface with the external world, each 8-bit
  byte has 2 extra bits added to it: a start-bit and a stop-bit.
  Without error correction, these extra start and stop bits usually go
  right thru the modem and out over the phone lines.  But when error
  correction is enabled, these extra bits are stripped off and the 8-bit
  bytes are put into packets.  This is more efficient and results in
  higher byte/sec flow in spite of the fact that there are a few more
  bytes added for packet headers and error correction purposes.

  4.8.  Data Flow (Speeds)

  Data (bytes representing letters, pictures, etc.) flows from your
  computer to your modem and then out on the telephone line (and
  conversely).  Flow rates (such as 56k (56000) bits/sec) are
  (incorrectly) called "speed".  But almost everyone says "speed"
  instead of "flow rate".  If there were no data compression the flow
  rate from the computer to the modem would be about the same as the
  flow rate over the telephone line.

  Actually there are two different speeds to consider at your end of the
  phone line:


  ·  The speed on the phone line itself (DCE speed) modem-to-modem

  ·  The speed from your computer's serial port to your modem (DTE
     speed)

  When you dial out and connect to another modem on the other end of the
  phone line, your modem often sends you a message like "CONNECT 28800"
  or "CONNECT 115200".  What do these mean?  Well, its either the DCE
  speed or the DTE speed. If it's higher than the advertised modem speed
  it must be the DTE modem-to-computer speed.  This is the case for the
  115200 speed shown above.  The 28800 must be a DCE (modem-to-modem)
  speed since the serial port has no such speed.  One may configure the
  modem to report either speed.  Some modems report both speeds and
  report the modem-to-modem speed as (for example): CARRIER 28800.

  If you have an internal modem you would not expect that there would be
  any speed limit on the DTE speed from your modem to your computer
  since you modem is inside your computer and is almost part of your
  computer.  But there usually is since the modem contains a dedicated
  serial port within it.  On some software modems there is no such speed
  limit.

  It's important to understand that the average speed is often less than
  the specified speed, especially on the short DTE computer-to-modem
  line.  Waits (or idle time) result in a lower average speed.  These
  waits may include long waits of perhaps a second due to ``Flow
  Control''.  At the other extreme there may be very short waits (idle
  time) of several micro-seconds separating the end of one byte and the
  start of the next byte.  In addition, modems will fallback to lower
  speeds if the telephone line conditions are less than pristine.

  For a discussion of what DTE speed is best to use see section ``What
  Speed Should I Use''.



  4.9.  Flow Control

  Flow control means the ability to slow down the flow of bytes in a
  wire.  For serial ports this means the ability to stop and then
  restart the flow without any loss of bytes.  Flow control is needed
  for modems and other hardware to allow a jump in instantaneous flow
  rates.


  4.9.1.  Example of Flow Control

  For example, consider the case where you connect a 33.6k external
  modem via a short cable to your serial port.  The modem sends and
  receives bytes over the phone line at  33.6k bits per second (bps).
  Assume it's not doing any data compression or error correction.  You
  have set the serial port speed to 115,200 bits/sec (bps), and you are
  sending data from your computer to the phone line.  Then the flow from
  the your computer to your modem over the short cable is at 115.2k bps.
  However the flow from your modem out the phone line is only 33.6k bps.
  Since a faster flow (115.2k) is going into your modem than is coming
  out of it, the modem is storing the excess flow (115.2k -33.6k = 81.6k
  bps) in one of its buffers.  This buffer would soon overrun (run out
  of free storage space) unless the high 115.2k flow is stopped.

  But now flow control comes to the rescue.  When the modem's buffer is
  almost full, the modem sends a stop signal to the serial port.  The
  serial port passes on the stop signal on to the device driver and the
  115.2k bps flow is halted.  Then the modem continues to send out data
  at 33.6k bps drawing on the data it previous accumulated in its
  buffer.  Since nothing is coming into this buffer, the number of bytes
  in it starts to drop.  When almost no bytes are left in the buffer,
  the modem sends a start signal to the serial port and the 115.2k flow
  from the computer to the modem resumes.  In effect, flow control
  creates an average flow rate in the short cable (in this case 33.6k)
  which is significantly less than the "on" flow rate of 115.2k bps.
  This is "start-stop" flow control.

  In the above simple example it was assumed that the modem did no data
  compression.  This could happen when the modem is sending a file which
  is already compressed and can't be compressed further.  Now let's
  consider the opposite extreme where the modem is compressing the data
  with a high compression ratio.  In such a case the modem might need an
  input flow rate of say 115.2k bps to provide an output (to the phone
  line) of 33.6k bps (compressed data).  This compression ratio is 3.43
  (115.2/33.6).   In this case the modem is able to compress the 115.2
  bps PC-to-modem flow and send the same data (in compressed form) out
  the phone line at 33.6bps.  There's no need for flow control here so
  long as the compression ratio remains higher that 3.43.  But the
  compression ratio varies from second to second and if it should drop
  below 3.43, flow control will be needed

  In the above example, the modem was an external modem.  But the same
  situation exists (as of early 2003) for most internal modems.  There
  is still a speed limit on the PC-to-modem speed even though this flow
  doesn't take place over an external cable.  This makes the internal
  modems compatible with the external modems.

  In the above example of flow control, the flow was from the computer
  to a modem.  But there is also flow control which is used for the
  opposite direction of flow: from a modem (or other device) to a
  computer.  Each direction of flow involves 3 buffers: 1. in the modem
  2. in the UART chip (called FIFOs) and 3. in main memory managed by
  the serial driver.  Flow control protects all buffers (except the
  FIFOs) from overflowing.

  Under Linux, the small UART FIFO buffers are not protected by flow
  control but instead rely on a fast response to the interrupts they
  issue.  Some UART chips can be set to do hardware flow control to
  protect their FIFOs but Linux (as of early 2003) doesn't seem to
  support it.  FIFO stand for "First In, First Out" which is the way it
  handles bytes in a queue.  All the 3 buffers use the FIFO rule but
  only the one in the UART is named "FIFO".   This is the essence of
  flow control but there are still some more details.

  If you have set the highest PC-to-modem speed, you may not need flow
  control in the direction from the modem to a PC.  For a complex
  example of a case where it's needed see "Complex Flow Control Example"
  in the Serial-HOWTO.  To slow down this incoming flow, your modem must
  tell the other modem to stop sending.  See ``Modem-to-Modem Flow
  Control''.

  4.9.2.  Hardware vs. Software Flow Control

  If feasible, it's best to use "hardware" flow control that uses two
  dedicated "modem control" wires to send the "stop" and "start"
  signals.  Hardware flow control at the serial port works like this:
  The two pins, RTS (Request to send) and CTS (Clear to send) are used.
  When the computer is ready to receive date it asserts RTS by putting a
  positive voltage on the RTS pin (meaning "Request To Send to me").
  When the computer is not able to receive any more bytes, it negates
  RTS by putting a negative voltage on the pin saying: "stop sending to
  me".  The RTS pin is connected by the serial cable to another pin on
  the modem, printer, terminal, etc.  This other pin's only function is
  to receive the signal.

  For the case of a modem this "other" pin will be the modem's RTS pin.
  But for a printer, another PC, or a non-modem device, it's usually a
  CTS pin so a "crossover" or "null modem" cable is required.  This
  cable connects the CTS pin at one end with the RTS pin at the other
  end (two wires since each end of the cable has a CTS pin).  For a
  modem, a straight-thru cable is used.

  For the opposite direction of flow a similar scheme is used.  For a
  modem, the CTS pin is used to send the flow control signal to the CTS
  pin on the PC.  For a non-modem, the RTS pin sends the signal.  Thus
  modems and non-modems have the roles of their RTS and CTS pins
  interchanged.  Some non-modems such as dumb terminals may use other
  pins for flow control such as the DTR pin instead of RTS.

  Software flow control uses the main receive and transmit wires to send
  the start and stop signals.  It uses the ASCII control characters DC1
  (start) and DC3 (stop) for this purpose.  They are just inserted into
  the regular stream of data.  Software flow control is not only slower
  in reacting but also does not allow the sending of binary data unless
  special precautions are taken.  Since binary data will likely contain
  DC1 and DC3, special means must be taken to distinguish between a DC3
  that means a flow control stop and a DC3 that is part of the binary
  code.  Likewise for DC1.  To get software flow control to work for
  binary data requires both modem (hardware) and software support


  4.9.3.  Symptoms of No Flow Control

  Understanding flow-control theory can be of practical use.  For
  example I used my modem to access the Internet and it seemed to work
  fine.  But after a few months, I tried to send out long files from my
  PC and a huge amount of retries and errors resulted but it finally
  succeeded.  Receiving in the other direction (from my ISP to me)
  worked fine.  The problem turned out to be a modem with flow control
  disabled.  My modem's buffer was overflowing (overrunning) on long
  outgoing files since no "stop" signal was ever sent to my computer to
  halt sending to the modem.  There was no problem in the direction from
  the modem to my computer since the capacity (say 115.2 kbps) was
  always higher than the flow from the telephone line.  But it was a
  problem in the other direction where the PC would send at 115.2 kbps
  and the modem couldn't handle this high flow resulting in overruns of
  the modem's buffer.  The fix was to enable flow control by putting
  into the modem's init string an enable-flow-control command.


  4.9.4.  Modem-to-Modem Flow Control

  This is the flow control of the data sent over the telephone lines
  between two modems.  It works as long as error correction is enabled.
  Actually, even without error correction it's possible to enable
  software flow control between modems but it may interfere with sending
  binary data so it's not often used.
  4.10.  Data Flow Path; Buffers

  It's been mention that there are 3 buffers for each direction of flow
  (3 pairs altogether): 16-byte FIFO buffers (in the UART), a pair of
  larger buffers inside a device connected to the serial port (such as a
  modem), and a pair of buffers (say 8k) in main memory.  When an
  application program sends bytes to the serial port they first get
  stashed in the transmit serial port buffer in main memory.  The other
  member of the pair consists of a receive buffer for the opposite
  direction of byte-flow.  Here's an example diagram for the case of
  browsing the Internet with a browser.  Transmit data flow is left to
  right while receive flow is right to left.  There is a separate buffer
  for each direction of flow.


  application     8k-byte         16-byte        1k-byte        tele-
  BROWSER ------- MEMORY -------- FIFO --------- MODEM -------- phone
  program         buffer          buffer         buffer         line



  For the transmit case, the serial device driver takes out say 16 bytes
  from this transmit buffer (in main memory), one byte at a time and
  puts them into the 16-byte transmit buffer in the serial UART for
  transmission.  Once in that transmit buffer, there is no way to stop
  them from being transmitted.  They are then transmitted to the modem
  or (other device connected to the serial port) which also has a fair
  sized (say 1k) buffer.  When the device driver (on orders from flow
  control) stops the flow of outgoing bytes from the computer, what it
  actually stops is the flow of outgoing bytes from the large transmit
  buffer in main memory.  Even after this has happened and the flow to
  the modem has stopped, an application program may keep sending bytes
  to the 8k transmit buffer until it becomes fill.  At the same time,
  the bytes stored in the FIFO and MODEM continue to be sent out until
  these buffers empty.

  When the memory buffer gets fill, the application program can't send
  any more bytes to it (a "write" statement in a C_program blocks) and
  the application program temporarily stops running and waits until some
  buffer space becomes available.  Thus a flow control "stop" is
  ultimately able to stop the program that is sending the bytes.  Even
  though this program stops, the computer does not necessarily stop
  computing since it may switch to running other processes while it's
  waiting at a flow control stop.

  The above was a little oversimplified in three ways.  First, some
  UARTs can do automatic hardware flow control which can stop the
  transmission out of the FIFO buffers if needed (not yet supported by
  Linux).  Second, while an application process is waiting to write to
  the transmit buffer, it could possibly perform other tasks.  Third,
  the serial driver (located between the memory buffer and the FIFO) has
  it's own buffer (in main memory) used to process characters.


  4.11.  Modem Commands

  Commands to the modem are sent to it from the communication software
  over the same conductor as used to send data.  The commands are short
  ASCII strings.  Examples are "AT&K3" for enabling hardware flow
  control (RTS/CTS) between your computer and modem; and "ATDT5393401
  for Dialing the number 5393401.  Note all commands are prefaced by
  "AT".   Some commands such as enabling flow control help configure the
  modem.  Other commands such as dialing a number actually do something.
  There are about a hundred or so different possible commands.  When
  your communication software starts running, it first sends an "init"
  string of commands to the modem to configure it.  All commands are
  sent on the ordinary data line before the modem dials (or receives a
  call).

  Once the modem is connected to another modem (on-line mode),
  everything that is sent from your computer to your modem goes directly
  to the other modem and is not interpreted by the modem as a command.
  There is a way to "escape" from this mode of operation and go back to
  command mode where everything sent to the modem will be interpreted as
  a command.  The computer just sends "+++" with a specified time
  spacing before and after it.  If this time spacing is correct, the
  modem reverts to command mode.  Another way to do this is by a signal
  on a certain modem control line.

  There are a number of lists of modem commands on the Internet.  The
  section ``Web Sites'' has links to a couple of such web-sites.
  Different models and brands of modems do not use exactly the same set
  of such commands.  So what works for one modem might not work for
  another.  Some common commands (not guaranteed to work on all modems)
  are listed in this HOWTO in the section ``Modem Configuration''



  4.12.  Serial Driver Module

  The device driver for the serial port is the software that operates
  the serial port.  It is now provided as a serial module.  From kernel
  2.2 on, this module will normally get loaded automatically if it's
  needed.  In earlier kernels, you had to have kerneld running in order
  to do auto-load modules on demand.  Otherwise the serial module needed
  to be explicitly listed in /etc/modules.  Before modules became
  popular with Linux, the serial driver was usually built into the
  kernel (and sometimes still is).  If it's built-in don't let the
  serial module load or else you will have two serial drivers running at
  the same time.  With 2 drivers there are all sorts of errors including
  a possible "I/O error" when attempting to open a serial port.  Use
  "lsmod" to see if the module is loaded.

  When the serial module is loaded it displays a message on the screen
  about the existing serial ports (often showing a wrong IRQ).  But once
  the module is used by setserial to tell the device driver the
  (hopefully) correct IRQ then you should see a second display similar
  to the first but with the correct IRQ, etc.  See "Serial Module" in
  the Serial-HOWTO.  See ``What is Setserial'' for more info on
  setserial.



  5.  Configuring Overview

  Since each modem has an associated serial port and the port has both
  hardware and software, there are three parts to configuring a modem:


  ·  Locate the serial port hardware: IO address, IRQ; Done by PnP
     methods or jumpers, setserial.  See ``Locating  the Serial Port: IO
     address IRQs'' ``What is  Setserial''

  ·  Configure the serial port driver (high-level): Done by the
     communication program (stty-like).  Sets speed, flow control, etc.
     See ``Configuring the Serial Driver  (high-level)'' See ``What is
     stty ?''

  ·  Configure the modem itself: Done by the communication program See
     ``Modem Configuration''
  The above omits a few other things that "setserial" can do besides
  locating the serial ports.  But normally you don't need to use them.
  Setserial may be used in the future to enable super-high speed.

  Communication programs include minicom, seyon, or wvdial (for PPP) and
  mgetty for dial-in.  Such communication programs require that you
  configure them although the default configuration they come with may
  only need a little tweaking.

  Unfortunately the communication program doesn't locate the serial
  port.  This "locating" is the low-level PnP configuring of the serial
  port: setting its IO address and IRQ in both the hardware and the
  driver.  If you are lucky, this will happen automatically when you
  boot Linux.  Setting these in the hardware was formerly done by
  jumpers and then running "setserial" but today it's done by "Plug-and-
  Play" software.  You may still need "setserial".  So if the serial
  driver can't find the serial port your modem is on, then you can try
  to find it yourself per the next section but it may not be easy.


  6.  Locating the Serial Port: IO address, IRQs

  6.1.  IO & IRQ Overview

  For a serial port to work properly, it must have both an IRQ and an IO
  address.  Without an IO address, it can't be located and will not work
  at all.  Without an IRQ it will need to use inefficient polling
  methods for which one must set the IRQ to 0.  So every serial port
  needs an IO address and IRQ.  In olden days this was set by jumpers on
  a serial port card.  Today it's set by digital signals sent to the
  hardware and this is part of "Plug-and-Play (PnP).

  The driver must also know both the IO address and IRQ so that it can
  locate the port chip.   Modern serial port drivers (kernel 2.4) try to
  determine this by PnP methods so one doesn't need to tell them (by
  using "setserial").   Such a driver might also set an IO address or
  enable the hardware.  But unfortunately, there is some PCI serial port
  hardware that the driver doesn't recognize so you may need to enable
  the port yourself.  See ``PCI: Enabling a disabled port''

  The driver also probes possible ISA serial port addresses to see if
  there are any serial ports there.  This works for the case of jumpers
  and sometimes works for a PnP port when the driver doesn't do PnP
  (prior to kernel 2.4).

  Locating the serial port by giving it an IRQ and IO address is low-
  level configuring.  It's often automatically done by the serial driver
  but sometimes you have to do it yourself.  What follows repeats what
  was said above but in more detail.

  The low-level configuring consists of assigning an IO address, IRQ,
  and name (such as ttyS2).  This IO-IRQ pair must be set in both the
  hardware and told to the serial driver.  Only the driver needs to know
  the name.We could call this "io-irq" configuring for short.  The
  "setserial" program is one way to tell the driver.  The other way is
  for the driver to use PnP methods to determine/set the IO/IRQ and then
  remember what it did.  For jumpers you must always use "setserial".
  If you need to configure but don't understand certain details it's
  easy to get into trouble.

  When Linux starts, some effort is made to detect and configure (low-
  level) a few serial ports.  Exactly what happens depends on your BIOS,
  hardware, Linux distribution, etc.  If the serial ports work OK, there
  may be no need for you to do any more low-level configuring.


  If you're having problems with the serial ports, then you may need to
  do low-level configuring.  If you have kernel 2.2 or lower, then you
  need to do it if you:


  ·  Plan to use more than 2 ISA serial ports

  ·  Are installing a new serial port (such as an internal modem)

  For kernel 2.2+ you may be able to use more that 2 serial ports
  without doing any low-level configuring by sharing interrupts.  All
  PCI ports should support this but for ISA it only works for some
  hardware.  It may be just as easy to give each port a unique interrupt
  if they is available.  See ``Interrupt sharing and Kernels 2.2+''

  The low-level configuring (setting the IRQ and IO address) seems to
  cause people more trouble than the high-level stuff, although for many
  it's fully automatic and there is no configuring to be done.  Until
  the serial driver knows the correct IRQ and IO address, the port will
  not usually not work at all.  Also, PnP ports can be disabled so that
  they can't be found (except with PnP tools such as lspci).
  Applications, and utilities such as "setserial" and "scanport" (Debian
  only ??) don't use PnP tools and thus can't detect disabled ports.

  Even if an ISA port can be found by the probing of the serial driver
  it may work extremely slow if the IRQ is wrong.  See ``Extremely Slow:
  Text appears on the screen slowly after long delays''.  PCI ports are
  less likely to get the IRQ wrong.

  In the Wintel world, the IO address and IRQ are called "resources" and
  we are thus configuring certain resources.  But there are many other
  types of "resources" so the term has many other meanings.  In summary,
  the low-level configuring consists of enabling the device, giving it a
  name (ttyS2 for example) and putting two values (an IRQ number and IO
  address) into two places:


  1. the device driver (often by running "setserial" at boot-time)

  2. configuration registers of the serial port hardware itself

  You may watch the start-up (= boot-time) messages.  They are usually
  correct.  But if you're having problems, your serial port may not show
  up at all or if you do see a message from  "setserial" it may not show
  the true configuration of the hardware (and it is not necessarily
  supposed to).  See ``I/O Address & IRQ: Boot-time messages''.


  6.2.  PCI Bus Support

  6.2.1.  Introduction

  Although some PCI modems are "winmodems" without a Linux driver (and
  will not work under Linux), other PCI modems will often work OK under
  Linux.  If it's a software modem, then you need to find a driver for
  it.  See Linmodem-HOWTO.

  If you have kernel 2.4, then there should be support for PnP (either
  built-in or by modules).  Some PCI serial ports can be automatically
  detected and low-level configured by the serial driver.  Others may
  not be.  No support exists in the serial driver for software modems.
  But separate drivers exist for many of them.Kernel 2.2 had no support
  for PCI serial ports (although some people got them working anyway).
  The 2.4 serial driver will read the id number digitally stored in the
  serial hardware to determine how to support it (if it knows how).  It
  should assign an I/O address to it, determine it's IRQ, etc.  So you
  don't need to use "setserial" for it.

  6.2.2.  More info on PCI

  PCI ports are not well standardized.  Some use main memory for
  communication with the PC.  Some require special enabling of the IRQ.
  The output of "lspci -vv" can help determine if one can be supported.
  If you see a 4-digit IO port, the port might work by just telling
  "setserial" the IO port and the IRQ.  Some people have gotten a 3COM
  3CP5610 PCI Modem to work that way.For example, if lspci shows IRQ 10,
  I/O at 0xecb8 and you decide to name it ttyS2 then the command is:

  setserial /dev/ttyS2 irq 10 port 0xecb8 autoconfig

  Note that the boot-time message "Probing PCI hardware" means reading
  the PnP configuration registers in the PCI cards which reveals the IO
  addresses and IRQs.  This is different that the probing of IO
  addresses by the serial driver which means reading certain IO
  addresses to see if what's read looks like there's a serial port at
  that address.


  6.3.  Common mistakes made re low-level configuring

  Here are some common mistakes people make:

  ·  setserial command: They run it (without the "autoconfig" and
     auto_irq options) and think it has checked the hardware to see if
     what it shows is correct (it hasn't).

  ·  setserial messages:  They see them displayed on the screen at boot-
     time (or by giving the setserial command) and erroneously think
     that the result always shows how their hardware is actually
     configured.

  ·  /proc/interrupts: When their serial device isn't in use they don't
     see its interrupt there, and erroneously conclude that their serial
     port can't be found (or doesn't have an interrupt set).

  ·  /proc/ioports and /proc/tty/driver/serial: People think this shows
     the actual hardware configuration when it only shows about the same
     info (possibly erroneous) as setserial.


  6.4.  IRQ & IO Address Must be Correct

  There are really two answers to the question "What is my IO and IRQ?"
  1. What the device driver thinks has been set (This is what setserial
  usually sets and shows.).  2. What is actually set in the hardware.
  Both 1. and 2. above should be the same.  If they're not it spells
  trouble since the driver has incorrect info on the physical serial
  port.  In some cases the hardware is disabled so it has no IO address
  or IRQ.

  If the driver has the wrong IO address it will try to send data to a
  non-existing serial port --or even worse, to some other device.  If it
  has the wrong IRQ the driver will not get interrupt service requests
  from the serial port, resulting in a very slow or no response.  See
  ``Extremely Slow: Text appears on the screen slowly after long
  delays''.  If it has the wrong model of UART there is also apt to be
  trouble.  To determine if both I0-IRQ pairs are identical you must
  find out how they are set in both the driver and the hardware.



  6.5.  What is the IO Address and IRQ per the driver ?

  6.5.1.  Introduction

  What the driver thinks is not necessarily how the hardware is actually
  set.  If everything works OK then what the driver thinks is likely
  correct (set in the hardware) and you don't need to investigate
  (unless you're curious or want to become a guru).  Ways to determine
  what the driver thinks include: boot-time messages ``I/O Address &
  IRQ: Boot-time messages'', the /proc directory "files" ``The /proc
  directory and setserial'', and the "setserial" command.



  6.5.2.  I/O Address & IRQ: Boot-time messages

  In many cases your ports will automatically get low-level configured
  at boot-time (but not always correctly).  To see what is happening,
  look at the start-up messages on the screen.  Don't neglect to check
  the messages from the BIOS before Linux is loaded (no examples shown
  here).  These BIOS messages may be frozen by pressing the Pause key.
  Use Shift-PageUp to scroll back to the messages after they have
  flashed by.  Shift-PageDown will scroll in the opposite direction.
  The dmesg command (or looking at logs in /var/log) will show some of
  the messages but they seem to  miss important ones from setserial.
  Here's an example of the start-up messages (as of mid 1999).  Note
  that ttyS00 is the same as /dev/ttyS0.



       At first you see what was detected (but the irq is only a wild guess):

       Serial driver version 4.27 with no serial options enabled
       ttyS00 at 0x03f8 (irq = 4) is a 16550A
       ttyS01 at 0x02f8 (irq = 3) is a 16550A
       ttyS02 at 0x03e8 (irq = 4) is a 16550A

       Later setserial shows you  what was saved, but it's not necessarily
       correct either:

       Loading the saved-state of the serial devices...
       /dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A
       /dev/ttyS1 at 0x02f8 (irq = 3) is a 16550A
       /dev/ttyS2 at 0x03e8 (irq = 5) is a 16550A



  Note that there is a slight disagreement: The first message shows
  ttyS2 at irq=4 while the second shows it at irq=5.  dmesg may not
  display the second message.  In most cases the second message is the
  correct one.  But if your having trouble it may be misleading.  Before
  reading the explanation of all of this complexity in the rest of this
  section, you might just try using your serial port and see if it works
  OK.  If so it may not be essential to read further.

  The second message is from the setserial program being run at boot-
  time.  It shows what the device driver thinks is the correct
  configuration.  But this too could be wrong.  For example, the irq
  could actually be set to irq=8 in the hardware (both messages wrong).
  The irq=5 could be there because someone incorrectly put this into a
  configuration file (or the like).  The fact that Linux sometimes gets
  IRQs wrong is because it doesn't by default probe for IRQs.  It just
  assumes the "standard" ones (first message) or accepts what you told
  it when you configured it (second message).  Neither of these is
  necessarily correct.  If the serial driver has the wrong IRQ, the
  serial port is very slow or doesn't seem to work at all.

  The first message is a result of Linux probing the serial port
  addresses but it doesn't probe for IRQs.  If a port shows up here it
  exists but the IRQ may be wrong.  Linux doesn't check IRQs because
  doing so is not foolproof.  It just assumes the IRQs are as shown
  because they are the "standard" values.  Your may check them manually
  with setserial using the autoconfig and auto_irq options but this
  isn't guaranteed to be correct either.

  The data shown by the BIOS messages (which you see at first before
  Linux is booted) is what is initially set in the hardware.  If your
  serial port is Plug-and-Play (PnP) then it's possible that "isapnp" or
  "setpci" will run and change these settings.  Look for messages about
  this after Linux starts.  The last serial port message shown in the
  example above should agree with the BIOS messages (as possibly
  modified by isapnp or setpci).  If they don't agree then you either
  need to change the setting in the port hardware or use setserial to
  tell the driver what is actually set in the hardware.

  Also, if you have Plug-and-Play (PnP) serial ports, they can only be
  found by PnP software unless the IRQ and IO has been set inside the
  hardware by Plug-and-Play software.  Prior to kernel 2.4 this was a
  common reason why the start-up messages did not show a serial port
  that physically exists.  A PnP BIOS may automatically low-level
  configure them.  PnP configuring will be explained later.


  6.5.3.  The /proc directory and setserial

  Type "setserial -g /dev/ttyS*".   There are some other ways to find
  this info by looking at "files" in the /proc directory.  Be warned
  that there is no guarantee that the same is set in the hardware.

  /proc/ioports will show the IO addresses that the drivers are using.
  /proc/interrupts shows the IRQs that are used by drivers of currently
  running processes (that have devices open).  It shows how many
  interrupts have actually be issued.  /proc/tty/driver/serial shows
  much of the above, plus the number of bytes that have been received
  and sent (even if the device is not now open).

  Note that for the IO addresses and IRQ assignments, you are only
  seeing what the driver thinks and not necessarily what is actually set
  in the hardware.  The data on the actual number of interrupts issued
  and bytes processed is real however.  If you see a large number of
  interrupts and/or bytes then it probably means that the device is (or
  was) working.  But the interrupts might be from another device.  If
  there are no bytes received (rx:0) but bytes were transmitted (tx:3749
  for example), then only one direction of flow is working (or being
  utilized).

  Sometimes a showing of just a few interrupts doesn't mean that the
  interrupt is actually being physically generated by any serial port.
  Thus if you see almost no interrupts for a port that you're trying to
  use, that interrupt might not be set in the hardware.  To view
  /proc/interrupts to check on a program that you're currently running
  (such as "minicom") you need to keep the program running while you
  view it.


  6.6.  What is the IO Address & IRQ of my Serial Port Hardware?

  6.6.1.  Introduction

  If it's PCI or ISA PnP then what's set in the hardware has been done
  by PnP methods.  Even if nothing has been set or the port disabled,
  PnP ports may still be found by using "lspci -v" or "isapnp
  --dumpregs".  Ports disabled by jumpers (or hardware failures) are
  completely lost.  See ``ISA PnP ports'', ``PCI: What IOs and IRQs have
  been set?'', ``PCI: Enabling a disabled port''

  PnP ports don't store their configuration in the hardware when the
  power is turned off.  This is in contrast to Jumpers (non-PnP) which
  remain the same with the power off.  That's why a PnP port is more
  likely to be found in a disabled state than an old non-PnP one.


  6.6.2.  PCI: What IOs and IRQs have been set?

  For PCI, the BIOS almost always sets the IRQ and may set the IO
  address as well.  To see how it's set use "lspci -vv" (best) or look
  in /proc/bus/pci (or for kernels <2.2 /proc/pci).  The modem's serial
  port is often called a "Communication controller".  Look for this.  If
  lspci shows "I/O ports at ... [disabled]" then the serial port is
  disabled and the hardware has no IO address so it's lost and can't be
  used.  See ``PCI: Enabling a disabled port'' for how to enable it.

  If more than one IO address is shown, the first one is more likely to
  be it.  You can't change the IRQ (at least not with "setpci")   This
  is because if one writes an IRQ it it's hardware register no action is
  taken on it.  It's the BIOS that should actually set up the IRQs and
  then write the correct value to this register for lspci to view.  If
  you must, change the IO address with "setpci" by changing the
  BASE_ADDRESS_0 or the like.


  6.6.3.  PCI: Enabling a disabled port

  If the port communicates via an IO address then "lspci -vv" should
  show "Control: I/O+ ..." with + meaning that the IO address is
  enabled.  If it shows "I/O-" (and "I/O ports at ... [disabled]") then
  you may need to use the setpci command to enable it.  For example
  "setpci -d 151f:000 command=101".  151f is the vendor id, and 000 is
  the device id both obtained from "lspci -n -v" or from /proc/bus/pci
  or from "scanpci -v".  The "command=101" means that 101 is put into
  the command register which is the same as the "Control" register
  displayed by "lspci".  The 101h sets two bits: the 1 sets I/O to + and
  the 100 part keeps SERR# set to +.   In this case only the SERR# bit
  of the Control register was initially observed to be + when the lspci
  command was run.  So we kept it enabled to + by setting bit 8 (where
  bit 0 is I/O) to 1 by the first 1 in 101.  Some serial cards don't use
  SERR# so if you see SERR#- then there's no need to enable it so then
  use: command=1.  Then you'll need to set up "setserial" to tell the
  driver the IO and IRQ.

  Bit 8 is actually the 9th bit since we started counting bits from 0.
  Don't be alarmed that lspci shows a lot of - signs showing that the
  card doesn't have many features available (or enabled).  Serial ports
  are relatively slow and don't need these features.

  Another way to enable it is to let the BIOS do it by telling the BIOS
  that you don't have a plug-and-play operating system.  Then the BIOS
  should enable it when you start your PC. If you have MS Windows9x on
  the same PC then doing this might cause problems with Windows (see
  Plug-and-Play-HOWTO).


  6.6.4.  ISA PnP ports

  For an ISA Plug-and-Play (PnP) port one may try the pnpdump program
  (part of isapnptools).  If you use the --dumpregs option then it
  should tell you the actual IO address and IRQ set in the port.  It
  should also find an ISA PnP port that is disabled.  The address it
  "trys" is not the device's IO address, but a special address used for
  communicating with PnP cards.


  6.6.5.  Finding a port that is not disabled (ISA, PCI, PnP, non-PnP)

  Perhaps the BIOS messages will tell you some info before Linux starts
  booting.  Use the shift-PageUp key to step back thru the boot-time
  messages and look at the very first ones which are from the BIOS.
  This is how it was before Linux started.  Setserial can't change it
  but isapnp or setpci can.  Starting with kernel 2.4, the serial driver
  can make such changes for many (but not all) serial ports.

  Using "scanport" (Debian only ??) will probe all I/O ports and will
  indicate what it thinks may be serial port.  After this you could try
  probing with setserial using the "autoconfig" option.  You'll need to
  guess the addresses to probe at (using clues from "scanport").  See
  ``What is Setserial''.

  For a port set with jumpers, the IO ports and IRQs are set per the
  jumpers. If the port is not Plug-and-Play (PnP) but has been setup by
  using a DOS program, then it's set at whatever the person who ran that
  program set it to.


  6.6.6.  Exploring via MS Windows (a last resort)

  For PnP ports, checking on how it's configured under DOS/Windows may
  (or may not) imply how it's under Linux.  MS Windows stores its
  configuration info in its Registry which is not used by Linux so they
  are not necessarily configured the same.  If you let a PnP BIOS
  automatically do the configuring when you start Linux (and have told
  the BIOS that you don't have a PnP operating system when starting
  Linux) then Linux should use whatever configuration is in the BIOS's
  non-volatile memory.  Windows also makes use of the same non-volatile
  memory but doesn't necessarily configure it that way.


  6.7.  Choosing Serial IRQs

  If you have Plug-and-Play ports then either a PnP BIOS or a serial
  driver may configure all your devices for you so then you may not need
  to choose any IRQs.  PnP software determines what it thinks is best
  and assigns them (but it's not always best).  But if you directly use
  isapnp (ISA bus) or jumpers then you have to choose.  If you already
  know what IRQ you want to use you could skip this section except that
  you may want to know that IRQ 0 has a special use (see the following
  paragraph).


  6.7.1.  IRQ 0 is not an IRQ

  While IRQ 0 is actually the timer (in hardware) it has a special
  meaning for setting a serial port with setserial.  It tells the driver
  that there is no interrupt for the port and the driver then will use
  polling methods.  Such polling puts more load on the CPU but can be
  tried if there is an interrupt conflict or mis-set interrupt.  The
  advantage of assigning IRQ 0 is that you don't need to know what
  interrupt is set in the hardware.  It should be used only as a
  temporary expedient until you are able to find a real interrupt to
  use.



  6.7.2.  Interrupt sharing,  Kernels 2.2+

  Sharing of IRQs is where two devices use the same IRQ.  As a general
  rule, this wasn't allowed for the ISA bus.   The PCI bus may share
  IRQs but one can't share the same IRQ between the ISA and the PCI bus.
  Most multi-port boards may share IRQs.  Sharing is not as efficient
  since every time a shared interrupt is given a check must be made to
  determine where it came from.  Thus if it's feasible, it's nicer to
  allocate every device its own interrupt.

  Prior to kernel 2.2, serial IRQs could not be shared with each other
  except for most multiport boards.  Starting with kernel 2.2 serial
  IRQs may be sometimes shared between serial ports.  In order for
  sharing to work in 2.2 the kernel must have been compiled with
  CONFIG_SERIAL_SHARE_IRQ, and the serial port hardware must support
  sharing (so that if two serial cards put different voltages on the
  same interrupt wire, only the voltage that means "this is an
  interrupt" will prevail).  Since the PCI bus specs permit sharing, any
  PCI card should allow sharing.


  6.7.3.  What IRQs to choose?

  The serial hardware often has only a limited number of IRQs.  Also you
  don't want IRQ conflicts.  So there may not be much of a choice.  Your
  PC may normally come with ttyS0 and ttyS2 at IRQ 4, and ttyS1 and
  ttyS3 at IRQ 3.  Looking at /proc/interrupts will show which IRQs are
  being used by programs currently running.  You likely don't want to
  use one of these.  Before IRQ 5 was used for sound cards, it was often
  used for a serial port.

  Here is how Greg (original author of Serial-HOWTO) set his up in
  /etc/rc.d/rc.serial.  rc.serial is a file (shell script) which runs at
  start-up (it may have a different name or location).  For versions of
  "setserial" after 2.15 it's not always done this way anymore but this
  example does show the choice of IRQs.



       /sbin/setserial /dev/ttyS0 irq 3        # my serial mouse
       /sbin/setserial /dev/ttyS1 irq 4        # my Wyse dumb terminal
       /sbin/setserial /dev/ttyS2 irq 5        # my Zoom modem
       /sbin/setserial /dev/ttyS3 irq 9        # my USR modem



  Standard IRQ assignments:

          IRQ  0    Timer channel 0 (May mean "no interrupt".  See below.)
          IRQ  1    Keyboard
          IRQ  2    Cascade for controller 2
          IRQ  3    Serial port 2
          IRQ  4    Serial port 1
          IRQ  5    Parallel port 2, Sound card
          IRQ  6    Floppy diskette
          IRQ  7    Parallel port 1
          IRQ  8    Real-time clock
          IRQ  9    Redirected to IRQ2
          IRQ 10    not assigned
          IRQ 11    not assigned
          IRQ 12    not assigned
          IRQ 13    Math coprocessor
          IRQ 14    Hard disk controller 1
          IRQ 15    Hard disk controller 2

  There is really no Right Thing to do when choosing interrupts.  Try to
  find one that isn't being used by the motherboard, or any other
  boards.  2, 3, 4, 5, 7, 10, 11, 12 or 15 are possible choices.  Note
  that IRQ 2 is the same as IRQ 9.  You can call it either 2 or 9, the
  serial driver is very understanding.  If you have a very old serial
  board it may not be able to use IRQs 8 and above.

  Make sure you don't use IRQs 1, 6, 8, 13 or 14!  These are used by
  your motherboard.  You will make her very unhappy by taking her IRQs.
  When you are done you might want to double-check /proc/interrupts when
  programs that use interrupts are being run and make sure there are no
  conflicts.


  6.8.  Choosing Addresses --Video card conflict with ttyS3

  Here's a problem with some old serial cards.  The IO address of the
  IBM 8514 video board (and others like it) is allegedly 0x?2e8 where ?
  is 2, 4, 8, or 9.  This may conflict with the IO address of ttyS3 at
  0x02e8.  Your may think that this shouldn't happen since the addresses
  are different in the high order digit (the leading 0 in 02e8).  You're
  right, but a poorly designed serial port may ignore the high order
  digit and respond to any address that ends in 2e8.  That is bad news
  if you try to use ttyS3 (ISA bus) at this IO address.

  For the ISA bus you should try to use the default addresses shown
  below.  PCI cards use different addresses so as not to conflict with
  ISA addresses.  The addresses shown below represent the first address
  of an 8-byte range.  For example 3f8 is really the range 3f8-3ff.
  Each serial device (as well as other types of devices that use IO
  addresses) needs its own unique address range.  There should be no
  overlaps (conflicts).  Here are the default addresses for commonly
  used serial ports on the ISA bus:



       ttyS0 address 0x3f8
       ttyS1 address 0x2f8
       ttyS2 address 0x3e8
       ttyS3 address 0x2e8



  Suppose there is an address conflict (as reported by setserial -g
  /dev/ttyS*) between a real serial port and another port which does not
  physically exist (and shows UART: unknown).  Such a conflict shouldn't
  cause problems but it sometimes does in older kernels.  To avoid this
  problem don't permit such address conflicts or delete /dev/ttySx if it
  doesn't physically exist.


  6.9.  Set IO Address & IRQ in the hardware (mostly for PnP)

  After it's set in the hardware don't forget to insure that it also
  gets set in the driver by using setserial.  For non-PnP serial ports
  they are either set in hardware by jumpers or by running a DOS program
  ("jumperless") to set them (it may disable PnP).  The rest of this
  subsection is only for PnP serial ports.  Here's a list of the
  possible methods of configuring PnP serial ports:


  ·  Using a PnP BIOS CMOS setup menu (usually only for external modems
     on ttyS0 (Com1) and ttyS1 (Com2))


  ·  Letting a PnP BIOS automatically configure a PnP serial port See
     ``Using a PnP BIOS to I0-IRQ Configure''

  ·  Doing nothing if the serial driver recognized your card OK

  ·  Using isapnp for a PnP serial port non-PCI)

  ·  Using setpci (pciutils or pcitools) for the PCI bus

  The IO address and IRQ must be set (by PnP) in their registers each
  time the system is powered on since PnP hardware doesn't remember how
  it was set when the power is shut off.  A simple way to do this is to
  let a PnP BIOS know that you don't have a PnP OS and the BIOS will
  automatically do this each time you start.  This might cause problems
  in Windows (which is a PnP OS) if you start Windows with the BIOS
  thinking that Windows is not a PnP OS.  See Plug-and-Play-HOWTO.

  Plug-and-Play (PnP) was designed to automate this io-irq configuring,
  but for Linux it initially made life much more complicated.  In modern
  Linux (2.4 kernels --partially in 2.2 kernels), each device driver has
  to do it's own PnP (using supplied software which it may utilize).
  There is unfortunately no centralized planning for assigning IO
  addresses and IRQs as there is in MS Windows.   But it usually works
  out OK in Linux anyway.


  6.9.1.  Using a PnP BIOS to I0-IRQ Configure

  While the explanation of how to use setpci or isapnp for io-irq
  configuring should come with such software, this is not the case if
  you want to let a PnP BIOS do such configuring.  Not all PnP BIOS can
  do this.  The BIOS usually has a CMOS menu for setting up the first
  two serial ports.  This menu may be hard to find.  For an "Award" BIOS
  it was found under "chipset features setup"  There is often little to
  choose from.  Unless otherwise indicated in a menu, these first two
  ports normally get set at the standard IO addresses and IRQs.  See
  ``Serial Port Device Names & Numbers''

  Whether you like it or not, when you start up a PC a PnP BIOS starts
  to do PnP (io-irq) configuring of hardware devices.  It may do the job
  partially and turn the rest over to a PnP OS (which Linux is in some
  sense) or if thinks you don't have a PnP OS it may fully configure all
  the PnP devices but not configure the device drivers.  This is what
  you want but it's not always easy to figure out exactly what the PnP
  BIOS has done.

  If you tell the BIOS that you don't have a PnP OS, then the PnP BIOS
  should do the configuring of all PnP serial ports --not just the first
  two.  An indirect way to control what the BIOS does (if you have
  Windows 9x on the same PC) is to "force" a configuration under
  Windows.  See Plug-and-Play-HOWTO and search for "forced".  It's
  easier to use the CMOS BIOS menu which may override what you "forced"
  under Windows.  There could be a BIOS option that can set or disable
  this "override" capability.

  If you add a new PnP device, the BIOS should PnP configure it.  It
  could even change the io-irq of existing devices if required to avoid
  any conflicts.  For this purpose, it keeps a list of non-PnP devices
  provided that you have told the BIOS how these non-PnP devices are io-
  irq configured.  One way to tell the BIOS this is by running a program
  called ICU under DOS/Windows.

  But how do you find out what the BIOS has done so that you set up the
  device drivers with this info?  The BIOS itself may provide some info,
  either in its setup menus of via messages on the screen when you turn
  on your computer.  See ``What is set in my serial port hardware?''.
  Other ways of finding out is to use lspci for the PCI bus or isapnp
  --dumpregs for the ISA bus.  The cryptic results it shows you may not
  be clear to a novice.


  6.10.  Giving the IRQ and IO Address to Setserial

  Once you've set the IRQ and IO address in the hardware (or arranged
  for it to be done by PnP) you also need to insure that the "setserial"
  command is run each time you start Linux.  See the subsection ``Boot-
  time Configuration''



  7.  Configuring the Serial Driver (high-level) "stty"

  7.1.  Introduction

  This configuring is normally done by your communications program such
  as wvdial.  It may do much of it without even letting you know what
  it's done.  In olden days it was done with the "stty" utility.  If you
  set something with stty, the communications program may change the
  setting so it's usually best to just let the communications program
  handle it.  See ``What is stty ?''


  7.2.  Hardware flow control (RTS/CTS)

  See ``Flow Control'' for an explanation of it.  You should always use
  hardware flow control if possible.  Your communication program or
  "getty" should have an option for setting it (and if you're in luck it
  might be enabled by default).  It needs to be set both inside your
  modem (by an init string or default) and in the device driver.  Your
  communication program should set both of these (if you configure it
  right).

  If none of the above will fully enable hardware flow control.  Then
  you must do it yourself.  For the modem, make sure that it's either
  done by the init string or is on by default.  If you need to tell the
  device driver to do it is best done on startup by putting it in a file
  that runs at boot-time.  See the subsection ``Boot-time
  Configuration'' You need to add the following to such a file for each
  serial port (example is ttyS2) you want to enable hardware flow
  control on:



       stty crtscts < /dev/ttyS2
       or
       stty -F /dev/ttyS2 crtscts



  If you want to see if flow control is enabled do the following:  In
  minicom (or the like) type AT&V (or ATI4 on 3Com modems) to see how
  the modem is configured and look for &K3 (or &H1 on 3Com modems) which
  means hardware flow control.  Then without exiting the communications
  program (such as minicom) see if the device driver knows about it by
  typing: stty -F /dev/ttyS2 -a.   Look for "crtscts" (without a
  disabling minus sign).



  7.3.  Speed Settings

  Besides flow control there is speed.  See ``What Speed Should I Use
  with My Modem''.  There's also are parity and bits-per-byte settings.
  Normally the port is set by the communications program at 8N1 (8-bits
  per byte, No parity, and 1 stop bit).  If you're running PPP then you
  must use 8N1.  So if you get a complaint that it's not 8-bit clean
  then it's likely not 8N1 as it should be.


  7.4.  Ignore CD Setting: clocal

  If the modem is not sending a CD signal and clocal is disabled (stty
  shows -clocal) then a program may not be able to open the serial port.
  If the port can't open, the program may just hang, waiting (often in
  vain) for a CD signal from the modem.  Actually, a skilled programmer
  can write the program in such a way as to force the port to open even
  when CD and -clocal say not to so it's not always a problem.

  One way to avoid any possible problems is to send  "AT&C" to the modem
  so that CD from the modem will always be on.  CD always-on is fine for
  dial-out but for dial-in the CD signal is sometimes (but rarely) used
  to detected an incoming call.

  Minicom sets clocal automatically when it starts up so there is no
  problem.  But version 6.0.192 of Kermit hung when I set -clocal and
  tried to "set line ...".

  Here's a problem that existed prior to the year 2000 or thereabouts.
  It's since been fixed.   If -clocal is set and there is no CD signal,
  then the "stty" command will hang and there is seemingly no way to set
  clocal (except by running minicom).  But minicom will restore -clocal
  when it exits.  One way to get out of this is to use minicom to send
  the "AT&C" to the modem (to get the CD signal) and then exit minicom
  with no reset so that the CD signal always remains on.  Then you may
  use stty again.


  7.5.  What is stty ?

  stty is something like setserial but it sets the speed (baud rate),
  hardware flow control,  and other parameters of a serial port.  Typing
  "stty -F /dev/ttyS2 -a" should show you how ttyS2 is configured.  Most
  of the stty settings are for things that you never need to use with
  modems.  Many of the setting are only needed for Text-Terminals (and
  some are only needed for antique terminals of the 1970s).  Your
  communication package should automatically set up the several settings
  needed for modems.  For this reason you normally don't need to use
  stty so it's not covered much in this Modem-HOWTO.  But stty is
  sometimes useful for trouble-shooting.   More is said about stty in
  the Serial-HOWTO or Text-Terminal-HOWTO..


  8.  Modem Configuration (excluding serial port)

  8.1.  Finding Your Modem

  Before spending a lot of time deciding how to configure your modem,
  you first need to make sure it can be found and that AT-commands and
  the like can be sent to it.  So I suggest you first give it a very
  simple configuration using the communication program you will be using
  on the port and see it it works.  If this works you may then want to
  improve on the configuration, If not then see ``My Modem is Physically
  There but Can't be Found''.  A winmodem may be hard to find and will
  not work under Linux.

  8.2.  AT Commands

  While the serial port on which a modem resides requires configuring,
  so does the modem itself.  The modem is configured by sending AT
  commands (or the like) to it on the same serial line that is used to
  send data.

  Most modems use an AT command set.  These are cryptic and short ASCII
  commands where all command strings must be prefaced by the letters AT.
  AT means: ATtention, expect a command to follow.  For example:
  ATZ&K3<return>  This is an AT-command string with two commands here: Z
  and &K3.  Z is short for Z0 and a few modems require that you use Z0
  instead of just Z.  It's like this for other commands ending in 0.
  The command string is terminated by a return character (use the
  <enter> key if you are manually typing it).  A string that's too long
  (40 or more characters) may not work on older modems.  You may use
  either uppercase or lowercase letters.

  Unfortunately there are many different variations of the AT command
  set so that what works for one modem may or may not work for another
  modem.  Thus there is no guarantee that the AT commands given in this
  section will work on your modem.

  Such command strings are either automatically sent to the modem by
  communication programs or are manually typed in by you.  Most
  communication programs provide a screen where you may change (edit)
  and save the init string that the communication program will use.  The
  modem itself has a stored configuration (profile) which is like a long
  init string.  It represents the configuration of the modem when it's
  first tuned on.  You may change it to suit your taste.  In most cases
  there are a few different such configurations (profiles) and there are
  ways to designate one of them to be active.

  If you have a manual for your modem (either on paper or on floppy
  disk) you might find AT-commands there.  3Com modems (and others ??)
  have AT-Command help files built into the modem so if you type say
  "AT$" to the modem it will display some "online help".

  You can also find info on AT commands on the Internet.  You should
  first try a site for your modem manufacturer.  If this doesn't work
  out then you can search the Internet using terms that are from AT
  commands such as &C1, &D3, etc.  This will tend to find sites that
  actually list AT-Commands instead of sites that just talk about them
  in general.  You might also try a few of the sites listed in the
  subsection ``Web Sites''.  Be warned that the AT-commands for a
  different brand of modem may be somewhat different.


  8.3.  Init Strings: Saving and Recalling

  The examples given in this subsection are from the Hayes AT modem
  command set.  All command strings must be prefaced by the two letters
  AT.  For example: AT&C1&D3^M (^M is the return character).  When a
  modem is powered on, it automatically configures itself with one of
  the configurations it has stored in its non-volatile memory.  If this
  configuration is satisfactory there is nothing further to do.

  If it's not satisfactory, then one may either alter the stored
  configuration or configure the modem each time you use it by sending
  it a string of commands known as an "init string" (= initialization
  string).  Normally, a communication program does this.  What it sends
  will depend on how you configured the communications program.  Your
  communication program should allow you to edit the init string and
  change it to whatever you want.  Sometimes the communications program
  will let you select the model of your modem and then it will use an
  init string that it thinks is best for that modem.
  The configuration of the modem when it's first powered on may be
  expressed by an init string.  You might think of this as the default
  "string" (called a profile).  If your communications program sends the
  modem another string (the init string), then this string will modify
  the default configuration.  For example, if the init string only
  contains two commands, then only those two items will be changed.
  However, some commands will recall a stored profile from inside the
  modem so a single such command in the init string can thereby change
  everything in the configuration.

  Modern modems have a few different stored profiles to choose from that
  are stored in the modem's non-volatile memory (it's still there when
  you turn it off).  In my modem there are two factory profiles (0 and
  1, neither of which you can change) and two user defined profiles (0
  and 1) that the user may set and store.  Your modem may have more.  To
  view some of these profiles send the command &V.  At power-up one of
  the user-defined profiles is loaded.  For example, if you type the
  command &Y0 (just Y0 for a 3Com modem) then in the future profile 0
  will be used at power-on.

  There are also commands to load (activate) any of the stored profiles.
  Such a load command may be put in an init string.  Of course if it
  loads the same profile that was automatically loaded at power-up,
  nothing is changed (unless the active profile has been modified since
  power-up).  Since your profile could have thus been modified it's a
  good idea to use some kind of an init string even if it does nothing
  more than load a stored profile.

  Examples of loading saved profiles:
  Z0 loads user-defined profile 0 and resets (hangs up, etc.)
  &F1 loads factory profile 1

  Once you have sent commands to the modem to configure it the way you
  want (such as loading a factory profile and modifying it a little) you
  may save this as a user-defined profile:
  &W0 saves the current configuration to user-profile 0.

  Many people don't bother saving a good configuration in their modem,
  but instead, send the modem a longer init string each time the modem
  is used.  Another method is to restore the factory default by &F1 at
  the start of the init string and then modify it a little by adding a
  few other commands to the end of the init string.  Since there is no
  way to modify the factory default this prevents anyone from changing
  the configuration by modifying (and saving) the user-defined profile.

  You may choose an init string supplied by someone else that they think
  is right for your modem.  Some communication programs have a library
  of init strings to select from.  The most difficult method (and one
  which will teach you the most about modems) is to study the modem
  manual and write one yourself.  You could save this configuration
  inside the modem so that you don't need an init string.  A third
  alternative is to start with an init string that someone else wrote,
  but modify it to suit your purposes.

  If you look at init strings used by communication programs you may see
  symbols which are not valid modem commands.  These symbols are
  commands to the communication program itself and will not be sent to
  the modem.  For example, ~ may mean to pause briefly.


  8.3.1.  Where is my "init string" so I can modify it ?

  This depends on your communication program (often a PPP program).  If
  this is the latest version of Modem-HOWTO send me info for other
  cases.

  ·  Gnome: run pppsetup

  ·  wvdial: edit /etc/wvdial.conf

  ·  minicom: hit ^Ao (or possibly ALT-o), then select "Modem and
     Dialing"


  8.4.  Other AT Modem Commands

  For dial-in see ``Dial-in Modem Configuration''.  The rest of this
  section is mostly what was in the old Serial-HOWTO.  All strings must
  start with AT.  Here's a few Hayes AT codes that should be in the
  string (if they are not set by using a factory default or by a saved
  configuration).


  ·  E1       command echo ON

  ·  Q0       result codes are reported

  ·  V1       result codes are verbose

  ·  S0=0     never answer (uugetty does this with the WAITFOR option)

  Here's some more AT commands for special purposes:

  ·  &C1   CD is only on when you're connected

  ·  &S0     DSR is always on

  ·  X3      Dial even if there is no dialtone (Use where dial-tones
     don't exist).


  Note: to get his old USR Courier V.34 modem to reset correctly when
  DTR drops, Greg Hankins had to set &D2 and S13=1 (this sets bit 0 of
  register S13).  This has been confirmed to work on USR Sportster V.34
  modems as well.


  Note: some old Supra modems treat CD differently than other modems.
  If you are using a Supra, try setting &C0 and not &C1.  You must also
  set &D2 to handle DTR correctly.


  8.5.  Blacklisting

  If phone number is dialed a few times with no success, some modems may
  blacklist a phone number.  After a certain time you may try again.
  Some countries require this to reduce needless repeated dialing.  To
  view the blacklist try %B.  To delete the blacklist use these AT
  commands:

  ·  SR Robotics o 3COM: s40=2 or if NG try s40=7

  ·  Lucent:  %t21,18,0

  ·  Rockwell:  %tcb

  ·  Cirrus Logic: *nc9



  8.6.  What AT Commands are Now Set in my Modem?

  You may try to use minicom for viewing your modem profile.  It's best
  not to have any other process running on the modem port when you do
  this.  If you have set up minicom for your modem, then you may type on
  the command line: minicom -o to start minicom without restoring the
  saved modem profile.  Then type at&v (or atI4 on 3Com modems) to
  display the profile.  To exit minicom without disturbing this profile,
  use the q (quit) command for exiting without resetting.

  The above may not work for various reasons.  If the modem has been set
  not to echo result codes it may not even display any profile.  If
  there is another process running on the modem port at the same time,
  some of what the modem sends to you is likely to be read by the other
  process so you will see only part of the profile.  Is there some way
  to temporarily stop the other process on the port so it will not
  interfere?  I tried the "stop" signal using the "kill" command but it
  didn't work.  If this is the latest version of this HOWTO, let me know
  if you find a way to do it.

  If you have at least one process running on the modem port and kill
  them, the modem's profile may be reset so you will not observe what
  the original profile was.  This will happen if you kill getty (or it's
  replacements: login or bash) and have &D3 set.  The killing of getty
  (or the like) will drop DTR and reset the modem's profile to the
  power-on state.   To keep getty from respawning when killed, comment
  it out in /etc/inittab and do an "init q".


  8.7.  Modem States (or Modes)

  Since the channel for sending AT commands to the modem is the same
  channel that is used for the flow of data (files, packets, etc.) then
  it's important to cleanly separate the AT commands from the data.

  When the modem is first turned on it's in the command mode (also
  called terminal mode, idle state or AT-command mode).  Anything sent
  to it from the PC is assumed to be an AT command and not data.  Then
  if a dial command is sent to it (ATD...), it dials and connects to
  another modem.  It's now in the on-line data mode (connected) and
  sends and receives data (such as Internet pages).  In this mode, any
  AT command one trys to send it will not work but will be transmitted
  to the other modem instead.  Except for the escape command.  This is
  +++ with a minimum time delay both at the start and end.  The time
  delay allows the modem to determine that it is likely a real escape
  and not just +++ in a file being transmitted.

  So we have two states so far: AT-command and on-line data.  But there
  is a third important state which is sort of a combination of these
  two.  It's the on-line command mode.  This is when the modem maintains
  a connection (without sending/receiving data) but anything sent from
  the PC is interpreted as an AT command.  This is the state reached
  with a +++ escape signal or by a DTR drop from the PC provided the &D1
  has been set.  Then one can send AT commands to the modem including
  commands which will leave this state and go to one of the other two
  states.

  There are other states also: dialing state and handshaking state but
  they normally lead to the connected (on-line) state.  If they don't
  then the modem should hang up, thereby returning to the initial AT-
  command (or idle) state.



  9.  Serial Port Devices /dev/ttyS2, (or /dev/ttys/2) etc.

  For creating devices in the device directory see: the Serial-HOWTO:
  "Creating Devices In the /dev directory".



  9.1.  Devfs (The new Device File System)

  This is a new type of device interface to Linux.  It's optional
  starting with kernel 2.4.  It's more efficient than the conventional
  interface and makes it easy to deal with a huge number of devices.
  The device names have all changed as well.  But there's an option to
  continue using the old names.  For a detailed description of it see:
  <http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html> Also see the
  kernel documentation tree: filesystems/devfs.

  The name changes (if used) are: ttyS2 becomes tts/2 (Serial port),
  tty3 becomes vc/3 (Virtual Console), ptyp1 becomes pty/m1 (PTY
  master), ttyp2 becomes pty/s2 (PTY slave).  "tts" looks like a
  directory which contains devices "files": 0, 1, 2, etc.  All of these
  new names should still be in the /dev directory although optionally
  one may put them elsewhere.


  9.2.  Serial Port Device Names & Numbers

  Devices in Linux have major and minor numbers (unless you use the new
  devfs).  The serial port ttySx (x=0,1,2, etc.) has major number 4.
  You may see this (and the minor numbers too) by typing: "ls -l ttyS*"
  in the /dev directory.

  There formerly was a "cua" name for each serial port and it behaved
  just a little differently.  For example, ttyS2 would correspond to
  cua2.  It was mainly used for modems.  The cua major number was 5 and
  minor numbers started at 64.  You may still have the cua devices in
  your /dev directory but they are now deprecated.  For details see
  Modem-HOWTO, section: cua Device Obsolete.

  Dos/Windows use the COM name while the setserial program uses tty00,
  tty01, etc.  Don't confuse these with dev/tty0, dev/tty1, etc.  which
  are used for the console (your PC monitor) but are not serial ports.
  The table below is for the "standard" case (but yours could be
  different).  The major/minor numbers don't exist with the devfs.  For
  the PCI bus the IO addresses are different.


                              ISA IO                              devfs usb
   dos            major minor address  devfs       devfs usb      acm modem
  COM1  /dev/ttyS0  4,  64;   3F8   /dev/tts/0  /dev/usb/tts/0 /dev/usb/acm/0
  COM2  /dev/ttyS1  4,  65;   2F8   /dev/tts/1  /dev/usb/tts/1 /dev/usb/acm/1
  COM3  /dev/ttyS2  4,  66;   3E8   /dev/tts/2  /dev/usb/tts/2 /dev/usb/acm/2
  COM4  /dev/ttyS3  4,  67;   2E8   /dev/tts/3  /dev/usb/tts/3 /dev/usb/acm/3



  9.3.  USB (Universal Serial Bus) Ports

  The serial ports on the USB are: /dev/ttyUSB0, /dev/ttyUSB1, etc.  The
  devfs names for these are: /dev/usb/tts/0, /dev/usb/tts/1, etc.  For
  many modems they are /dev/usb/acm/0, etc. (in devfs notation).  For
  more info see the usb subdirectory in the kernel documentation
  directory for files: acm and usb-serial.


  9.4.  Link ttySN to /dev/modem

  On some installations, two extra devices will be created, /dev/modem
  for your modem and /dev/mouse for a mouse.  Both of these are symbolic
  links to the appropriate serial device in /dev which you specified
  during the installation Except if you have a bus mouse, then
  /dev/mouse will point to the bus mouse device).

  Formerly (in the 1990s) the use of /dev/modem was discouraged since
  lock files might not realize that it was really say /dev/ttyS2.  The
  newer lock file system doesn't fall into this trap so it's now OK to
  use such links.



  9.5.  cua Device Obsolete

  Each ttyS device has a corresponding cua device.  But the cua device
  is deprecated so it's best to use ttyS (unless cua is required).
  There is a difference between cua and ttyS but a savvy programmer can
  make a ttyS port behave just like a cua port so there is no real need
  for the cua anymore.  Except that some older programs may need to use
  the cua.

  What's the difference?  The main difference between cua and ttyS has
  to do with what happens in a C-program when an ordinary "open" command
  tries to open the port. If a cua port has been set to check modem
  control signals, the port can be opened even if the CD modem control
  signal says not to.  Astute programming (by adding additional lines to
  the program) can force a ttyS port to behave this way also.  But a cua
  port can be more easily programmed to open for dialing out on a modem
  even when the modem fails to assert CD (since no one has called into
  it and there's no carrier).  That's why cua was once used for dial-out
  and ttyS used for dial-in.

  Starting with Linux kernel 2.2, a warning message is put in the kernel
  log when one uses cua.  This is an omen that cua is defunct and should
  be avoided if possible.


  10.  Interesting Programs You Should Know About

  10.1.  What is setserial ?

  This part is in 3 HOWTOs: Modem, Serial, and Text-Terminal.  There are
  some minor differences, depending on which HOWTO it appears in.


  10.1.1.  Introduction

  The setserial program doesn't seem to work with serial ports used with
  linmodems such as ttySHCF0.  If you have a Laptop (PCMCIA) don't use
  setserial until you read ``Laptops: PCMCIA''.  setserial is a program
  which allows you to tell the device driver software the I/O address of
  the serial port, which interrupt (IRQ) is set in the port's hardware,
  what type of UART you have, etc.  Since there's a good chance that the
  serial ports will be automatically detected and set, many people never
  need to use setserial.  In any case setserial will not work without
  either serial support built into the kernel or loaded as a module.
  The module may get loaded automatically if you (or a script) tries to
  use setserial.

  Setserial can also show how the driver is currently set.  In addition,
  it can be made to probe the hardware I0 port addresses to try to
  determine the UART type and IRQ, but this has severe limitations.  See
  ``Probing''.  Note that it can't set the IRQ or the port address in
  the hardware of PnP serial ports (but the plug-and-play features of
  the serial driver may do this).  It can't read the PnP data stored in
  configuration registers in the hardware.

  If you only have one or two built-in serial ports, they will usually
  get set up correctly without using setserial.  Otherwise, if you add
  more serial ports (such as a modem card) you may need to deal with
  setserial.  Besides the man page for setserial, check out info in
  /usr/doc/setserial.../ or /usr/share/doc/setserial.  It should tell
  you how setserial is handled in your distribution of Linux.

  Setserial is often run automatically at boot-time by a start-up shell-
  script for the purpose of assigning IRQs, etc. to the driver.
  Setserial will only work if the serial module is loaded (or if the
  equivalent was compiled into your kernel).  If the serial module gets
  unloaded later on, the changes previously made by setserial will be
  forgotten by the kernel.  But recent (2000) distributions may contain
  scripts that save and restore this.  If not, then setserial must be
  run again to reestablish them.  In addition to running via a start-up
  script, something akin to setserial also runs earlier when the serial
  module is loaded (or the like).  Thus when you watch the start-up
  messages on the screen it may look like it ran twice, and in fact it
  has.



  Setserial does not set either IRQ's nor I/O addresses in the serial
  port hardware itself.  That is done either by jumpers or by plug-and-
  play.  You must tell setserial the identical values that have been set
  in the hardware.  Do not just invent some values that you think would
  be nice to use and then tell them to setserial.  However, if you know
  the I/O address but don't know the IRQ you may command setserial to
  attempt to determine the IRQ.

  You can see a list of possible commands by just typing setserial with
  no arguments.  This fails to show you the one-letter options such as
  -v for verbose which you should normally use when troubleshooting.
  Note that setserial calls an IO address a "port".  If you type:


       setserial -g /dev/ttyS*



  you'll see some info about how that device driver is configured for
  your ports.  Note that where it says "UART: unknown" it probably means
  that no uart exists.  In other words you probably have no such serial
  port and the other info shown about the port is meaningless and should
  be ignored.  If you really do have such a serial port, setserial
  doesn't recognize it and that needs to be fixed.

  If you add -a to the option -g you will see more info although few
  people need to deal with (or understand) this additional info since
  the default settings you see usually work fine.  In normal cases the
  hardware is set up the same way as "setserial" reports, but if you are
  having problems there is a good chance that "setserial" has it wrong.
  In fact, you can run "setserial" and assign a purely fictitious I/O
  port address, any IRQ, and whatever uart type you would like to have.
  Then the next time you type "setserial ..." it will display these
  bogus values without complaint.  They will also be officially
  registered with the kernel as displayed (at the top of the screen) by
  the "scanport" command (Debian).  Of course the serial port driver
  will not work correctly (if at all) if you attempt to use such a port.
  Thus when giving parameters to "setserial" anything goes.  Well
  almost.  If you assign one port a base address that is already
  assigned (such as 3e8) it may not accept it.  But if you use 3e9 it
  will accept it.  Unfortunately 3e9 is already assigned since it is
  within the range starting at base address 3e8.  Thus the moral of the
  story is to make sure your data is correct before assigning resources
  with setserial.

  While assignments made by setserial are lost when the PC is powered
  off, a configuration file may restore them (or a previous
  configuration) when the PC is started up again.  In newer versions,
  what you change by setserial may get automatically saved to a
  configuration file.  In older versions, the configuration file only
  changes if you edit it manually so the configuration always remains
  the same from boot to boot.  See ``Configuration Scripts/Files''


  10.1.2.  Probing

  Prior to probing with "setserial", one may run the "scanport" (Debian)
  command to check all possible ports in one scan.  It makes crude
  guesses as to what is on some ports but doesn't determine the IRQ.
  But it's a fast first start.  It may hang your PC but so far it's
  worked fine for me.  Note that non-Debian distributions don't seem to
  supply "scanport".  Is there an another scan program?

  With appropriate options, setserial can probe (at a given I/O address)
  for a serial port but you must guess the I/O address.  If you ask it
  to probe for /dev/ttyS2 for example, it will only probe at the address
  it thinks ttyS2 is at (2F8).  If you tell setserial that ttyS2 is at a
  different address, then it will probe at that address, etc.  See
  ``Probing''

  The purpose of this is to see if there is a uart there, and if so,
  what its IRQ is.  Use "setserial" mainly as a last resort as there are
  faster ways to attempt it such as wvdialconf to detect modems, looking
  at very early boot-time messages, or using pnpdump --dumpregs.  To try
  to detect the physical hardware use for example :
  setserial /dev/ttyS2 -v autoconfig
  If the resulting message shows a uart type such as 16550A, then you're
  OK.  If instead it shows "unknown" for the uart type, then there is
  supposedly no serial port at all at that I/O address.  Some cheap
  serial ports don't identify themselves correctly so if you see
  "unknown" you still might have a serial port there.

  Besides auto-probing for a uart type, setserial can auto-probe for
  IRQ's but this doesn't always work right either.  In one case it first
  gave the wrong irq but when the command was repeated it found the
  correct irq.  In versions of setserial >= 2.15, the results of your
  last probe test could be automatically saved and put into the
  configuration file /etc/serial.conf or /var/lib/serial.conf which will
  be used next time you start Linux.  At boot-time when the serial
  module loads (or the like), a probe for UARTs is made automatically
  and reported on the screen.  But the IRQs shown may be wrong.  The
  second report of the same is the result of a script which usually does
  no probing and thus provides no reliable information as to how the
  hardware is actually set.  It only shows configuration data someone
  wrote into the script or data that got saved in /etc/serial.conf.

  It may be that two serial ports both have the same IO address set in
  the hardware.  Of course this is not permitted but it sometimes
  happens anyway.  Probing detects one serial port when actually there
  are two.  However if they have different IRQs, then the probe for IRQs
  may show IRQ = 0.  For me it only did this if I first used setserial
  to give the IRQ a fictitious value.

  10.1.3.  Boot-time Configuration

  When the kernel loads the serial module (or if the "module equivalent"
  is built into the kernel) then only ttyS{0-3} are auto-detected and
  the driver is set to use only IRQs 4 and 3 (regardless of what IRQs
  are actually set in the hardware).  You see this as a boot-time
  message just like as if setserial had been run.

  To correct possible errors in IRQs (or for other reasons) there may be
  a file somewhere that runs setserial again.  Unfortunately, if this
  file has some IRQs wrong, the kernel will still have incorrect info
  about the IRQs.  This file should run early at boot-time before any
  process uses the serial port.  In fact, your distribution may have set
  things up so that the setserial program runs automatically from a
  start-up script at boot-time.  More info about how to handle this
  situation for your particular distribution might be found in file
  named "setserial..." or the like located in directory /usr/doc/ or
  /usr/share/doc/.

  Before modifying a configuration file, you can test out a "proposed"
  setserial command by just typing it on the command line.  In some
  cases the results of this use of setserial will automatically get
  saved in /etc/serial.conf when you shutdown.  So if it worked OK (and
  solved your problem) then there's no need to modify any configuration
  file.  See ``New configuration method using /etc/serial.conf''.


  10.1.4.  Configuration Scripts/Files

  Your objective is to modify (or create) a script file in the /etc tree
  that runs setserial at boot-time.  Most distributions provide such a
  file (but it may not initially reside in the /etc tree).  In addition,
  setserial 2.15 and higher often have an /etc/serial.conf file that is
  used by the above script so that you don't need to directly edit the
  script that runs setserial.  In addition just using setserial on the
  command line (2.15+) may ultimately alter this configuration file.

  So prior to version 2.15 all you do is edit a script.  After 2.15 you
  may need to either do one of three things: 1. edit a script.  2. edit
  /etc/serial.conf or 3. run "setserial" on the command line which may
  result in /etc/serial.conf automatically being edited.  Which one of
  these you need to do depends on both your particular distribution, and
  how you have set it up.


  10.1.5.  Edit a script (required prior to version 2.15)

  Prior to setserial 2.15 (1999) there was no /etc/serial.conf file to
  configure setserial.   Thus you need to find the file that runs
  "setserial" at boot time and edit it.  If it doesn't exist, you need
  to create one (or place the commands in a file that runs early at
  boot-time).  If such a file is currently being used it's likely
  somewhere in the /etc directory-tree.  But Redhat <6.0 has supplied it
  in /usr/doc/setserial/ but you need to move it to the /etc tree before
  using it.   You might use "locate" to try to find such a file.  For
  example, you could type: locate "*serial*".

  The script /etc/rc.d/rc.serial was commonly used in the past.  The
  Debian distribution used /etc/rc.boot/0setserial.  Another file once
  used was /etc/rc.d/rc.local but it's not a good idea to use this since
  it may not be run early enough.  It's been reported that other
  processes may try to open the serial port before rc.local runs
  resulting in serial communication failure.  Today it's most likely in
  /etc/init.d/ but it isn't normally intended to be edited.


  If such a file is supplied, it should contain a number of commented-
  out examples.  By uncommenting some of these and/or modifying them,
  you should be able to set things up correctly.  Make sure that you are
  using a valid path for setserial, and a valid device name.  You could
  do a test by executing this file manually (just type its name as the
  super-user) to see if it works right.  Testing like this is a lot
  faster than doing repeated reboots to get it right.

  For versions >= 2.15 (provided your distribution implemented the
  change, Redhat didn't) it may be more tricky to do since the file that
  runs setserial on startup, /etc/init.d/setserial or the like was not
  intended to be edited by the user.  See ``New configuration method
  using /etc/serial.conf''.

  If you want setserial to automatically determine the uart and the IRQ
  for ttyS3 you would add something like:



       /sbin/setserial  /dev/ttyS3 auto_irq skip_test autoconfig



  Do this for every serial port you want to auto configure.  Be sure to
  give a device name that really does exist on your machine.  In some
  cases this will not work right due to the hardware.  If you know what
  the uart and irq actually are, you may want to assign them explicitly
  with "setserial".  For example:



       /sbin/setserial /dev/ttyS3 irq 5 uart 16550A  skip_test



  10.1.6.  New configuration method using /etc/serial.conf

  Prior to setserial version 2.15, the way to configure setserial was to
  manually edit the shell-script that ran setserial at boot-time.  See
  ``Edit a script (after version 2.15: perhaps not)''.  Starting with
  version 2.15 (1999) of setserial this shell-script is not edited but
  instead gets its data from a configuration file: /etc/serial.conf.
  Furthermore you may not even need to edit serial.conf because using
  the "setserial" command on the command line may automatically cause
  serial.conf to be edited appropriately.

  This was intended so that you don't need to edit any file in order to
  set up (or change) what setserial does each time that Linux is booted.
  But there are serious pitfalls because it's not really "setserial"
  that edits serial.conf.  Confusion is compounded because different
  distributions handle this differently.  In addition, you may modify it
  so that it works differently.

  What often happens is this:  When you shut down your PC the script
  that runs "setserial" at boot-time is run again, but this time it only
  does what the part for the "stop" case says to do:  It uses
  "setserial" to find out what the current state of "setserial" is, and
  it puts that info into the serial.conf file.  Thus when you run
  "setserial" to change the serial.conf file, it doesn't get changed
  immediately but only when and if you shut down normally.


  Now you can perhaps guess what problems might occur.  Suppose you
  don't shut down normally (someone turns the power off, etc.) and the
  changes don't get saved.  Suppose you experiment with "setserial" and
  forget to run it a final time to restore the original state (or make a
  mistake in restoring the original state).  Then your "experimental"
  settings are saved.

  If you manually edit serial.conf, then your editing is destroyed when
  you shut down because it gets changed back to the state of setserial
  at shutdown.  There is a way to disable the changing of serial.conf at
  shutdown and that is to remove "###AUTOSAVE###" or the like from first
  line of serial.conf.  In at least one distribution, the removal of
  "###AUTOSAVE###" from the first line is automatically done after the
  first time you shutdown just after installation.  The serial.conf file
  should contain some comments to explain this.

  The file most commonly used to run setserial at boot-time (in
  conformance with the configuration file) is now /etc/init.d/setserial
  (Debian) or /etc/init.d/serial (Redhat), or etc.,  but it should not
  normally be edited.  For 2.15, Redhat 6.0 just had a file
  /usr/doc/setserial-2.15/rc.serial which you have to move to
  /etc/init.d/ if you want setserial to run at boot-time.

  To disable a port, use setserial to set it to "uart none".  The format
  of /etc/serial.conf appears to be just like that of the parameters
  placed after "setserial" on the command line with one line for each
  port.  If you don't use autosave, you may edit /etc/serial.conf
  manually.

  BUG: As of July 1999 there is a bug/problem since with ###AUTOSAVE###
  only the setserial parameters displayed by "setserial -Gg /dev/ttyS*"
  get saved but the other parameters don't get saved.  Use the -a flag
  to "setserial" to see all parameters.  This will only affect a small
  minority of users since the defaults for the parameters not saved are
  usually OK for most situations.  It's been reported as a bug and may
  be fixed by now.

  In order to force the current settings set by setserial to be saved to
  the configuration file (serial.conf) without shutting down, do what
  normally happens when you shutdown: Run the shell-script
  /etc/init.d/{set}serial stop.  The "stop" command will save the
  current configuration but the serial ports still keep working OK.

  In some cases you may wind up with both the old and new configuration
  methods installed but hopefully only one of them runs at boot-time.
  Debian labeled obsolete files with "...pre-2.15".


  10.1.7.  IRQs

  By default, both ttyS0 and ttyS2 will share IRQ 4, while ttyS1 and
  ttyS3 share IRQ 3.  But actually sharing serial interrupts (using them
  in running programs) is not permitted unless you: 1. have kernel 2.2
  or better, and 2. you've complied in support for this, and 3. your
  serial hardware supports it.  See


  ``Interrupt sharing and Kernels 2.2+'' If you only have two serial
  ports, ttyS0 and ttyS1, you're still OK since IRQ sharing conflicts
  don't exist for non-existent devices.

  If you add an internal modem and retain ttyS0 and ttyS1, then you
  should attempt to find an unused IRQ and set it both on your serial
  port (or modem card) and then use setserial to assign it to your
  device driver.  If IRQ 5 is not being used for a sound card, this may
  be one you can use for a modem.  To set the IRQ in hardware you may
  need to use isapnp, a PnP BIOS, or patch Linux to make it PnP.  To
  help you determine which spare IRQ's you might have, type "man
  setserial" and search for say: "IRQ 11".


  10.1.8.  Laptops: PCMCIA

  If you have a Laptop, read PCMCIA-HOWTO for info on the serial
  configuration.  For serial ports on the motherboard, setserial is used
  just like it is for a desktop.  But for PCMCIA cards (such as a modem)
  it's a different story.  The configuring of the PCMCIA system should
  automatically run setserial so you shouldn't need to run it.  If you
  do run it (by a script file or by /etc/serial.conf) it might be
  different and cause trouble.  The autosave feature for serial.conf
  shouldn't save anything for PCMCIA cards (but Debian did until
  2.15-7).  Of course, it's always OK to use setserial to find out how
  the driver is configured for PCMCIA cards.



  10.2.  What is isapnp ?

  isapnp is a program to configure Plug-and-Play (PnP) devices on the
  ISA bus including internal modems.  It comes in a package called
  "isapnptools" and includes another program, "pnpdump" which finds all
  your ISA PnP devices and shows you options for configuring them in a
  format which may be added to the PnP configuration file:
  /etc/isapnp.conf.  It may also be used with the --dumpregs option to
  show the current IO address and IRQ of the modem's serial port.  The
  isapnp command may be put into a startup file so that it runs each
  time you start the computer and thus will configure ISA PnP devices.
  It is able to do this even if your BIOS doesn't support PnP.  See
  Plug-and-Play-HOWTO.


  10.3.  What is wvdialconf ?

  wvdialconf will try to find which serial port (ttyS?) has a modem on
  it.  It also creates a configuration program for the wvdial program.
  wvdial is used for simplified dialing out using the PPP protocol to an
  ISP.  But you don't need to install PPP in order to use wvdialconf.
  It will only find  modems which are not in use.  It will also
  automatically devise a "suitable" init strings but sometimes gets it
  wrong.  Since this command has no options, it's simple to use but you
  must give it the name of a file to put the init string (and other
  data) into.  For example type: wvdialconf my_config_file_name.


  11.  Trying Out Your Modem (Dialing Out)

  11.1.  Are You Ready to Dial Out ?

  Once you've plugged in your modem and know which serial port it's on
  you're ready to try using it.  If you already have an account with an
  ISP to connect to the Internet, you could try using a program like
  "wvdial" to connect to the Internet using the PPP protocol.

  As an alternative to taking one big step using PPP to connect to the
  Internet, you could do a two step process:  First just test out your
  modem without using PPP (using Minicom or Kermit).  Then if your modem
  works OK, use "wvdial" or other ppp dialer to connect to the Internet.
  A different strategy is to first try a ppp dialer and then if that
  doesn't work out, fallback to Minicom or Kermit to see if your modem
  works OK.  Knowing how to use either Minicom or Kermit is handy for
  dialing out to some places directly without going thru the Internet.
  If you are going to use Minicom or Kermit you must find a phone number
  to dial that will accept phone calls from a computer (without using
  PPP).  Perhaps a local library has such a phone number for its on-line
  catalog.

  Then make sure you are ready to phone.  Do you know what serial port
  (such as ttyS2) your modem is on?  You should have found this out when
  you io-irq configured your serial ports.  Have you decided what speed
  you are going to use for this port?  See ``Speed Table'' for a quick
  selection or ``What Speed Should I Use with My Modem'' for more
  details.  If you have no clue of what speed to set, try setting it a
  few times faster than the advertised speed of your modem.  Also
  remember that if you see a menu where an option is "hardware flow
  control" and/or "RTS/CTS" or the like, select it.  Is a live telephone
  cable plugged in to your modem?  You may want to connect this cable to
  a real telephone to make sure that it can produce a dial tone.

  Now you need to select a communication (dialing) program to use to
  dial out.  Internet dialing programs (using PPP) include wvdial, kppp
  (for KDE), and for Gnome: gnome-ppp or "modem lights".   Non-internet
  dialing programs include: minicom, seyon (X Window), and kermit.  See
  section ``Communications Programs'' about some communications
  programs.  Three examples are presented next: ``Dialing Out with
  wvdial'' ``Dialing Out with Minicom'' and ``Dialing Out with Kermit''


  11.2.  Dialing Out with wvdial

  Wvdial is a program with not only dials out, but starts PPP and logs
  you in to an ISP where you get to the Internet.  Wvdial may be
  configured during the installation process or by using the program
  "wvdialconf".  See the man pages for both "wvdialconf" and "wvdial".
  However, before using wvdial you must do two other tasks not covered
  by the wvdial documentation:

  ·  set up your network on your PC. The old HOWTO, "ISP-Hookup-HOWTO"
     has some info on how to do this but fails to mention wvdial which
     replaces "chatscripts".

  ·  configure your browser


  11.3.  Dialing Out with Minicom

  Minicom comes with most Linux distributions.  To configure it you
  should be the root user.  As root, type "minicom -s" to configure.
  This will take you directly to the configuration (set-up) menus.  This
  allows you to use the configuration immediately.   If you just type
  "minicom" and then configure, you'll need to leave and restart minicom
  for the configuration to take effect.  Within minicom type ^A to see
  the bottom status line.  This shows to type ^A Z for help (you've
  already typed the ^A so just type z).

  Most of the options don't need to be set for just simply dialing out.
  To configure you have to supply a few basic items: the name of the
  serial port your modem is on such as /dev/ttyS2 and the speed such as
  115200.  These are set at the serial port menu.  Go to it and set
  them.  Also (if possible) set hardware flow control (RTS/CTS).  Then
  save them.  When typing in the speed, you should also see something
  like "8N1" which you should leave alone.  It means: 8-bit bytes, No
  parity, 1 stop-bit appended to each byte.  If you can't find the speed
  you want, a lower speed will always work for a test.   Exit (hit
  return) when done and save the configuration as default (dfl) using
  the menu.  Unless you've used the -s option when you called minicom,
  you'll need to exit minicom and start it again so it can now find the
  serial port and initialize the modem.
  Now you are ready to dial.  But first at the main screen you get after
  you first type "minicom" make sure there's a modem there by typing AT
  and then hit the <enter> key.  It should display OK.  If it doesn't
  something is wrong and there is no point of trying to dial.

  If you got the "OK" go back to help and select the dialing directory.
  You may edit it and type in a phone number, etc. into the directory
  and then select "dial" to dial it.  Alternatively, you may just dial
  manually (by selecting "manual" and then type the number at the
  keyboard).  If it doesn't work, carefully note any error messages and
  try to figure out what went wrong.


  11.4.  Dialing Out with Kermit

  You can find the latest version of kermit at
  http://www.columbia.edu/kermit/.  For example, say your modem was on
  ttyS3, and its speed was 115200 bps.  You would do the following:


       linux# kermit
       C-Kermit 6.0.192, 6 Sep 96, for Linux
        Copyright (C) 1985, 1996,
         Trustees of Columbia University in the City of New York.
       Default file-transfer mode is BINARY
       Type ? or HELP for help.
       C-Kermit>set line /dev/ttyS3
       C-Kermit>set carrier-watch off
       C-Kermit>set speed 115200
       /dev/ttyS3, 115200 bps
       C-Kermit>c
       Connecting to /dev/ttyS3, speed 115200.
       The escape character is Ctrl-\ (ASCII 28, FS)
       Type the escape character followed by C to get back,
       or followed by ? to see other options.
       ATE1Q0V1                           ; you type this and then the Enter key
       OK                                 ; modem should respond with this



  If your modem responds to AT commands, you can assume your modem is
  working correctly on the Linux side.  Now try calling another modem by
  typing:


       ATDT7654321



  where 7654321 is a phone number.  Use ATDP instead of ATDT if you have
  a pulse line.  If the call goes through, your modem is working.

  To get back to the kermit prompt, hold down the Ctrl key, press the
  backslash key, then let go of the Ctrl key, then press the C key:


       Ctrl-\-C
       (Back at linux)
       C-Kermit>quit
       linux#



  This was just a test using the primitive "by-hand" dialing method.
  The normal method is to let kermit do the dialing for you with its
  built-in modem database and automatic dialing features, for example
  using a US Robotics (USR) modem:


       linux# kermit
       C-Kermit 6.0.192, 6 Sep 1997, for Linux
        Copyright (C) 1985, 1996,
         Trustees of Columbia University in the City of New York.
       Default file-transfer mode is BINARY
       Type ? or HELP for help
       C-Kermit>set modem type usr        ; Select modem type
       C-Kermit>set line /dev/ttyS3       ; Select communication device
       C-Kermit>set speed 115200          ; Set the dialing speed
       C-Kermit>dial 7654321              ; Dial
        Number: 7654321
        Device=/dev/ttyS3, modem=usr, speed=115200
        Call completed.<BEEP>
       Connecting to /dev/ttyS3, speed 115200
       The escape character is Ctrl-\ (ASCII 28, FS).
       Type the escape character followed by C to get back,
       or followed by ? to see other options.

       Welcome to ...

       login:



  12.  Dial-In

  12.1.  Dial-In Overview

  Dial-in is where you set up your PC so that others may dial in to your
  PC (at your phone number) and use your PC.  Unfortunately some use the
  term "dial-in" when what they actually mean is just the opposite:
  dial-out.  Dial-in works like this.  Someone with a modem dials your
  telephone number.  Your modem answers the phone ring and connects.
  Once the caller is connected the getty program is notified and starts
  the login process for the caller.  After the caller has logged in, the
  caller then may use your PC.  It could be almost as if they were
  sitting at the monitor-console.

  The caller may use a script to automatically log in.  This script will
  be of the expect-send type.  For example it expects "login:" and then
  (after it detects "login:") will send the users login name.  It next
  expects the password and then sends the password, etc.  Then once the
  user has been automatically logged in, the /etc/passwd (password file)
  might specify that a shell (such as bash) will be started for the
  user.  Or it might specify that PPP is to start so that the user may
  be connected to the Internet.  See the PPP-HOWTO for more details.
  The program that you use at your PC to handle dialin is called getty
  or mgetty.  See ``Getty''

  An advanced getty program such as mgetty can watch to see if PPP is
  started by the PC on the other end.  If so, the login prompt would be
  skipped, a PPP connection would be made, and login would take place
  automatically over the PPP connection.



  12.2.  What Happens when Someone Dials In ?

  Here's a more detailed description of dialin.  This all assumes that
  you are using either mgetty or uugetty.  Agetty is inferior and
  doesn't work exactly as described below (see ``About agetty'')

  For dialin to work, the modem must be listening for a ring and getty
  must be running and ready to respond to the call.  Your modem is
  normally listening for incoming calls, but what it does when it gets a
  ring depends on how it's configured.  The modem can either
  automatically answer the phone or not directly answer it.  In the
  latter case the modem sends a "RING" message to getty and then getty
  tells the modem to answer the ring.  In either case, it may be set up
  to answer on say the 4th ring.  This means that if the call is not for
  the modem, one must walk/run to the phone and pick it up manually
  before the 4th ring.  Then they can talk to the other person.  If they
  get to the phone too late they will hear the screeching noise of the
  modem which has answered the call.

  Once the modem answers the call it sends tones to the other modem (and
  conversely).  The two modems negotiate how they will communicate and
  when this is completed your modem sends a "CONNECT" message (or the
  like) to getty.  When getty gets this message, it sends a login prompt
  out the serial port.  Once a user name is given to this prompt getty
  may just call on a program named login to handle the logging in from
  there on.  While getty usually starts running at boot-time it should
  wait until a connection is made before sending out a "login" prompt.

  Now for more details on the two methods of answering the call.  For
  the first method where the modem automatically answers the call, the
  number of times it will ring before answering is controlled by the S0
  register of the modem.  If S0 is set to 3, the modem will
  automatically answer on the 3rd ring.  If it's set to 0 then the modem
  will only answer the call if getty sends it an "A" (= Answer) AT
  command to the modem while the phone is ringing.  (Actually an "ATA"
  is sent since all modem commands are prefixed by "AT".)  This is known
  as "manual" answering since the modem itself doesn't do it
  automatically (but getty does).  You might think it best to utilize
  the ability of the modem hardware to automatically answer the call,
  but it's actually better if getty answers it "manually".

  For the "manual" answer case, getty opens the port at boot-time and
  listens.  When the phone rings, a "RING" message is sent to the
  listening getty.  Then if getty wants to answer this ring, it sends
  the modem an "A" command.  Note that getty may be set to answer only
  after say 4 "RING" messages (the 4th ring) similar to the automatic
  answer method.  The modem then makes a connection and sends a "CONNECT
  ..." message to getty which then sends a login prompt to the caller.
  It's not all quite this simple as are some special tricks used to
  allow dial-out when waiting for a call.  See ``Dialing Out while
  Waiting for an Incoming Call''

  The automatic answer case uses the CD (Carrier Detect aka DCD) wire
  from the modem to the serial port to tell when a connection is made.
  It works like this.  At boot-time getty tries to open the serial port
  but the attempt fails since the modem has negated CD (the modem is
  idle).  Then the getty program waits at the open statement in the
  program until a CD signal is asserted.  When a CD signal arrives
  (perhaps hours later) then the port is opened and getty sends the
  login prompt.  While getty is waiting (sleeping) at the open
  statement, other processes can run so it doesn't degrade computer
  performance.  What actually wakes getty up is an interrupt which is
  issued when the CD line from the modem changes its state to on.

  You may wonder how getty is able to open the serial port in the
  "manual"-answer case since CD may be negated.  Well, there's a way to
  write a program to force the port to open even if there is no CD
  signal asserted.


  12.3.  56k Doesn't Work for Dialin

  If you expect that people will be able to dial-in to you at 56k, it
  can't be done unless you have all the following:

  1. You have a digital connection to the telephone company such as a
     trunkside-T1 or ISDN line

  2. You use special digital modems (see ``Digital Modems'')

  3. You have a "... concentrator", or the like to interface your
     digital-modems to the digital lines of the telephone company.

     A "... concentrator" may  be called a "modem concentrator" or a
     "remote access concentrator" or it could be included in a "remote
     access server" (RAS) which includes the digital modems, etc.  This
     type of setup is used by ISPs (Internet Service Providers).


  12.4.  Getty

  12.4.1.  Introduction to Getty

  getty is the program you run for dialin.  You don't need it for
  dialout.  In addition to presenting a login prompt, it also may help
  answer the telephone.  Originally getty was used for logging in to a
  computer from a dumb terminal.  A major use of it today is for logging
  in to a Linux console.  There are several different getty programs but
  a few of these work OK with modems for dialin.  The getty program is
  usually started at boot-time.  It must be called from the /etc/inittab
  file.  In this file you may find some examples which you will likely
  need to edit a bit.  Hopefully these examples will be for the flavor
  of getty installed on your PC.

  There are four different getty programs to choose from that may be
  used with modems for dial-in: mgetty, uugetty, getty_em, and agetty.
  A brief overview is given in the following subsections.  agetty is the
  weakest of the four and it's mainly for use with directly connected
  text-terminals. mgetty has support for fax and voice mail but uugetty
  doesn't.  mgetty allegedly lacks a few of the features of uugetty.
  getty_em is a simplified version of uugetty.  Thus mgetty is likely
  your best choice unless you are already familiar with uugetty (or find
  it difficult to get mgetty).  The syntax for these getty programs
  differs, so be sure to check that you are using the correct syntax in
  /etc/inittab for whichever getty you use.

  In order to see what documentation exists about the various gettys on
  your computer, use the "locate" command.  Type: locate "*getty*"
  (including the quotes may help).  Note that many distributions just
  call the program getty even though it may actually be agetty, uugetty,
  etc.  But if you read the man page (type: man getty), it might
  disclose which getty it is.  This should be the getty program with
  path /sbin/getty.


  12.4.2.  How getty respawns

  After you log in you will notice (by using "top", "ps -ax", or
  "ptree") that the getty process is no longer running.  What happened
  to it?  Why does getty restart again if your shell is killed?  Here's
  why.

  After you type in your user name, getty takes it and calls the login
  program telling it your user name.  The getty process is replaced by
  the login process.  The login process asks for your password, checks
  it and starts whatever process is specified in your password file.
  This process is often the bash shell.  If so, bash starts and replaces
  the login process.  Note that one process replaces another and that
  the bash shell process originally started as the getty process.  The
  implications of this will be explained below.

  Now in the /etc/inittab file, getty is supposed to respawn (restart)
  if killed.  It says so on the line that calls getty.  But if the bash
  shell (or the login process) is killed, getty respawns (restarts).
  Why?  Well, both the login process and bash are replacements for getty
  and inherit the signal connections establish by their predecessors.
  In fact if you observe the details you will notice that the
  replacement process will have the same process ID as the original
  process.  Thus bash is sort of getty in disguise with the same process
  ID number.  If bash is killed it is just like getty was killed (even
  though getty isn't running anymore).  This results in getty
  respawning.

  When one logs out, all the processes on that serial port are killed
  including the bash shell.  This may also happen (if enabled) if a
  hangup signal is sent to the serial port by a drop of DCD voltage by
  the modem.  Either the logout or drop in DCD will result in getty
  respawning.  One may force getty to respawn by manually killing bash
  (or login) either by hitting the k key, etc. while in "top" or with
  the "kill" command.  You will likely need to kill it with signal 9
  (which can't be ignored).



  12.4.3.  About mgetty

  mgetty was written as a replacement for uugetty which was in existence
  long before mgetty.  Both are for use with modems but mgetty is best
  (unless you already are committed to uugetty).  mgetty may be also
  used for directly connected terminals.  In addition to allowing dialup
  logins, mgetty also provides FAX support, auto PPP detection, and
  caller-id support.  It permits dialing out when mgetty is waiting for
  an incoming phone call.  There is a supplemental program called vgetty
  which handles voicemail for some modems.  mgetty documentation is fair
  (except for voice mail), and is not supplemented in this HOWTO.  To
  automatically start PPP one must edit /etc/mgetty/login.conf to use
  "AutoPPP" (has example).  You can find the latest information on
  mgetty at http://www.leo.org/~doering/mgetty/ and
  <http://alpha.greenie.net/mgetty/>


  12.4.4.  About uugetty

  getty_ps  contains two programs: getty is used for console and
  terminal devices, and uugetty for modems.  Greg Hankins (former author
  of Serial-HOWTO) used uugetty so his writings about it are included
  here.  See ``Uugetty''.


  12.4.5.  About getty_em

  This is a simplified version of ``uugetty''.  It was written by Vern
  Hoxie after he became fully confused with complex support files needed
  for getty_ps and uugetty.

  It is part of the collection of serial port utilities and information
  by Vern Hoxie available via ftp from  <scicom.alphacdc.com/pub/linux>.
  The name of the collection is ``serial_suite.tgz''.
  12.4.6.  About agetty

  This subsection is long since the author tried using agetty for
  dialin.  agetty is seemingly simple since there are no initialization
  files.  But when I tried it, it opened the serial port even when there
  was no CD signal present.  It then sent both a login prompt and the
  /etc/issue file to the modem in the AT-command state before a
  connection was made.  The modem thinks all this an AT command and if
  it does contain any "at" strings (by accident) it is likely to
  adversely modify your modem profile.  Echo wars can start where getty
  and the modem send the same string back and forth over and over.  You
  may see a "respawning too rapidly" error message if this happens.  To
  prevent this you need to disable all echoing and result codes from the
  modem (E0 and Q1).  Also use the -i option with agetty to prevent any
  /etc/issue file from being sent.

  If you start getty on the modem port and a few seconds later find that
  you have the login process running on that port instead of getty, it
  means that a bogus user name has been sent to agetty from the modem.
  To keep this from happening, I had to save my dial-in profile in the
  modem so that it become effective at power-on.  The other saved
  profile is for dial-out.  Then any dial-out programs which use the
  modem must use a Z, Z0, or Z1 in their init string to initialize the
  modem for dial-out (by loading the saved dial-out profile).  If the
  1-profile is for dial-in you use Z1 to load it, etc.  If you want to
  listen for dial-in later on, then the modem needs to be reset to the
  dial-in profile.  Not all dial-out programs can do this reset upon
  exit from them.

  Thus while agetty may work OK if you set up a dial-in profile
  correctly in the modem hardware, it's probably best suited for virtual
  consoles or terminals rather than modems.  If agetty is running for
  dialin, there's no easy way to dial out.  When someone first dials in
  to agetty, they should hit the return key to get the login prompt.
  agetty in the Debian distribution is just named getty.


  12.4.7.  About mingetty, and fbgetty

  mingetty is a small getty that will work only for monitors (the usual
  console) so you can't use it with modems for dialin.  fbgetty is as
  above but supports framebuffers.


  12.5.  Why "Manual" Answer is Best

  The difference between the two ways of answering is exhibited when the
  computer happens to be down but the modem is still working.  For the
  manual case, the "RING" message is sent to getty but since the
  computer is down, getty isn't there and the phone never gets answered.
  There are no telephone charges when there is no answer.  For the
  automatic answer case, the modem (which is still on) answers the phone
  but no login message is ever sent since the computer is down.  The
  phone bill runs up as the waiting continues.  If the phone call is
  toll-free, it doesn't make much difference, although it may be
  frustrating waiting for a login prompt that never arrives.  mgetty
  uses manual answer.  Uugetty can do this too using a configuration
  script.


  12.6.  Dialing Out while Waiting for an Incoming Call

  Here's what could go wrong with a simple-minded manual-answer
  situation.  Suppose another process dials out while getty is listening
  for a "RING" message from its modem on the serial wire.  Then incoming
  bytes for the dial-out process flow from the modem to the serial port.
  For example, your modem may send a "CONNECT" message to your serial
  port when the dial-out process connects.  If getty reads this there's
  trouble since reads are destructive reads.  Once getty reads it, then
  the dial-out process that is expecting "CONNECT" (or something else)
  can't read it.  Thus the dial-out process is likely to fail.

  There's a way to avoid this and here's how mgetty does it.  When
  mgetty is listing for an incoming call, it doesn't read anything from
  the port until it thinks that the characters are for mgetty.  Mgetty
  monitors the port and if characters arrive, it doesn't read them right
  away.  Instead, it first checks to see if another process is using the
  port.  If so, mgetty backs off and closes the port (but the port
  remains open for the other process).  Thus if another process dials
  out, mgetty doesn't interfere with it.  When the other process finally
  closes the port, then mgetty resumes "listening".  It's a special type
  of "listening" that refrains from reading until mgetty believes that
  what it will read is for mgetty (hopefully a "RING" message).

  When mgetty checks to see if another process is using the port, it
  actually checks for valid lockfiles on the port.  If the other process
  failed to use lockfiles, too bad for it.  For more details see the
  mgetty documentation: "How mgetty works".  For programmers only:
  "listening" is actually using the system calls "poll" or "select" to
  monitor the port.  They are likely also used to monitor the port when
  a non-mgetty process is using the port.

  With auto-answer, getty is waiting for CD to be asserted so that it
  can open the port.  One may dial out, but once a connection is made
  the modem's CD is asserted.  If getty were to then read the port it
  would eat the characters intended to be read by the dial-out
  connection.  While agetty will have this problem, it's claimed that
  uugetty will check lockfiles before reading (similar to mgetty).


  12.7.  Ending a Dial-in Call

  There are two major ways to end a dial-in call.  The caller may either
  logout or just hang up.  For the hangup case see ``Caller hangs up''


  12.7.1.  Caller logs out

  When the call is over the normal way to end the connection is for the
  user to log out.  This will kill the remote user's shell on your PC.
  Now since there is nothing running on this port, the port is closed
  and sends a hangup signal to the modem by negating DTR.  This will
  only happen if stty -a shows hupcl (hang up on close) but this should
  be the default.

  The modem getting this hangup (negated DTR signal) will then hang up
  the phone line (provided the modem has been configured to do this
  --see below).  The modem should then be ready to answer any new
  incoming calls.  Killing the user's shell also causes getty to respawn
  and wait for the next call.

  As an alternative to using DTR to tell the modem to hang up the phone
  line, a script used after getty respawns may send the unique escape
  code sequence +++ to the modem to put it into AT command mode.  The
  +++ must have both an initial and final time delay.  Once in AT
  command mode, a hangup command (H0) may be sent to the modem as well
  as other AT commands.  If the PC fails to successfully signal the
  modem when a logout happens (or to use the +++ escape when restarting
  getty), then the modem is apt to remain in on-line mode and no more
  incoming calls can be received.


  12.7.2.  When DTR drops (is negated)

  When DTR (the "hang-up" signal) is negated, what the modem does
  depends on the value of the &D option in the modem's profile.  If it's
  &D0 nothing at all happens (the modem ignores the negation of DTR).
  Here's what happens when DTR drops:

  &D2: The modem will hang up and go into AT command mode (off-line) to
  wait for the next call.  Except that it will not be able to
  automatically answer the phone until DTR is asserted again.  But since
  getty automatically respawns (if so set in /etc/inittab) then getty
  will immediately restart after a logout and this will assert DTR.  So
  what happens when someone logs out is that DTR only is negated for a
  fraction of a second (winks) before it gets asserted again.  For the
  above to happen, the DTR must be negated for at least the time
  specified by register S25.

  &D3: In this case the modem does a hard reset: It hangs up and
  restores the saved profile as specified by &Y.  It should now be in
  the same state it was in when first powered on and it's ready for
  incoming calls.  The S25 limit may have no effect so even a very short
  "wink" is detected.  Another brand of modem says the S25 limit is
  still valid.  Thus &D3 is a stronger "reset" than &D2 which doesn't
  restore the saved profile and could require a longer wink to work.

  If you don't know which of the above two to use try using &D3 first.
  Under favorable conditions, either one should work OK.  It's reported
  that for a few modems only &D2 works OK.


  12.7.3.  Caller hangs up

  Instead of logging out the normal way, a caller may just hang up.
  This results in a lost connection and of course a loss of carrier.
  Other problems could also cause a loss of carrier.  The "NO CARRIER"
  result code is displayed.  The modem hangs up and waits for the next
  call.  Except that there is no getty running yet to start the login
  process.

  Here's how getty gets started again:  The loss of carrier should
  negate the CD signal sent by the modem to the serial port (provided
  &C1 has been set).  When the PC's serial port gets the negated CD
  signal it should kill the shell and then getty should respawn.

  This paragraph is about other things that happen but do nothing.  Only
  the curious need read it.  When the shell is killed, a DTR wink is
  sent to the modem but since the modem is not on-line anymore and has
  already hung up, the modem ignores the negation of DTR (hang up).  The
  loss of carrier also negates the DSR signal sent by the modem to the
  serial port (provided &S1 or &S2 is set) but this signal is ignored
  (by Linux).


  12.8.  Dial-in Modem Configuration

  The getty programs have a provision for sending an init string to the
  modem to configure it.  But you may need to edit it.  Another method
  is to save a suitable init string inside the modem (see ``Init
  Strings: Saving and Recalling'' for how to save it in the modem).

  The configuration for dial-in depends both on the getty you use and
  perhaps on your modem.  If you can't find suggested configurations in
  other documentation here are some hints using Hayes AT commands:



  ·  &C1  Make the CD line to the serial port track the actual state of
     the carrier (CD asserted only when there's carrier).  Getty_em
     requires &C0 (CD always asserted)

  ·  &D3  Do a hard reset of the modem when someone logs out (or hangs
     up).  For some modems it's reported that &D2 is required since they
     can't tolerate a hard reset ??

  ·  E0  Don't echo AT commands back to the serial port.  This is a must
     for agetty.  Some suggest E1 (echo AT commands) for mgetty.  For
     dial-out you want E1 so you can see what was sent.

  ·  &K3 Use hardware flow control

  ·  Q0  Echo results words (such as CONNECT).  Most gettys use them.
     But it's reported an AT&T version of uugetty and agetty require Q2
     (no result words for dial-in).

  ·  S0=?  mgetty suggests S0=0 (manual answer).  If you set S0=3 the
     modem will auto-answer on the 3rd ring, etc.  Agetty uses auto-
     answer.  So does uugetty (usually).

  ·  V1  Display results (such as CONNECT) in words (and not in code)

  ·  X4  Check for dialtone and busy signal


  12.9.  Callback

  Callback is where someone first dials in to your modem.  Then, you get
  a little info from the caller and then call it right back.  Why would
  you want to do this?  One reason is to save on telephone bills if you
  can call the caller cheaper than the caller can call you.  Another is
  to make sure that the caller really is who it claims to be.  If a
  caller calls you and claims to be calling from its usual phone number,
  then one way to verify this is to actually place a new call to that
  number.

  There's a program for Linux called "callback" that works with mgetty.
  It's at  <ftp://ftp.rug.nl/contrib/frank/software/linux/callback/>
  Step-by-step instructions on how someone installed it (and PPP) is at
  <http://www.stokely.com/unix.serial.port.resources/callback.html>


  12.10.  Voice Mail

  Voice mail is like an answering machine run by a computer.  To do this
  you must have a modem that supports "voice" and supporting software.
  Instead of storing the messages on tape, they are stored in digital
  format on a hard-drive.  When a person phones you, they hear a
  "greeting" message and can then leave a message for you.  More
  advanced systems would have caller-selectable mail boxes and caller-
  selectable messages to listen to.  Free software is available in Linux
  for simple answering, but doesn't seem to be available yet for the
  more advanced stuff.

  I know of two different voicemail packages for Linux.  One is a very
  minimal package (see ``Voicemail Software'').  The other, more
  advanced, but currently poorly documented, is vgetty.  It's an
  optional addition to the well documented and widely distributed mgetty
  program.  It supports ZyXEL-like voice modem commands.  In the Debian
  distribution, you must get the mgetty-voice package in addition to the
  mgetty package and mgetty-doc package.



  12.11.  Simple Manual Dial-In

  This is really doing it manually!  It doesn't even permit the caller
  to login but the caller may "chat" with you, etc.  It's a way to
  answer a call without bothering to edit any configuration files for
  dial-in or enabling getty.  To do it you run a terminal program such
  as minicom.  Make sure it's connected to your modem by typing "AT
  <enter>" and expect "OK".  Then wait for the call.  Then you really
  answer the call manually by typing "ATA" when the phone is ringing.
  This doesn't run getty and the caller can't login.  But if the caller
  is calling in with a terminal program they may type a message to your
  screen (and conversely).  You both may send files back and forth by
  using the commands built into the terminal programs (such as minicom).
  Another way to answer such a call would be to type say "ATS0=3" just
  before the call comes in to enable the modem to auto-answer on the
  third ring.

  This is one way to crudely transfer files with someone on a MS Windows
  PC who uses HyperTerminal or Terminal (for Windows 3.x or DOS).  These
  two MS programs are something like minicom.  Using this simple manual
  method (for Linux-to-Linux or MS-to-Linux) requires two people to be
  present, one one each end of the phone line connection running a
  terminal communications program.  Be warned that if both people type
  at the same time it's chaos.  It's a "last resort" way to transfer
  files between any two people that have PCs (either Linux or MS
  Windows).  It could also be used for testing your modem or as a
  preliminary test before setting up dial-in.


  12.12.  Complex GUI Dial-In, VNC

  At the opposite extreme to the simple (but labor intensive) manual
  dial-in described above, is one that results in GUI graphical
  interface to the Linux PC.  This generally requires that a network
  running TCP/IP protocol exist between the two computers.  One way to
  get such a "network" is to dial-out to a PC set for dial-in and then
  run PPP on the phone line.  PPP will use TCP/IP protocol encapsulated
  inside the PPP packets.  Both sides must run PPP and mgetty can be
  configured to start PPP as soon as the caller does.  The caller may
  use a PPP-dialer program just like they were dialing an ISP.  Programs
  such as wvdial, eznet, or chat scripts should do it.

  Instead of this tiny network over a phone connection a much larger
  network (the entire world) is reached via an ISP.  For their lowest-
  rate service many of them use proxy servers that will not give you
  access to the ports you need to use.   Even if they don't use proxy
  servers, the IP address they give you is only temporary for the
  session, so you'll need to email this IP to whomever wants to reach
  you.  If you get a more expensive ISP service, then you can avoid
  these problems.

  One way to get a GUI interface from the remote PC is to run the GPLed
  program: Virtual Network Computer (VNC) from AT&T.  It has a server
  part which you run on your Linux PC for dial-in and a viewer (client)
  part used for dial-out.  Neither of these actually does any dialing or
  login but assumes that you have a network already set up.  The VNC
  server has an X-server built in and may use Linux's twm window
  manager.  See the article on VNC in Linux Magazine:
  <http://www.linux-mag.com/2000-11/desktop_03.html>.  The AT&T site for
  VNC is:  <http://www.uk.research.att.com/vnc/>.

  With VNC one can also connect to remote Windows PCs, get the Windows
  GUI on a Linux PC, and run Windows programs on the remote Windows PC.
  Of course the Windows PC must be running VNC (as a server).
  Obviously, a GUI connection over a modem will be slower than a text-
  only connection especially if you run KDE or GNOME or want 16-bit
  color.


  12.13.  Interoperability with MS Windows

  Once you have dial-in set up, others may call in to you using minicom
  (or the like) from Unix-like systems.  From MS Windows one may call
  you using "HyperTerminal (or just "Terminal" in Windows 3.1 or DOS).

  If in Windows one wants to use dial-up with a network protocol over
  the phone line it's called "Dial-up Networking".  But it probably will
  not be able to communicate with Linux.  For setting up such dial-in in
  Windows one clicks on "server" while dial-out is the "client.  Such
  dial-in is often called "remote control" meaning that the caller can
  use your PC, run programs on it, and thus control it remotely.

  While it's easy to call in to a text-based Linux system from MS
  Windows, it's not so easy the other way around (partly because Windows
  is not text-based and would need to put the caller into DOS where
  files wouldn't be protected like they are in Linux.

  However Windows "Dial-up Networking" can establish a dial-in provided
  the caller uses certain network protocols over the phone line: MS's or
  Novel's (two protocols not liked by Linux).  So if someone with
  Windows enables their Dial-up networking server in Windows 98, you
  can't just dial in directly to it from Linux.  This type of dial-in
  doesn't permit the caller to run most of the programs on the host like
  Linux does.  It's called "remote access" and one may transfer files,
  use the hosts printer, access databases, etc.  Is there some way to
  interface to Dial-up Networking from Linux??

  It is possible for two people to crudely chat and send files using
  Minicom on the Linux end and HyperTerminal on the Windows end.  It's
  all done manually by two live persons, one on each end of the phone
  connection.  See ``Simple Manual Dial-In''.

  At the opposite extreme, one would like to run a dial-in so that the
  person calling would get a GUI interface.  For that a network protocol
  is normally used.  It's possible using PC Anywhere for Windows or VNC
  for both Linux and Windows.  But PC Anywhere doesn't seem to talk to
  Linux ??  Other Window programs for "remote control" include Laplink,
  Co-Session, and Microcom.  Do any such programs support Linux besides
  VNC ??


  13.  Uugetty for Dial-In (from the old Serial-HOWTO)

  Be aware that you could use mgetty as a (better?) alternative to
  uugetty.  mgetty is newer and more popular than uugetty.  See
  ``Getty'' for a brief comparison of these 2 gettys.


  13.1.  Installing getty_ps

  Since uugetty is part of getty_ps you'll first have to install
  getty_ps.  If you don't have it, get the latest version from
  metalab.unc.edu:/pub/Linux/system/serial.  In particular, if you want
  to use high speeds (57600 and 115200 bps), you must get version 2.0.7j
  or later.  You must also have libc 5.x or greater.


  By default, getty_ps will be configured to be Linux FSSTND (File
  System Standard) compliant, which means that the binaries will be in
  /sbin, and the config files will be named /etc/conf.{uu}getty.ttySN.
  This is not apparent from the documentation!  It will also expect lock
  files to go in /var/lock.  Make sure you have the /var/lock directory.
  If you don't want FSSTND compliance, binaries will go in /etc, config
  files will go in /etc/default/{uu}getty.ttySN, and lock files will go
  in /usr/spool/uucp.  I recommend doing things this way if you are
  using UUCP, because UUCP will have problems if you move the lock files
  to where it isn't looking for them.

  getty_ps can also use syslogd to log messages.  See the man pages for
  syslogd(1) and syslog.conf(5) for setting up syslogd, if you don't
  have it running already.  Messages are logged with priority LOG_AUTH,
  errors use LOG_ERR, and debugging uses LOG_DEBUG.  If you don't want
  to use syslogd you can edit tune.h in the getty_ps source files to use
  a log file for messages instead, namely /var/adm/getty.log by default.

  Decide on if you want FSSTND compliance and syslog capability.  You
  can also choose a combination of the two.  Edit the Makefile, tune.h
  and config.h to reflect your decisions.  Then compile and install
  according to the instructions included with the package.


  13.2.  Setting up uugetty

  With uugetty you may dial out with your modem while uugetty is
  watching the port for logins.  uugetty does important lock file
  checking.  Update /etc/gettydefs to include an entry for your modem.
  For help with the meaning of the entries that you put into
  /etc/gettydefs, see the "serial_suite" collected by Vern Hoxie.  How
  to get it is in section See``About getty_em''.  When you are done
  editing /etc/gettydefs, you can verify that the syntax is correct by
  doing:



       linux# getty -c /etc/gettydefs



  13.2.1.  Modern Modems

  If you have a 9600 bps or faster modem with data compression, you can
  lock your serial port to one speed.  For example:


       # 115200 fixed speed
       F115200# B115200 CS8 # B115200 SANE -ISTRIP HUPCL #@S @L @B login: #F115200



  If you have your modem set up to do RTS/CTS hardware flow control, you
  can add CRTSCTS to the entries:


       # 115200 fixed speed with hardware flow control
       F115200# B115200 CS8 CRTSCTS # B115200 SANE -ISTRIP HUPCL CRTSCTS #@S @L @B login: #F115200



  13.2.2.  Old slow modems

  If you have a slow modem (under 9600 bps) Then, instead of one line
  for a single speed, your need several lines to try a number of speeds.
  Note that these lines are linked to each other by the last "word" in
  the line such as #4800.  Blank lines are needed between each entry.
  Are the higher modem-to-serial_port speeds in this example really
  needed for a slow modem ??  The uugetty documentation shows them so
  I'm not yet deleting them.



       # Modem entries
       115200# B115200 CS8 # B115200 SANE -ISTRIP HUPCL #@S @L @B login: #57600

       57600# B57600 CS8 # B57600 SANE -ISTRIP HUPCL #@S @L @B login: #38400

       38400# B38400 CS8 # B38400 SANE -ISTRIP HUPCL #@S @L @B login: #19200

       19200# B19200 CS8 # B19200 SANE -ISTRIP HUPCL #@S @L @B login: #9600

       9600# B9600 CS8 # B9600 SANE -ISTRIP HUPCL #@S @L @B login: #4800

       4800# B4800 CS8 # B4800 SANE -ISTRIP HUPCL #@S @L @B login: #2400

       2400# B2400 CS8 # B2400 SANE -ISTRIP HUPCL #@S @L @B login: #1200

       1200# B1200 CS8 # B1200 SANE -ISTRIP HUPCL #@S @L @B login: #115200



  13.2.3.  Login Banner

  If you want, you can make uugetty print interesting things in the
  login banner.  In Greg's  examples, he has the system name, the serial
  line, and the current bps rate.  You can add other things:



              @B    The current (evaluated at the time the @B is seen) bps rate.
              @D    The current date, in MM/DD/YY.
              @L    The serial line to which uugetty is attached.
              @S    The system name.
              @T    The current time, in HH:MM:SS (24-hour).
              @U    The number of currently signed-on users.  This is  a
                    count of the number of entries in the /etc/utmp file
                    that have a non-null ut_name field.
              @V    The value of VERSION, as given in the defaults file.
              To display a single '@' character, use either '\@' or '@@'.



  13.3.  Customizing uugetty

  There are lots of parameters you can tweak for each port you have.
  These are implemented in separate config files for each port.  The
  file /etc/conf.uugetty will be used by all instances of uugetty, and
  /etc/conf.uugetty.ttySN will only be used by that one port.  Sample
  default config files can be found with the getty_ps source files,
  which come with most Linux distributions.  Due to space concerns, they
  are not listed here.  Note that if you are using older versions of
  uugetty (older than 2.0.7e), or aren't using FSSTND, then the default
  file will be /etc/default/uugetty.ttySN.  Greg's
  /etc/conf.uugetty.ttyS3 looked like this:



  # sample uugetty configuration file for a Hayes compatible modem to allow
  # incoming modem connections
  #
  # line to initialize
  INITLINE=ttyS3
  # timeout to disconnect if idle...
  TIMEOUT=60
  # modem initialization string...
  # format: <expect> <send> ... (chat sequence)
  INIT="" AT\r OK\r\n
  WAITFOR=RING
  CONNECT="" ATA\r CONNECT\s\A
  # this line sets the time to delay before sending the login banner
  DELAY=1
  #DEBUG=010



  Add the following line to your /etc/inittab, so that uugetty is run on
  your serial port, substituting in the correct information for your
  environment - run-levels (2345 or 345, etc.)  config file location,
  port, speed, and default terminal type:



       S3:2345:respawn:/sbin/uugetty -d /etc/default/uugetty.ttyS3 ttyS3 F115200 vt100



  Restart init:


       linux# init q



  For the speed parameter in your /etc/inittab, you want to use the
  highest bps rate that your modem supports.

  Now Linux will be watching your serial port for connections.  Dial in
  from another machine and login to you Linux system.


  uugetty has a lot more options, see the man page for uugetty) (often
  just called getty) for a full description.  Among other things there
  is a scheduling feature, and a ringback feature.


  14.  What Speed Should I Use with My Modem?

  By "speed" we really mean the "data flow rate" but almost everybody
  incorrectly calls it speed.  For all modern modems you have no choice
  of the speed that the modem uses on the telephone line since it will
  automatically choose the highest possible speed that is feasible under
  the circumstances.  If one modem is slower than the other, then the
  faster modem will operate at the slower modem's speed.  On a noisy
  line, the speed will drop still lower.

  While the above speeds are selected automatically by the modems you do
  have a choice as to what speed will be used between your modem and
  your computer (PC-to-modem speed).  This is sometimes called "DTE
  speed" where "DTE" stands for Data Terminal Equipment (Your computer
  is a DTE.)  You need to set this speed high enough so this part of the
  signal path will not be a bottleneck.  The setting for the DTE speed
  is the maximum speed of this link.  Most of the time it will likely
  actually operate at lower speeds.

  For an external modem, DTE speed is the speed (in bits/sec) of the
  flow over the cable between you modem and PC.  For an internal modem,
  it's the same idea since the modem also emulates a serial port.  It
  may seem ridiculous having a speed limit on communication between a
  computer and a modem card that is directly connected inside the
  computer to a much higher speed bus.  But it's usually that way since
  the modem card probably includes a dedicated serial port which does
  have speed limits (and settable speeds).  However, some software
  modems have no such speed limits.


  14.1.  Speed and Data Compression

  What speed do you choose?  If it were not for "data compression" one
  might try to choose a DTE speed exactly the same as the modem speed.
  Data compression takes the bytes sent to the modem from your computer
  and encodes them into a fewer number of bytes.  For example, if the
  flow (speed) from the PC to the modem was 20,000 bytes/sec (bps) and
  the compression ratio was 2 to 1, then only 10,000 bytes/sec would
  flow over the telephone line.  Thus for a 2:1 compression ratio you
  would need to set the DTE speed to double the maximum modem speed on
  the phone line.  If the compression ratio were 3 to 1 you would need
  to set it 3 times faster, etc.


  14.2.  Where do I Set Speed ?

  This DTE (PC-to-modem) speed is normally set by a menu in your
  communications program or by an option given to the getty command if
  someone is dialing in.  You can't set the DCE modem-to-modem speed
  since this is set automatically by the modem to the highest feasible
  speed after negotiation with the other modem.  Well, actually you can
  set the modem-to-modem speed with the S37 register but you shouldn't
  do it.  If the two modems on a connection were to be set this way to
  different speeds, then they couldn't communicate with each other.


  14.3.  Can't Set a High Enough Speed

  14.3.1.  Speeds over 115.2k

  The top speed of 115.2k has been standard since the mid 1990's.  But
  by the year 2000, most new serial ports supported higher speeds of
  230.4k and 460.8k.  Some also support 921.6k.  Unfortunately Linux
  seldom uses these speeds due to lack of drivers.  Thus such ports
  behave just like 115.2k ports unless the higher speeds are enabled by
  special software.  To get these speeds you need to compile the kernel
  with special patches until support is built into the kernel's serial
  driver.

  Unfortunately serial port manufacturers never got together on a
  standard way to support high speeds, so the serial driver needs to
  support a variety of hardware.  Once high speed is enabled, a standard
  way to choose it is to set baud_base to the highest speed with
  setserial (unless the serial driver does this for you).  The software
  will then use a divisor of 1 to set the highest speed.  All this will
  hopefully be supported by the Linux kernel sometime in 2002.

  A non-standard way that some manufacturers have implemented high speed
  is to use a very large number for the divisor to get the high speed.
  This number isn't really a divisor at all since it doesn't divide
  anything.  It's just serves as a code number to tell the hardware what
  speed to use.  In such cases you need to compile the kernel with
  special patches.

  One patch to support this second type of high-speed hardware is called
  shsmod (Super High Speed Mode).  There are both Windows and Linux
  versions of this patch.  See  <http://www.devdrv.com/shsmod/>.  There
  is also a module for the VIA VT82C686 chip
  <http://www.kati.fi/viahss/>.  Using it may result in buffer overflow.

  For internal modems, only a minority of them advertise that they
  support speeds of over 115.2k for their built-in serial ports.  Does
  shsmod support these ??


  14.3.2.  How speed is set in hardware: the divisor and baud_base

  Speed is set by having the serial port's clock change frequency.  But
  this change happens not by actually changing the frequency of the
  oscillator driving the clock but by "dividing" the clock's frequency.
  For example, to divide by two, just ignore every other clock tick.
  This cuts the speed in half.  Dividing by 3 makes the clock run at 1/3
  frequency, etc.  So to slow the clock down (meaning set speed), we
  just send the clock a divisor.  It's sent by the serial driver to a
  register in the port.  Thus speed is set by a divisor.

  If the clock runs at a top speed of 115,000 bps (common), then here
  are the divisors for various speeds (assuming a maximum speed of
  115,200): 1 (115.2k), 2 (57.6k), 3 (38.4k), 6 (19.2k), 12 (9.6k), 24
  (4.8k), 48 (2.4k), 96 (1.2k), etc.  The serial driver sets the speed
  in the hardware by sending the hardware only a "divisor" (a positive
  integer).  This "divisor" divides the "maximum speed" of the hardware
  resulting in a slower speed (except a divisor of 1 obviously tells the
  hardware to run at maximum speed).

  There are exceptions to the above since for certain serial port
  hardware, speeds above 115.2k are set by using a very high divisor.
  Keep that exception in mind as you read the rest of this section.
  Normally, if you specify a speed of 115.2k (in your communication
  program or by stty) then the serial driver sets the port hardware to
  divisor 1 which sets the highest speed.

  Besides using a very high divisor to set high speed the conventional
  way to do it is as follows: If you happen to have hardware with a
  maximum speed of say 230.4k (and the 230.4k speed has been enabled),
  then specifying 115.2k will result in divisor 1.  For some hardware
  this will actually give you 230.4k.  This is double the speed that you
  set.  In fact, for any speed you set, the actual speed will be double.
  If you had hardware that could run at 460.8k then the actual speed
  would be quadruple what you set.  All the above assumes that you don't
  use "setserial" to modify things.


  14.3.3.  Setting the divisor, speed accounting

  To correct this accounting (but not always fix the problem) you may
  use "setserial" to change the baud_base to the actual maximal speed of
  your port such as 230.4k.  Then if you set the speed (by your
  application or by stty) to 230.4k, a divisor of 1 will be used and
  you'll get the same speed as you set.

  If you have old software which will not permit such a high speed (but
  your hardware has it enabled) then you might want to look into using
  the "spd_cust" parameter for setserial with "divisor 1".  Then when
  you tell the application that the speed it 38,400, it will use divisor
  1 and get the highest speed.

  There are some brands of UARTs that uses a very high divisor to set
  high speeds.  There isn't any satisfactory way to use "setserial" (say
  set "divisor 32770") to get such a speed since then setserial would
  then think that the speed is very low and disable the FIFO in the
  UART.


  14.3.4.  Crystal frequency is higher than baud_base

  Note that the baud_base setting is usually much lower than the
  frequency of the crystal oscillator since the crystal frequency of say
  1.8432 MHz is divided by 16 in the hardware to get the actual top
  speed of 115.2k.  The reason the crystal frequency needs to be higher
  is so that this high crystal speed can generate clock ticks to take a
  number of samples of each  bit to determine if it's a 1 or a 0.

  Actually, the 1.8432 MHz "crystal frequency" is obtained from a 18.432
  MHz crystal oscillator by dividing by 10 before being fed to the UART.
  Other schemes are also possible as long as the UART performs properly.



  14.4.  Speed Table

  It's best to have at least a 16650 UART for a 56k modem but few modems
  or serial ports provide it.  Second best is a 16550 that has been
  tweaked to give 230,400 bps (230.4 kbps).  Most people still use a
  16550 that is only 115.2 kbps but it's claimed to only slow down
  thruput by a few percent (on average).  This is because a typical
  compression ratio is 2 to 1 and for downloading compressed files
  (packages) it's 1 to 1.  There's no degradation for these cases.  Here
  are some suggested speeds to set your serial line if your modem speed
  is:


  ·  56k (V.92): use 115.2 kbps or 230.4 kbps (best)

  ·  56k (V.90): use 115.2 kbps or 230.4 kbps (best)

  ·  33.6k (V.34bis): use 115.2 kbps

  ·  28.8k (V.34): use 115.2 kbps

  ·  14.4k (V.32bis): use 57600 bps

  ·  9.6k (V.32): use 38400 bps

  ·  slower than a 9600 bps (V.32) modem: Set the speed to the same
     speed as the modem (unless you have data compression).

  All the above speeds may use V.42bis data compression and V.42 error
  correction.  If data compression is not used then the speed may be set
  lower so long as it's above the modem speed.


  15.  Communications Programs And Utilities

  While PPP is used for Internet access you also need a dialer program
  (or script) that will dial a phone number and then start PPP once a
  connection is made.  When the other side answers the phone, then three
  things happen: a modem connection is established (CONNECT), PPP is
  started at both ends, and you get logged in automatically.  The exact
  sequence of the last 2 events may vary.  Dialer programs for ppp
  include wvdial, chap scripts, kppp, RP3 (front end to wvdial and
  ifup), gnome-ppp, and "modem lights" (Gnome).  Linuxconf configures
  some dialers.

  There are also older dialer programs which can dial out (via a modem)
  but don't connect to the Internet.  Instead, you get connected to a
  computer somewhere that puts a text image on your screen.  This was
  much used in the past to connect to Bulletin Boards.  See ``PCs and
  BBSs''  Today, it might be used to connect to a remote computer that
  you may login to (including a PC at home).  Programs for this are:
  minicom (the most popular), Seyon (X-Windows only) and Kermit.  Some
  people have likely also used these programs for dialing out with ppp
  for the Internet but it's not what they were originally designed for.


  15.1.  Minicom vs. Kermit

  Minicom is only a communications program while Kermit is both a
  communications program and a file transfer protocol.  But one may use
  the Kermit protocol from within Minicom (provided one has Kermit
  installed on one's PC).  Minicom is menu based while Kermit is command
  line based (interactive at the special Kermit prompt).  While the
  Kermit program is free software, the documentation is not all free.
  There is no detailed manual supplied and it is suggested that you
  purchase a book as the manual.  However Kermit has interactive online
  help which tells all but lacks tutorial explanations for the beginner.
  Commands may be put in a script file so you don't have to type them
  over again each time.  Kermit (as a communications program) is more
  powerful than Minicom.

  Although all Minicom documentation is free, it's not as extensive as
  Kermit's.  Since permission is required to include Kermit in a
  commercial distribution, and since the documentation is not entirely
  free, some distributions don't include Kermit.  In my opinion it's
  easier to set up Minicom, there is less to learn, and you can still
  use kermit from within Minicom.


  15.2.  List of Communication Software

  Here is a list of some communication software you can choose from, If
  they didn't come with your distribution they should be available via
  FTP.  I would like comparative comments on the dialout programs.  Are
  the least popular ones obsolete?


  15.2.1.  Least Popular Dialout



  ·  ecu - a communications program

  ·  pcomm - procomm-like communications program with zmodem

  ·  xc - xcomm communication package


  15.2.2.  Most Popular Dialout



  ·  PPP dialers for getting on the internet: wvdial, eznet, chat, pon
     (uses chat),

  ·  minicom - telix-like communications program.  Can work with
     scripts, zmodem, kermit


  ·  C-Kermit <http://www.columbia.edu/kermit/> - portable, scriptable,
     serial and TCP/IP communications including file transfer,
     character-set translation, and zmodem support

  ·  seyon - X based communication program


  15.2.3.  Fax

  By using a fax program, you may use most modems to send faxes.  In
  this case you dial out directly and not via ppp and an ISP.  You also
  pay any long-distance telephone charges.  email is more efficient.


  ·  efax is a small fax program

  ·  hylafax is a large fax program based on the client-server model.

  ·  mgetty+fax handles fax stuff and login for dial-ins

  ·  A fax protocol tutorial
     <http://www.iec.org/online/tutorials/vfoip/topic08.html>


  15.2.4.  Voicemail Software



  ·  vgetty is an extension to mgetty that handles voicemail for some
     modems.  It should come with recent releases of mgetty.

  ·  VOCP <http://vocp.sourceforge.net/>is a "complete voice messaging"
     system for Linux.


  15.2.5.  Dial-in (uses getty)



  ·  mgetty+fax is for modems and is well documented (except for
     voicemail as of early 1999).  It also handles fax stuff and
     provides an alternative to uugetty.  It's incorporating voicemail
     (using vgetty) features.  See ``About mgetty''

  ·  uugetty is also for modems.  It comes as a part of the ps_getty
     package.  See ``About getty_ps''


  15.2.6.  Other



  ·  callback is where you dial out to a remote modem and then that
     modem hangs up and calls you back (to save on phone bills).

  ·  SLiRP and term provide a PPP-like service that you can run in user
     space on a remote computer with a shell account.  See ``term and
     SLiRP'' for more details

  ·  ZyXEL is a control program for ZyXEL U-1496 modems.  It handles
     dialin, dialout, dial back security, FAXing, and voice mailbox
     functions.

  ·  SLIP and PPP software can be found at
      ftp://metalab.unc.edu/pub/Linux/system/network/serial/.

  ·  Other things can be found on
     ftp://metalab.unc.edu/pub/Linux/system/serial and
     ftp://metalab.unc.edu/pub/Linux/apps/serialcomm or one of the many
     mirrors.  These are the directories where serial programs are kept.


  15.3.  SLiRP and term

  SLiRP and term are programs which are of use if you only have a dial-
  up shell account on a Unix-like machine and want to get the equivalent
  of a PPP account (or the like) without being authorized to have it
  (possibly because you don't want to pay extra for it, etc.).  SLiRP is
  more popular than term which is almost obsolete.

  To use SLiRP you install it in your shell account on the remote
  computer.  Then you dial up the account and run SLiRP on the remote
  and PPP on your local PC.  You now have a PPP connection over which
  you may run a web browser on your local PC such as Netscape, etc.
  There may be some problems as SLiRP is not as good as a real PPP
  account.  Some accounts may provide SLiRP since it saves on IP
  addresses (You have no IP address while using SLiRP).

  term is something like SLiRP only you need to run term on both the
  local and remote computer.  There is no PPP on the phone line since
  term uses its own protocol.  To use term from your PC you need to use
  a term-aware version of ftp to do ftp, etc.  Thus it's easier to use
  SLiRP since the ordinary version of ftp works fine with SLiRP.  There
  is an unmaintained Term HOWTO.


  15.4.  MS Windows

  If you want someone who uses MS Windows to dial in to your Linux PC
  then if they use:

  ·  Windows 3.x: use Terminal

  ·  Windows 95/98/2000: use HyperTerminal

  Third party dial-out programs include HyperTerminal Private Edition.


  16.  Two Modems (Modem Doubling)

  16.1.  Introduction

  By using two modems at the same time, the flow of data can be doubled.
  It takes two modems and two phone lines.  There are two methods of
  doing this.  One is "modem bonding" where software at both ends of the
  modem-to-modem connection enables the paired modems to work like a
  single channel.

  The second method is called "modem teaming.  Only one end of the
  connection uses software to make 2 different connections to the
  internet.  Then when a file is to be downloaded, one modem gets the
  first half of the file.  The second modems simultaneously gets the
  last half of the same file by pretending that it's resuming a download
  that was interrupted in the middle of the file.  Is there any modem
  teaming support in Linux ??


  16.2.  Modem Bonding

  There are two ways to do this in Linux: EQL and multilink.  These are
  provided as part of the Linux kernel (provided they've been selected
  when the kernel was compiled).  For multilink the kernel must be at
  least v.2.4.  Both ends of the connection must run them.  Few (if any)
  ISPs provide EQL but many provide Multilink.

  The way it works is something like multiplexing only it's the other
  way around.  Thus it's called inverse-multiplexing.  For the multilink
  case, suppose you're sending some packets.  The first packet goes out
  on modem1 while the second packet is going out on modem2.  Then the
  third packet follows the first packet on modem1.  The forth packet
  goes on modem2, etc.  To keep each modem busy, it may be necessary to
  send out more packets on one modem than the other.  Since EQL is not
  packet based, it doesn't split up the flow on packet boundaries.


  16.2.1.  EQL

  EQL is "serial line load balancing" which has been available for Linux
  since at least 1995.  An old (1995) howto on it is in the kernel
  documentation (in the networking subdirectory).  Unfortunately, ISPs
  don't seem to provide EQL.


  16.2.2.  Multilink

  Staring with kernel 2.4 in 2000, experimental support is provided for
  multilink.  It must be selected when compiling the kernel and it only
  works with PPP.


  17.  Troubleshooting

  17.1.  My Modem is Physically There but Can't be Found

  An error messages could be something like "No modem detected", "Modem
  not responding".  There could be no error message.  If you have
  installed an internal modem (serial port is builtin) or are using an
  external one and don't know what serial port it's connected to then
  the problem is to find the serial port.  See ``My Serial Port is
  Physically There but Can't be Found''.  This section is about finding
  out which serial port has the modem on it.

  There's a program that looks for modems on commonly used serial ports
  called "wvdialconf".  Just type "wvdialconf <a-new-file-name>".  It
  will create the new file as a configuration file but you don't need
  this file unless you are going to use "wvdial" for dialing.  See
  ``What is wvdialconf ?''  Unfortunately, if your modem is in "online
  data" mode, wvdialconf will report "No modem detected"  See ``No
  response to AT''

  Your problem could be due to a winmodem (or the like) which usually
  can't be used with Linux.  See ``Software-based Modems (winmodems)''.
  The "setserial program may be used to detect serial ports but will not
  detect modems on them.  Thus "wvdialconf" is best to try first.

  Another way try to find out if there's a modem on a port is to start
  "minicom" on the port (after first setting up minicom for the correct
  serial port --you will need to save the setup and then exit minicom
  and start it again).  Then type "AT" and you should see "OK" (or 0 if
  it's set for "digit result codes").  If you don't immediately see "OK"
  then if:

  ·  No response.  See ``No response to AT''

  ·  It takes many seconds to get an expected truncated response
     (including only the cursor moving down one line).  See ``Extremely
     Slow: Text appears on the screen slowly after long delays''

  ·  Some strange characters appear but they are not in response to AT.
     This likely means that your modem is still connected to something
     at the other end of the phone line which is sending some cryptic
     packets or the like.


  17.1.1.  No response to AT

  The modem should send you "OK" in response to your "AT" which you type
  to the modem (using minicom or the like).  If you don't see "OK" (and
  in most cases don't even see the "AT" you typed either) then the modem
  is not responding (often because what you type doesn't even get to the
  modem).

  A common cause is that there is no modem on the serial port you are
  typing to.  For the case of an internal modem, that serial port likely
  doesn't exist either.  That's because the PnP modem card (which has a
  built-in serial port) has either not been configured (by isapnp or the
  like) or has been configured incorrectly.  See ``My Serial Port is
  Physically There but Can't be Found''.

  If what you type is really getting thru to a modem, then the lack of
  response could be due to the modem being in "online data" mode where
  it can't accept any AT commands.  You may have been using the modem
  and then abruptly disconnected (such as killing the process with
  signal 9).  In that case your modem did not get reset to "command
  mode" where it can interact to AT commands.  "Minicom" may display
  "You are already online.  Hangup first."  (For another meaning of this
  minicom message see ``You are already online!  Hang up first.'')
  Well, you are sort of online but you are may not be connected to
  anything over the phone line.  Wvdial will report "modem not
  responding" for the same situation.

  To fix this as a last resort you could reboot the computer.  Another
  way to try to fix this is to send +++ to the modem to tell it to
  escape back to "command mode" from "online data mode".  On both sides
  of the +++ sequence there must be about 1 second of delay (nothing
  sent during "guard time").  This may not work if another process is
  using the modem since the +++ sequence could wind up with other
  characters inserted in between them or after the +++ (during the guard
  time).  Ironically, even if the modem line is idle, typing an
  unexpected +++ is likely to set off an exchange of control packets
  (that you never see) that will violate the required guard time so that
  the +++ doesn't do what you wanted.  +++ is usually in the string that
  is named "hangup string" so if you command minicom (or the like) to
  hangup it might work.  Another way to do this is to just exit minicom
  and then run minicom again.


  17.2.  "Modem is busy"

  What this means depends on what program sent it.  The modem could
  actually be in use (busy).  Another cause reported for the SuSE
  distribution is that there may be two serial drivers present instead
  of one.  One driver was built into the kernel and the second was a
  module.

  In kppp, this message is sent when an attempt to get/set the serial
  port "stty" parameters fails.  (It's similar to the "Input/output
  error" one may get when trying to use "stty -F /dev/ttySx").  To get a
  few of these stty parameters, the true address of the port must be
  known to the driver.  So the driver may have the wrong address.  The
  setserial" command will display what the driver thinks but it's likely
  wrong in this case.  So what the "modem busy" often means is that the
  serial port (and thus the modem) can't be found.

  If you have a pci modem, then use one of these commands: lspci, or cat
  /proc/pci, or dmesg to find the correct address and irq of the modem's
  serial port.  Then check to see if "setserial" shows the same thing.
  If not, you need to run a script at boot-time which contains a
  setserial command that will tell the driver the correct address and
  irq.  The reason that the driver has it wrong may be due to failure of
  the kernel to understand the lspci data correctly.  You might notice
  this in a boot-time message.


  17.3.  "You are already online!  Hang up first." (from minicom)

  The modem has its CD signal on.  Either you are actually online (a
  remote modem is sending you a carrier) or your modem has been setup to
  always send the CD signal.  In minicom, type at&v to see if &C or &C0
  is set.  If so, CD will be on even if you are offline and you'll
  erroneously get this error message.  The fix is to set &C1 in the init
  string or save it in the modem.


  17.4.  I can't get near 56k on my 56k modem

  There must be very low noise on the line for it to work at even close
  to 56k.  Some phone lines are so bad that the speeds obtainable are
  much slower than 56k (like 28.8k or even slower).  Sometimes extension
  phones connected to the same line can cause problems.  To test this
  you might connect your modem directly at the point where the telephone
  line enters the building with the feeds for everything else on that
  line disconnected (if others can tolerate such a test).


  17.5.  Uploading (downloading) files is broken/slow

  Flow control (both at your PC and/or modem-to-modem) may not be
  enabled.  For the uploading case: If you have set a high DTE speed
  (like 115.2k) then flow from your modem to your PC may work OK but
  uploading flow in the other direction will not all get thru due to the
  telephone line bottleneck.  This will result in many errors and the
  resending of packets.  It may thus take far too long to send a file.
  In some cases, files don't make it thru at all.

  For the downloading case: If you're downloading long uncompressed
  files or web pages (and your modem uses data compression) or if you've
  set a low DTE speed, then downloading may also be broken due to no
  flow control.


  17.6.  For Dial-in I Keep Getting "line NNN of inittab invalid"

  Make sure you are using the correct syntax for your version of init.
  The different init's that are out there use different syntax in the
  /etc/inittab file.  Make sure you are using the correct syntax for
  your version of getty.


  17.7.  I Keep Getting: ``Id "S3" respawning too fast: disabled for 5
  minutes''

  Id "S3" is just an example.  In this case look on the line which
  starts with "S3" in /etc/inittab and calls getty.  This line causes
  the problem.  Make sure the syntax for this line is correct and that
  the device (ttyS3) exists and can be found.  If the modem has negated
  CD and getty opens the port, you'll get this error message since
  negated CD will kill getty.  Then getty will respawn only to be killed
  again, etc.  Thus it respawns over and over (too fast).  It seems that
  if the cable to the modem is disconnected or you have the wrong serial
  port, it's just like CD is negated.  All this can occur when your
  modem is chatting with getty.  Make sure your modem is configured
  correctly.  Look at AT commands E and Q.


  If you use uugetty, verify that your /etc/gettydefs syntax is correct
  by doing the following:


       linux# getty -c /etc/gettydefs



  This can also happen when the uugetty initialization is failing.  See
  section ``uugetty Still Doesn't Work''.


  17.8.  My Modem is Hosed after Someone Hangs Up, or uugetty doesn't
  respawn

  This can happen when your modem doesn't reset when DTR is dropped.
  Greg Hankins saw his RD and SD LEDs go crazy when this happened.  You
  need to have your modem reset.  Most Hayes compatible modems do this
  with &D3, but for USR Courier, SupraFax, and other modems, you must
  set &D2 (and S13=1 for USR Courier).  Check your modem manual (if you
  have one).


  17.9.  NO DIALTONE

  It means exactly what it says.  Someone else may be using another
  telephone on the same line.  You also get this error if there is no
  phone line plugged into the modem, or if the phone line is somehow
  broken.  Try plugging a real telephone into the phone cord used by the
  modem.  Check it for a dialtone.

  If for some reason your modem doesn't detect a dialtone, then you can
  force it to dial anyway by putting X3 in the init string.


  17.10.  NO CARRIER

  This means that the analog sine wave (the carrier) from the other
  modem isn't present like it should be.  If you were already connected,
  this means that the connection has been lost.  There may have been
  noise on the line or a bad connection.  The other modem may have hung
  up on you for some reason:   Perhaps the automatic login process
  didn't work out OK.  Perhaps PPP didn't get started OK.  Perhaps a
  time limit was exceeded.

  If you get this error before you get connected, it means that the
  carrier of the other modem wasn't detected by your modem.  This may
  happen if there is there is no properly working modem on the other
  end.  For example, an answering machine could have picked up your call
  instead of a modem.  NO CARRIER will also happen if the modems fail to
  negotiate a protocol to use.  This can happen if you have an early
  V.90 modem that first tries to negotiate a high speed X2 or K56flex
  protocol.  These two protocols are obsolete and some ISP servers will
  drop the connection (hang up) when this happens since they have no
  understanding of such protocols and don't wait around long enough for
  the calling modem to fallback to V.90.

  If you force your modem to drop the connection by dropping the DTR
  signal or sending your modem the hangup signal (ATH) you may get this
  error message.  But in this case you (or your software) wanted to drop
  the connection so there should be no problem.  In this case you are
  only supposed to get NO CARRIER if data was lost.   So for most cases
  of dropping a connection by hangup (or by dropping DTR) you only get
  an OK message.  Your modem dialer program may not even display that to
  you.


  17.11.  uugetty Still Doesn't Work

  There is a DEBUG option that comes with getty_ps.  Edit your config
  file /etc/conf.{uu}getty.ttySN and add DEBUG=NNN.  Where NNN is one of
  the following combination of numbers according to what you are trying
  to debug:


       D_OPT   001            option settings
       D_DEF   002            defaults file processing
       D_UTMP  004            utmp/wtmp processing
       D_INIT  010            line initialization (INIT)
       D_GTAB  020            gettytab file processing
       D_RUN   040            other runtime diagnostics
       D_RB    100            ringback debugging
       D_LOCK  200            uugetty lockfile processing
       D_SCH   400            schedule processing
       D_ALL   777            everything



  Setting DEBUG=010 is a good place to start.

  If you are running syslogd, debugging info will appear in your log
  files.  If you aren't running syslogd info will appear in
  /tmp/getty:ttySN for debugging getty and /tmp/uugetty:ttySN for
  uugetty, and in /var/adm/getty.log.  Look at the debugging info and
  see what is going on.  Most likely, you will need to tune some of the
  parameters in your config file, and reconfigure your modem.

  You could also try mgetty.  Some people have better luck with it.


  17.12.  (The following subsections are in both the Serial and Modem
  HOWTOs)

  17.13.  My Serial Port is Physically There but Can't be Found

  If a physical device (such as a modem) doesn't work at all it's often
  because it's disabled and has no address (PnP hasn't enabled it) or
  that it is enabled but is not at the I/O address that setserial thinks
  it's at.  Thus it can't be found.

  First check BIOS messages at boot-time (and possible the BIOS menu for
  the serial port).  For the PCI bus use lspci or scanpci.  If it's an
  ISA bus PnP serial port, try "pnpdump --dumpregs" and/or see Plug-and-
  Play-HOWTO.  If the port happens to be enabled then the following two
  paragraphs may help find it:

  For a non-PnP ISA legacy port, using "scanport" (Debian only ??) will
  scan all bus ports and may discover an unknown port that could be a
  serial port (but it doesn't probe the port).  It could hang your PC.
  You may try probing with setserial.  See ``Probing''.

  If nothing seems to get thru the port it may be accessible but have a
  bad interrupt.  See ``Extremely Slow: Text appears on the screen
  slowly after long delays''.  Use setserial -g to see what the serial
  driver thinks and check for IRQ and I0 address conflicts.  Even if you
  see no conflicts the driver may have incorrect information (view it by
  "setserial") and conflicts may still exist.

  If two ports have the same IO address then probing it will erroneously
  indicate only one port.  Plug-and-play detection will find both ports
  so this should only be a problem if at least one port is not plug-and-
  play.  All sorts of errors may be reported/observed for devices
  illegally "sharing" a port but the fact that there are two devices on
  the same a port doesn't seem to get detected (except hopefully by
  you).  In the above case, if the IRQs are different then probing for
  IRQs with setserial might "detect" this situation by failing to detect
  any IRQ.  See ``Probing''.


  17.14.  Extremely Slow: Text appears on the screen slowly after long
  delays

  It's likely mis-set/conflicting interrupts.  Here are some of the
  symptoms which will happen the first time you try to use a modem,
  terminal, or serial printer.  In some cases you type something but
  nothing appears on the screen until many seconds later.  Only the last
  character typed may show up.  It may be just an invisible <return>
  character so all you notice is that the cursor jumps down one line.
  In other cases where a lot of data should appear on the screen, only a
  batch of about 16 characters appear.  Then there is a long wait of
  many seconds for the next batch of characters.  You might also get
  "input overrun" error messages (or find them in logs).

  For more details on the symptoms and why this happens see the Serial-
  HOWTO section: "Interrupt Problem Details".

  If it involves Plug-and-Play devices, see also Plug-and-Play-HOWTO.

  As a quick check to see if it really is an interrupt problem, set the
  IRQ to 0 with "setserial".  This will tell the driver to use polling
  instead of interrupts.  If this seems to fix the "slow" problem then
  you had an interrupt problem.  You should still try to solve the
  problem since polling uses excessive computer resources.

  Checking to find the interrupt conflict may not be easy since Linux
  supposedly doesn't permit any interrupt conflicts and will send you a
  ``/dev/ttyS?: Device or resource busy'' error message if it thinks you
  are attempting to create a conflict.  But a real conflict can be
  created if "setserial" has told the kernel incorrect info.  The kernel
  has been lied to and thus doesn't think there is any conflict.  Thus
  using "setserial" will not reveal the conflict (nor will looking at
  /proc/interrupts which bases its info on "setserial").  You still need
  to know what "setserial" thinks so that you can pinpoint where it's
  wrong and change it when you determine what's really set in the
  hardware.

  What you need to do is to check how the hardware is set by checking
  jumpers or using PnP software to check how the hardware is actually
  set.  For PnP run either "pnpdump --dumpregs" (if ISA bus) or run
  "lspci" (if PCI bus).  Compare this to how Linux (e.g. "setserial")
  thinks the hardware is set.


  17.15.  Somewhat Slow: I expected it to be a few times faster

  An obvious reason is that the baud rate is actually set too slow.
  It's claimed that this happened by trying to set the baud rate to a
  speed higher than the hardware can support (such as 230400).

  Another reason may be that whatever is on the serial port (such as a
  modem, terminal, printer) doesn't work as fast as you thought it did.
  A 56k Modem seldom works at 56k and the Internet often has congestion
  and bottlenecks that slow things down.  If the modem on the other end
  does not have a digital connection to the phone line (and uses a
  special "digital modem" not sold in most computer stores), then speeds
  above 33.6k are not possible.

  Another possible reason is that you have an obsolete serial port: UART
  8250, 16450 or early 16550 (or the serial driver thinks you do).  See
  "What are UARTS" in the Serial-HOWTO.

  Use "setserial -g /dev/ttyS*".  If it shows anything less than a
  16550A, this may be your problem.  If you think that "setserial" has
  it wrong check it out.  See ``What is Setserial'' for more info.  If
  you really do have an obsolete serial port, lying about it to
  setserial will only make things worse.


  17.16.  The Startup Screen Show Wrong IRQs for the Serial Ports.

  For non-PnP ports, Linux does not do any IRQ detection on startup.
  When the serial module loads it only does serial device detection.
  Thus, disregard what it says about the IRQ, because it's just assuming
  the standard IRQs.  This is done, because IRQ detection is unreliable,
  and can be fooled.  But if and when setserial runs from a start-up
  script, it changes the IRQ's and displays the new (and hopefully
  correct) state on on the startup screen.  If the wrong IRQ is not
  corrected by a later display on the screen, then you've got a problem.

  So, even though I have my ttyS2 set at IRQ 5, I still see


       ttyS02 at 0x03e8 (irq = 4) is a 16550A



  at first when Linux boots.  (Older kernels may show "ttyS02" as
  "tty02" which is the same as ttyS2).  You may need to use setserial to
  tell Linux the IRQ you are using.


  17.17.  "Cannot open /dev/ttyS?: Permission denied"

  Check the file permissions on this port with "ls -l /dev/ttyS?"_ If
  you own the ttyS? then you need read and write permissions: crw with
  the c (Character device) in col. 1.  It you don't own it then it
  should show rw- in cols. 8 & 9 which means that everyone has read and
  write permission on it.  Use "chmod" to change permissions.  There are
  more complicated ways to get access like belonging to a "group" that
  has group permission.


  17.18.  "Operation not supported by device" for ttyS?

  This means that an operation requested by setserial, stty, etc.
  couldn't be done because the kernel doesn't support doing it.
  Formerly this was often due to the "serial" module not being loaded.
  But with the advent of PnP, it may likely mean that there is no modem
  (or other serial device) at the address where the driver (and
  setserial) thinks it is.  If there is no modem there, commands (for
  operations) sent to that address obviously don't get done.  See ``What
  is set in my serial port hardware?''

  If the "serial" module wasn't loaded but "lsmod" shows you it's now
  loaded it might be the case that it's loaded now but wasn't loaded
  when you got the error message.  In many cases the module will
  automatically loaded when needed (if it can be found).  To force
  loading of the "serial" module it may be listed in the file:
  /etc/modules.conf or /etc/modules.  The actual module should reside
  in: /lib/modules/.../misc/serial.o.


  17.19.  "Cannot create lockfile. Sorry"

  When a port is "opened" by a program a lockfile is created in
  /var/lock/.  Wrong permissions for the lock directory will not allow a
  lockfile to be created there.  Use "ls -ld /var/lock" to see if the
  permissions are OK.  Giving rwx permissions for the root owner and the
  group should work, provided that the users that need to dialout belong
  to that group.  Others should have r-x permission.  Even with this
  scheme, there may be a security risk.  Use "chmod" to change
  permissions and "chgrp" to change groups.  Of course, if there is no
  "lock" directory no lockfile can be created there.  For more info on
  lockfiles see the Serial-HOWTO subsection: "What Are Lock Files".


  17.20.  "Device /dev/ttyS? is locked."

  This means that someone else (or some other process) is supposedly
  using the serial port.  There are various ways to try to find out what
  process is "using" it.  One way is to look at the contents of the
  lockfile (/var/lock/LCK...).  It should be the process id.  If the
  process id is say 100 type "ps 100" to find out what it is.  Then if
  the process is no longer needed, it may be gracefully killed by "kill
  100".  If it refuses to be killed use "kill -9 100" to force it to be
  killed, but then the lockfile will not be removed and you'll need to
  delete it manually.  Of course if there is no such process as 100 then
  you may just remove the lockfile but in most cases the lockfile should
  have been automatically removed if it contained a stale process id
  (such as 100).


  17.21.  "/dev/tty? Device or resource busy"

  This means that the device you are trying to access (or use) is
  supposedly busy (in use) or that a resource it needs (such as an IRQ)
  is supposedly being used by another device and can't be shared.  This
  message is easy to understand if it only means that the device is busy
  (in use).  But it sometimes means that a needed resource is already in
  use (busy).  What makes it even more confusing is that in some cases
  neither the device nor the resources that it needs are actually
  "busy".

  In olden days, if a PC was shutdown by just turning off the power, a
  bogus lockfile might remain and then later on one would get this bogus
  message and not be able to use the serial port.  Software today is
  supposed to automatically remove such bogus lockfiles, but as of 2003
  there is still a problem with the "wvdial" dialer program related to
  lockfiles.  If wvdial can't create a lockfile because it doesn't have
  write permission in the /var/lock/ directory, you will see this
  erroneous message.

  The following example is where interrupts can't be shared (at least
  one of the interrupts is on the ISA bus).  The ``resource busy'' part
  often means (example for ttyS2) ``You can't use ttyS2 since another
  device is using ttyS2's interrupt.'' The potential interrupt conflict
  is inferred from what "setserial" thinks.  A more accurate error
  message would be ``Can't use ttyS2 since the setserial data (and
  kernel data) indicates that another device is using ttyS2's
  interrupt''.  If two devices use the same IRQ and you start up only
  one of the devices, everything is OK because there is no conflict yet.
  But when you next try to start the second device (without quitting the
  first device) you get a "... busy" error message.  This is because the
  kernel only keeps track of what IRQs are actually in use and actual
  conflicts don't happen unless the devices are in use (open).   The
  situation for I/O address (such as 0x3f8) conflict is similar.

  This error is sometimes due to having two serial drivers: one a module
  and the other compiled into the kernel.  Both drivers try to grab the
  same resources and one driver finds them "busy".

  There are two possible cases when you see this message:

  1. There may be a real resource conflict that is being avoided.

  2. Setserial has it wrong and the only reason ttyS2 can't be used is
     that setserial erroneously predicts a conflict.

  What you need to do is to find the interrupt setserial thinks ttyS2 is
  using.  Look at /proc/tty/driver/serial.  You should also be able to
  find it with the "setserial" command for ttyS2.

  Bug in old versions:  Prior to 2001 there was a bug which wouldn't let
  you see it with "setserial".  Trying to see it would give the same
  "... busy" error message.

  To try to resolve this problem reboot or: exit or gracefully kill all
  likely conflicting processes.   If you reboot: 1. Watch the boot-time
  messages for the serial ports.  2. Hope that the file that runs
  "setserial" at boot-time doesn't (by itself) create the same conflict
  again.

  If you think you know what IRQ say ttyS2 is using then you may look at
  /proc/interrupts to find what else (besides another serial port) is
  currently using this IRQ.  You might also want to double check that
  any suspicious IRQs shown here (and by "setserial") are correct (the
  same as set in the hardware).  A way to test whether or not it's a
  potential interrupt conflict is to set the IRQ to 0 (polling) using
  "setserial".  Then if the busy message goes away, it was likely a
  potential interrupt conflict.  It's not a good idea to leave it
  permanently set at 0 since it will put more load on the CPU.


  17.22.  "Input/output error" from setserial, stty, pppd, etc.

  This means that communication with the serial port isn't working
  right.  It could mean that there isn't any serial port at the IO
  address that setserial thinks your port is at.  It could also be an
  interrupt conflict (or an IO address conflict).  It also may mean that
  the serial port is in use (busy or opened) and thus the attempt to
  get/set parameters by setserial or stty failed.  It will also happen
  if you make a typo in the serial port name such as typing "ttys"
  instead of "ttyS".


  17.23.  "LSR safety check engaged"

  LSR is the name of a hardware register.  It's claimed that this means
  there is no serial port at the specified address.


  17.24.  Overrun errors on serial port

  This is an overrun of the hardware FIFO buffer and you can't increase
  its size.  Bug note (reported in 2002): Due to a bug in some kernel
  2.4 versions, the port number may be missing and you will only see
  "ttyS" (no port number).  But if devfs notation such as "tts/2" is
  being used, there is no bug.  See "Higher Serial Thruput" in the
  Serial-HOWTO.


  17.25.  Modem doesn't pick up incoming calls

  This paragraph is for the case where a modem is used for both dial-in
  and dial-out.  If the modem generates a DCD (=CD) signal, some
  programs (but not mgetty) will think that the modem is busy.  This
  will cause a problem when you are trying to dial out with a modem and
  the modem's DCD or DTR are not implemented correctly.  The modem
  should assert DCD only when there is an actual connection (ie someone
  has dialed in), not when getty is watching the port.  Check to make
  sure that your modem is configured to only assert DCD when there is a
  connection (&C1).  DTR should be on (asserted) by the communications
  program whenever something is using, or watching the line, like getty,
  kermit, or some other comm program.


  17.26.  Port get characters only sporadically

  There could be some other program running on the port.  Use "top"
  (provided you've set it to display the port number) or "ps -alxw".
  Look at the results to see if the port is being used by another
  program.  Be on the lookout for the gpm mouse program which often runs
  on a serial port.


  17.27.  Troubleshooting Tools

  These are some of the programs you might want to use in
  troubleshooting:

  ·  "lsof /dev/ttyS*" will list serial ports which are open.

  ·  "setserial" shows and sets the low-level hardware configuration of
     a port (what the driver thinks it is).  See ``What is Setserial''

  ·  "stty" shows and sets the configuration of a port (except for that
     handled by "setserial").  See the Serial-HOWTO section: "Stty".

  ·  "modemstat" or "statserial" will show the current state of various
     modem signal lines (such as DTR, CTS, etc.)

  ·  "irqtune" will give serial port interrupts higher priority to
     improve performance.

  ·  "hdparm" for hard-disk tuning may help some more.

  ·  "lspci" shows the actual IRQs, etc. of hardware on the PCI bus.

  ·  "pnpdump --dumpregs" shows the actual IRQs, etc. of hardware for
     PnP devices on the ISA bus.

  ·  Some "files" in the /proc tree (such as ioports, interrupts, and
     tty/driver/serial).



  18.  Flash Upgrades

  Many modems can be upgraded by reprogramming their flash memories with
  an upgrade program which you get from the Internet.  By sending this
  "program" from the PC via the serial port to the modem, the modem will
  store this program in its non-volatile memory (it's still there when
  the power is turned off).  The instructions on installing it are
  usually on how to do in under Windows so you'll need to figure out how
  to do the equivalent under Linux (unless you want to install the
  upgrade under Windows).  Sending the program to the modem is often
  called a download.

  If the latest version of this HOWTO still contains this request (see
  ``New Versions of this HOWTO'') please send me your experiences with
  installing such upgrades that will be helpful to others.

  Here's the general idea of doing an upgrade.  First, there may be a
  command that you need to send your modem to tell it that what follows
  is a flash ROM upgrade.  In one case this was AT**  You can do this by
  starting a communications program (such as minicom) and type.  First
  type AT <enter> to see if your modem is there and answers "OK".

  Next, you need to send an file (sometimes two files) directly to the
  modem.  Communication programs (such as minicom) often use zmodem or
  kermit to send files to the modem (and beyond) but these put the file
  into packets which append headers and you want the exact file sent to
  the modem, not a modified one.  But the kermit communications program
  has a "transmit" command that will send the file directly (without
  using the kermit packets) so this is one way to send a file directly.
  Minicom didn't have this feature in 1998.

  Another way to send the file(s) would be to escape from the
  communications program to the shell (in minicom this is ^AJ) and then:
  cat upgrade_file_name > /dev/ttyS2  (if your serial port is ttyS2).
  Then go back to the communication program (type fg at the command line
  prompt in minicom) to see what happened.

  Here's an example session for a certain Rockwell modem (C-a is ^A):

  - Run minicom
  - Type AT** : see "Download initiated ..."
  - C-a J
  - cat FLASH.S37 > /dev/modem
  - fg : see "Download flash code ..."
  - C-a J
  - cat 283P1722.S37 > /dev/modem
  - fg : see "Device successfully programmed"



  19.  Other Sources of Information

  19.1.  Misc


  ·  man pages for: agetty(8), getty(1m), gettydefs(5), init(1),
     isapnp(8), login(1), mgetty(8), setserial(8)

  ·  Your modem manual (if it exists).  Some modems come without
     manuals.

  ·  Serial  Suite <ftp://scicom.alphacdc.com/pub/linux/> by Vern Hoxie
     is a collection of blurbs about the care and feeding of the Linux
     serial port plus some simple programs.

  ·  The Linux serial mailing list.  To subscribe, send email to
     majordomo@vger.rutgers.edu, with ``subscribe linux-serial'' in the
     message body.  If you send ``help'' in the message body, you get a
     help message.  The server also serves many other Linux lists.  Send
     the ``lists'' command for a list of mailing lists.


  19.2.  Books

  I've been unable to find a good up-to-date book on modems.

  ·  The Complete Modem Reference by Gilbert Held, 1997.  Contains too
     much info about obsolete topics.  More up-to-date info may be found
     on the Internet.

  ·  Modems For Dummies by Tina Rathbone, 1996.  (Have never seen it.)

  ·  The Modem Technical Guide by Douglas Anderson, 1996.

  ·  Ultimate Modem Handbook by Cass R. Lewart, 1998.

  ·  Black, Uyless D.: Physical Layer Interfaces & Protocols, IEEE
     Computer Society Press, Los Alamitos, CA, 1996.


  19.3.  HOWTOs



  ·  Cable-Modem mini-howto

  ·  SuSE ISDN Howto (not a LDP Howto)
     <http://sol.parkland.cc.il.us/sdb/en/html/isdn.html> or
     <http://brenner.chemietechnik.uni-
     dortmund.de/doc/sdb/en/html/isdn.html>

  ·  Linux-Modem-Sharing mini-howto.  Computers on a network share a
     single modem for dial-out (like a shared printer).

  ·  Modems-HOWTO: In French (Not used in creating this Modem-HOWTO)

  ·  NET-3-4-HOWTO: all about networking, including SLIP, CSLIP, and PPP

  ·  PPP-HOWTO: help with PPP including modem set-up

  ·  Serial-HOWTO has info on Multiport Serial Cards used for both
     terminals and banks of modems.  Covers the serial port in more
     detail than in the HOWTO.

  ·  Serial-Programming-HOWTO: for some aspects of serial-port
     programming

  ·  Text-Terminal-HOWTO: (including connecting up with modems)

  ·  UUCP-HOWTO: for information on setting up UUCP


  19.4.  Usenet newsgroups



  ·  comp.os.linux.answers; FAQs, How-To's, READMEs, etc. about Linux.

  ·  comp.os.linux.hardware; Hardware compatibility with the Linux
     operating system.

  ·  comp.os.linux.setup; Linux installation and system administration.

  ·  comp.dcom.modems; Modems for all OS's



  19.5.  Web Sites



  ·  Modem List of modems which work/don't_work under Linux
     <http://personal.myvine.com/~gromitkc/winmodem.html>

  ·  Linux Serial  Driver home page <http://serial.sourceforge.net/>
     Includes info about support for PCI modems.

  ·  Hayes AT modem commands Technical Reference for Hayes (tm) Modem
     Users <http://www-dcg.fnal.gov/Net/HYSTRM20.TXT>

  ·  AT Command Set and Register Summary for Analog Modem Modules
     (Cisco)
     <http://www.cisco.com/univercd/cc/td/doc/product/access/acs_mod/cis3600/analogfw/analogat.htm>

  ·  Controlling your Modem with AT Commands
     <http://www.zoltrix.com/support_html/modem/USEMODEM.HTM>

  ·  Modem FAQs:
     Navas 28800-56K Modem FAQ <http://modemfaq.home.att.net/>

  ·  Curt's High Speed  Modem Page <http://www.net-boy.com>

  ·  Much info on 56k modems 56k Modem = v.Unreliable
     <http://modemsite.com/56k/>

  ·  Links to modem manufacturers
     <http://www.56k.com/links/Modem_Manufacturers/>

  ·  More Links to modem  manufacturers <http://modems.rosenet.net/>

  ·  < http://search.fcc.gov/> name="Search for manufacturer by FCC ID">


  20.  Appendix A:  How Analog Modems Work (technical) (unfinished)

  20.1.  Modulation Details

  20.1.1.  Intro to Modulation

  This part describes the modulation methods used for conventional
  modems.  It doesn't cover the high speed PCM methods (modulus
  conversion) sometimes used by ``56k Modems (V.90, V.92)''.  But 56k
  modems also use the modulation methods described here.

  Modulation is the conversion of a digital signal represented by binary
  binary (0 or 1) into an analog signal something like a sine wave.  The
  modulated signal consists pure sine wave "carrier" signal which is
  modified to convey information.  A pure carrier sine wave, unchanging
  in frequency and voltage, provides no flow of information at all
  (except that a carrier is present).  To make it convey information we
  modify (or modulate) this carrier.  There are 3 basic types of
  modulation: frequency, amplitude, and phase.  They will be explained
  next.


  20.1.2.  Frequency Modulation

  The simplest modulation method is frequency modulation.  Frequency is
  measured in cycles per second (of a sine wave).  It's the count of the
  number of times the sine wave shape repeats itself in a second.  This
  is the same as the number of times it reaches it peak value during a
  second.  The word "Hertz" (abbreviated Hz) is used to mean "cycles per
  second".
  A simple example of frequency modulation is where one frequency means
  a binary 0 and another means a 1.  For example, for some obsolete 300
  baud modems 1070 Hz meant a binary 0 while 1270 Hz meant a binary 1.
  This was called "frequency shift keying".  Instead of just two
  possible frequencies, more could be used to allow more information to
  be transmitted.  If we had 4 different frequencies (call them A, B, C,
  and D) then each frequency could stand for a pair of bits.  For
  example, to send 00 one would use frequency A.  To send 01, use
  frequency B; for 10 use C; for 11 use D.  In like manner, by using 8
  different frequencies we could send 3 bits with each shift in
  frequency.  Each time we double the number of possible frequencies we
  increase the number of bits it can represent by 1.


  20.1.3.  Amplitude Modulation

  Once one understands frequency modulation example above including the
  possibilities of representing a few bits by a single shift in
  frequency, it's easier to understand both amplitude modulation and
  phase modulation.  For amplitude modulation, one just changes the
  height (voltage) of the sine wave analogous to changing the frequency
  of the sine wave.  For a simple case there could only be 2 allowed
  amplitude levels, one representing a 0-bit and another representing a
  1-bit.  As explained for the case of frequency modulation, having more
  possible amplitudes will result in more information being transmitted
  per change in amplitude.


  20.1.4.  Phase Modulation

  To change the phase of a sine wave at a certain instant of time, we
  stop sending this old sine wave and immediately begin sending a new
  sine wave of the same frequency and amplitude.  If we started sending
  the new sine wave at the same voltage level (and slope) as existed
  when we stopped sending the old sine wave, there would be no change in
  phase (and no detectable change at all).  But suppose that we started
  up the new sine wave at a different point on the sine wave curve.
  Then there would likely be a sudden voltage jump at the point in time
  where the old sine wave stopped and the new sine wave began.  This is
  a phase shift and it's measured in degrees (deg.)  A 0 deg. (or a 360
  deg.) phase shift means no change at all while a 180 deg. phase shift
  just reverses the voltage (and slope) of the sine wave.  Put another
  way, a 180 deg. phase shift just skips over a half-period (180 deg.)
  at the point of transition.  Of course we could just skip over say 90
  deg. or 135 deg. etc.  As in the example for frequency modulation, the
  more possible phase shifts, the more bits a single shift in phase can
  represent.


  20.1.5.  Combination Modulation

  Instead of just selecting either frequency, amplitude, or phase
  modulation, we may chose to combine modulation methods.  Suppose that
  we have 256 possible frequencies and thus can send a byte (8 bits) for
  each shift in frequency (since 2 to the 8 power is 256).  Suppose also
  that we have another 256 different amplitudes so that each shift in
  amplitude represents a byte.  Also suppose there are 256 possible
  phase shifts.  Then a certain points in time we may make a shift in
  all 3 things: frequency, amplitude and phase.  This would send out 3
  bytes for each such transition.

  No modulation method in use today actually does this.  It's not
  practical due to the relatively long time it would take to detect all
  3 types of changes.  The main problem is that frequent shifts in phase
  can make it appear that a shift in frequency has happened when it
  actually didn't.
  To avoid this difficulty one may simultaneous change only the phase
  and amplitude (with no change in frequency).  This is called phase-
  amplitude modulation.   It is also called quadrature amplitude
  modulation (= QAM) since there were only 4 possible phases
  (quadrature) in early versions of it.  This method is used today for
  the common modem speeds of 14.4k, 28.8k, and 33.6k.  The only
  significant case where this modulation method is not used today is for
  56k modems.  But even 56k modems exclusively use QAM (phase-amplitude
  modulation) in the direction from your PC out the telephone line.
  Sometimes even the other direction will also fall back to QAM when
  line conditions are not good enough.  Thus QAM (phase-amplitude
  modulation) still remains the most widely used method on ordinary
  telephone lines.


  20.2.  56k Modems (V.90, V.92)

  The "modulation" method used for speeds above 33.6k is entirely
  different than the common phase-amplitude modulation used at 33.6k and
  below.  Since ordinary telephone calls are converted to digital
  signals at the local offices of the telephone company, the fastest
  speed that you can send digital data by an ordinary telephone call is
  the same speed that the telephone company uses over its digital
  portion of its network (for a  phone call).  What is this speed?
  Well, it's close to 64kbps.  It would be 64k but sometimes bits are
  "stolen" for signalling purposes.  But if the phone Co. knows that the
  link is not for voice, bits may not get stolen.  The case of 64k will
  be presented and then it will be explained why the actual speed is
  lower (56k or less --often significantly less).

  Thus 64k is the absolute top speed possible (not counting date
  compression) for an ordinary telephone call using the digital portion
  of the circuit that was designed to send digital encodings of the
  human voice.  In order to use 64k, the modems need to either have
  direct access to the digital portion of the circuit or be able to
  determine the exact digital signal that generated a received analog
  signal (and conversely).  This task is far too error prone if both
  sides of a telephone call have only an analog interface to the
  telephone company.  But if one side has a digital interface, then it's
  possible (in one direction for V.90 and in both directions for V.92).
  Thus if your ISP has a digital interface to the phone company, the ISP
  may send out a certain digital signal over the phone lines toward your
  PC.  The digital signal from the ISP gets converted to analog at the
  local telephone office near your PC's location (perhaps near your
  home).  Then it's your modem's task to try to figure out exactly what
  that digital signal was.  If it could do this then transmission at 64k
  (the speed of the telephone company's digital signal) is possible in
  this direction.

  What method does the telephone company use to digitally encode analog
  signals?  It uses a method of sampling the amplitude of the analog
  signal at a rate of 8000 samples per second.  Each sample amplitude is
  encoded as a 8-bit byte.  (Note: 8 x 8000 = 64k)  This is called
  "Pulse Code Modulation" = PCM.  These bytes are then sent digitally on
  the telephone company's digital circuits where many calls share a
  single circuit using a time-sharing scheme known as "time division
  multiplexing".  Then finally at a local telephone office near your
  home, the digital signal is de-multiplexed resulting in the same
  digital signal as was originally created by PCM.  Then this signal is
  converted back to analog and sent to your home.  This analog to
  digital conversion (and conversely) is done by telephone company
  hardware called a "codec" (coder/decoder).   Each PCM 8-bit byte
  creates a certain amplitude of the analog signal.  Your modem's task
  is to determine just what that PCM 8-bit byte was, based on the analog
  amplitude it detects.

  This was originally called is called "modulus conversion".  It's now
  often called "PCM"-something since its just like encoding/decoding PCM
  but with the added problem of sampling at the precise time that the
  codec generated the analog voltage from the digital PCM code.

  In order to determine the digital codes the telephone Co. used to
  create the analog signal, the modem must sample this analog signal
  amplitude at exactly the same points in time the phone Co. did when it
  created the analog signal.  To do this an 8kHz clock timing signal is
  generated with help from a residual 4kHz signal on the analog phone
  line.  The creation of amplitudes to go out to your home/office at 8k
  amplitudes/sec sort of creates a 4kHz signal.  Suppose every other
  amplitude was of opposite polarity.  Then there would be a 4kHz sine-
  like wave created.  Each amplitude is in a sense a 8-bit symbol and
  when to sample amplitudes is known as "symbol timing".  The modem's
  task is to insure that it's 8kHz clock runs at precisely twice the
  speed of the 4kHz signal (which could drift slightly off 4kHz) and
  that the modem's clock is synchronized with that used by the telephone
  company's codec.  The actual electronics may use much higher frequency
  clocks (dividing them down) and take more than a single sample.  If
  you know how this synchronization works, let me know (if this is a
  recent Modem-HOWTO).

  Now the encoding of amplitudes in PCM is not linear.  At low
  amplitudes an increment of 1 in the PCM byte value represents a much
  smaller increment (delta) in analog signal amplitude than would be the
  case if the amplitude being sampled were much higher.  Thus for low
  amplitudes it's difficult to distinguish between adjacent byte values.
  To make it easier to do this (for 56k modems) certain PCM codes
  representing very low amplitudes are not used.  This gives a larger
  delta between possible amplitudes and makes correct detection of them
  by your modem easier.  Thus half of the amplitude levels are not used
  (in the downstream direction) by V.90 or V.92.  This is tantamount to
  each symbol (valid amplitude level) representing 7 bits instead of 8.
  This is where 56k comes from: 7 bits/symbol x 8k symbols/sec = 56k
  bps.  Of course each amplitude symbol is actually generated by 8-bits
  but only 128 bytes of the possible 256 bytes are actually used by the
  ISP sender.  There is a code table mapping these 128 8-bit bytes to
  128 7-bit bytes.  It's not just a simple mapping like ignoring the
  last bit.  Thus to send 7 normal data bytes (8-bits) will take 8 of
  the above mentioned bytes.

  But it's a little more complicated that this.  If the line conditions
  are not nearly perfect or if the direction is upstream (V.92 only),
  then even fewer possible levels (symbols) are used resulting in speeds
  under 56k.  Also due to US government rules prohibiting high power
  levels on phone lines, certain high amplitudes levels can't be used
  resulting in only about 53.3k at best for "56k" modems in the
  downstream direction.

  Note that the digital part of the telephone network is bi-directional.
  Two such circuits are used for a phone call, one in each direction.
  For V.90, the 56k signal is only used in one of these directions: from
  your ISP to your PC (called the "downstream" direction).  For this
  V.90, the other direction (upstream, from your home/office to the ISP)
  uses the conventional phase-amplitude modulation scheme with a maximum
  of 36.6kbps (and not 53.3kbps).  For V.92, this upstream direction
  also uses the PCM method and supports up to 48 kbps.  The analog
  portion of the circuit from your home/office to the nearest telephone
  Co. office was never intended to be bi-directional since it's only a
  single twisted pair.  But due to sophisticated cancellation methods
  it's able to convey data simultaneously in both directions as
  explained in the next subsection.  It's claimed that with V.92, it's
  almost impossible to get maximum thruput in both directions
  simultaneously due to the difficulties of bi-directional flow on a
  single circuit.
  20.3.  Full Duplex on One Circuit

  Modern modems are able to both send and receive signals
  simultaneously.  One could call this "bidirectional" or "full duplex".
  This was once done by using one frequency for sending and another for
  receiving.  Today, the same frequency is used for both sending and
  receiving.  How this works is not easy to comprehend.

  Most of the telephone system "main lines" are digital with two
  channels in use when you make a telephone call.  What you say goes
  over one digital channel and what the other person says goes over the
  other (reverse) digital channel.  Unfortunately, the part of the
  telephone system which goes to homes (and many offices) is not digital
  but only a single analog channel.  If both modems were directly
  connected to the digital part of the phone system then bidirectional
  communication (sending and receiving at the same time) would be no
  problem because two channels would be available.

  But the end portion of the signal path goes over just one circuit.
  How can there be two-way communication on it simultaneously?  It works
  something like this.  Suppose your modem is receiving a signal from
  the other modem and is not transmitting.  Then there's no problem.
  But if your modem were to start transmitting (with the other received
  signal still flowing into your modem) it would drown out the received
  signal.  If the transmitted signal was a "solid" voltage wave applied
  to the end of the line then there is no way any received signal could
  be present at that point.

  But the transmitter has "internal impedance" and the transmitted
  signal applied to the end of the line is not solid (or strong enough)
  to completely eliminate the received signal coming from the other end.
  Thus while the voltage at the end of the line is mostly the stronger
  transmitted signal a small part of it is the desired received signal.
  All that is needed is to filter out this stronger transmitted signal
  and then what remains will be the signal from the other end which we
  want.  To do this, one only needs to get the pure transmitted signal
  directly from the transmitter (before it's applied to the line)
  amplify it a determined amount, and then subtract it from the total
  signal present at the end of the line.  Doing this in the receiver
  circuits leaves a signal which mostly came from the other end of the
  line.


  20.4.  Echo Cancellation

  An analog signal traveling down a line in one direction may encounter
  changes in the line that will cause part of the signal to echo back in
  the opposite direction.  Since the same circuit is used for bi-
  directional flow of data, such echos will result in garbled reception.
  One way to ameliorate this problem is to send training signals once in
  a while to determine the echo characteristic of the line.  This will
  enable one to predict the echos that will be generated by any given
  signal.  Then this prediction method is used to predict what echos the
  transmitted signal will cause.   Then this predicted echo signal is
  subtracted from the received signal.  This cancels out the echoes.


  21.  Appendix B: Analog Voice Infeasible Over Non-Voice Modem

  Sometimes people look for software that will allow them to transmit
  ordinary analog voice over a modem that doesn't support voice.  It
  doesn't exist since it's just not very feasible to do this.  Of
  course, one can use VOIP to send digital voice over a modem.  And when
  one makes a call and the other side picks up the line, one might be
  able to hear voice for a few seconds until negotiations for a
  connection begin.
  But once a modem is connected, sending analog voice over it just isn't
  feasible.  For phase-amplitude modulation, carrier frequencies of
  fixed values are used which doesn't allow the continuously variable
  frequencies required in analog voice.  V.90 and V.92 might be
  feasible, but if line conditions deteriorate, they fall back to phase-
  amplitude modulation which won't work.  Furthermore, V.90 uses phase-
  amplitude in one direction.  Also, both V.90 and V.92 don't permit all
  amplitudes to be used, which limits the waveshapes which can be
  created.


  22.  Appendix C: "baud" vs. "bps"

  22.1.  A simple example

  ``baud'' and ``bps'' are perhaps one of the most misused terms in the
  computing and telecommunications field.  Many people use these terms
  interchangeably, when in fact they are not!  bps is simply the number
  of bits transmitted per second.  The baud rate is a measure of how
  many times per second a signal changes (or could change).  For a
  typical serial port a 1-bit is -12 volts and a 0-bit is +12 v (volts).
  If the bps is 38,400 a sequence of 010101... would also be 38,400 baud
  since the voltage shifts back and forth from positive to negative to
  positive, etc. and there are 38,400 shifts per second.  For another
  sequence say 111000111... there will be fewer shifts of voltage since
  for three 1's in sequence the voltage just stays at -12 volts yet we
  say that its still 38,400 baud since there is a possibility that the
  number of changes per second will be that high.

  Looked at another way, put an imaginary tic mark separating each bit
  (even though the voltage may not change).  38,400 baud then means
  38,400 tic marks per second.  The tic marks at at the instants of
  permitted change and are actually marked by a synchronized clock
  signal generated in the hardware but not sent over the external cable.

  Suppose that a "change" may have more than the two possible outcomes
  of the previous example (of +- 12 v).  Suppose it has 4 possible
  outcomes, each represented by a unique voltage level.  Each level may
  represent a pair of bits (such as 01).  For example, -12v could be 00,
  -6v 01, +6v 10 and +12v 11.  Here the bit rate is double the baud
  rate.  For example, 3000 changes per second will generate 2 bits for
  each change resulting in 6000 bits per second (bps).  In other words
  3000 baud results in 6000 bps.


  22.2.  Real examples

  The above example is overly simple.  Real examples are more
  complicated but based on the same idea.  This explains how a modem
  running at 2400 baud, can send 14400 bps (or higher).  The modem
  achieves a bps rate greater than baud rate by encoding many bits in
  each signal change (or transition).  Thus, when 2 or more bits are
  encoded per baud, the bps rate exceeds the baud rate. If your modem-
  to-modem connection is at 14400 bps, it's going to be sending 6 bits
  per signal transition (or symbol) at 2400 baud.  A speed of 28800 bps
  is obtained by 3200 baud at 9 bits/baud.  When people misuse the word
  baud, they may mean the modem speed (such as 33.6k).

  Common modem bps rates were formerly  50, 75, 110, 300, 1200, 2400,
  9600.  These were also the bps rates over the serial_port-to-modem
  cables.  Today the bps modem-to-modem (maximum) rates are 14.4k,
  28.8k, 33.6k, and 56k, but the common rates over the serialPort-to-
  modem cables are not the same but are: 19.2k, 38.4k, 57.6k, 115.2k,
  230.4k.  The high speed of 230.4k is (as of late 2000) unfortunately
  not provided by most new (and old) hardware.  Using modems with
  V.42bis compression (max 4:1 compression), rates up to 115.2k bps are
  possible for 33.6k modems.  203.2k (4 x 53.3k) is possible for 56k
  modems.

  Except for 56k modems, most modems run at 2400, 3000, or 3200 baud.
  Even the 56k modems use these bauds for transmission and sometimes
  fall back to them for reception.  Because of the bandwidth limitations
  on voice-grade phone lines, baud rates greater than 2400 are harder to
  achieve, and only work under conditions of good phone line quality.

  How did this confusion between bps and baud start?  Well, back when
  antique low speed modems were high speed modems, the bps rate actually
  did equal the baud rate.  One bit would be encoded per phase change.
  People would use bps and baud interchangeably, because they were the
  same number.  For example, a 300 bps modem also had a baud rate of
  300. This all changed when faster modems came around, and the bit rate
  exceeded the baud rate.  ``baud'' is named after Emile Baudot, the
  inventor of the asynchronous telegraph printer.  One way this problem
  gets resolved is to use the term "symbol rate" instead of "baud" and
  thus avoid using the term "baud".  However when talking about the
  "speeds" between the modem and the serial port (DTE speed) baud and
  the symbol rate are the same.  And even "speed" is a misnomer since we
  really mean flow rate.


  23.  Appendix D: Terminal Server Connection

  This section was adapted from Text-Terminal-HOWTO.

  A terminal server is something like an intelligent switch that can
  connect many modems (or terminals) to one or more computers.  It's not
  a mechanical switch so it may change the speeds and protocols of the
  streams of data that go thru it.  A number of companies make terminal
  servers: Xyplex, Cisco, 3Com, Computone, Livingston, etc.  There are
  many different types and capabilities.  Another HOWTO is needed to
  compare and describe them (including the possibility of creating your
  own terminal server with a Linux PC).  Most are used for modem
  connections rather than directly connected terminals.

  One use for them is to connect many modems (or terminals) to a high
  speed network which connects to host computers.  Of course the
  terminal server must have the computing power and software to run
  network protocols so it is in some ways like a computer.  The terminal
  server may interact with the user and ask what computer to connect to,
  etc. or it may connect without asking.  One may sometimes send jobs to
  a printer thru a terminal server.

  A PC today has enough computing power to act like a terminal server
  except that each serial port should have its own hardware interrupt.
  PC's only have a few spare interrupts for this purpose and since they
  are hard-wired you can't create more by software.  A solution is to
  use an advanced multiport serial card which has its own system of
  interrupts (or on lower cost models, shares one of the PC's interrupts
  between a number of ports).  See Serial-HOWTO for more info.  If such
  a PC runs Linux with getty running on many serial ports it might be
  thought of as a terminal server.  It is in effect a terminal server if
  it's linked to other PC's over a network and if its job is mainly to
  pass thru data and handle the serial port interrupts every 14 (or so)
  bytes.  Software called "radius" is sometimes used.

  Today real terminal servers serve more than just terminals.  They also
  serve PC's which emulate terminals, and are sometimes connected to a
  bank of modems connected to phone lines.  Some even include built-in
  modems.  If a terminal (or PC emulating one) is connected directly to
  a modem, the modem at the other end of the line could be connected to
  a terminal server.  In some cases the terminal server by default
  expects the callers to use PPP packets, something that real text
  terminals don't generate.


  24.  Appendix E: Digital Modems: ISDN, DSL, RAS

  24.1.  Introduction

  This HOWTO only deals with the common type of analog modem used to
  connect PC's to ordinary analog telephone lines.  The standard
  definition of a modem is sometimes broadened to include digital
  "modems".  Today direct digital service is now being provided to many
  homes and offices so that a computer there sends out digital signals
  directly (well almost) into the telephone lines.  But a device is
  still needed to convert the computer digital signal into the type of
  digital signal used telephone circuits.  This device is sometimes
  called a modem.  While this HOWTO doesn't cover such modems, some
  links to documents that do may be found at the start of this HOWTO.
  The next 3 sub-sections: ISDN, DSL and 56k, briefly mention such
  "modems".


  24.2.  ISDN "Modems"

  Such a "modem" is  really a Terminal Adapter (TA).  Support for some
  of them can be built into the kernel 2.4 or added as a module.  The
  kernel documentation has an isdn subdirectory.  Configuration might
  use "isdn-config" GUI.  A Debian package "isdnutils" is available.
  There is SuSE ISDN Howto (not a LDP Howto) which is translated from
  German  <http://sol.parkland.cc.il.us/sdb/en/html/isdn.html> There is
  an isdn4linux package and a newsgroup: de.alt.comm.isdn4linux.  Many
  of the postings are in German.  You might try using a search engine to
  look for "isdn4linux".


  24.3.  Digital Subscriber Line (DSL)

  DSL uses the existing twisted pair line from your home (etc.) to the
  local telephone office.  This can be used if your telephone line can
  accept significantly higher speeds than an ordinary modem would use.
  It replaces the analog-to-digital converter at the local telephone
  office with a converter which can accept a much faster flow of data
  (in a different format of course).  The device which converts the
  digital signals from your computer to the signal used to represent
  digital data on the local telephone line is also called a modem.


  24.4.  56k Digital-Modems

  These are not the 56k modems that people use in their PCs.  They are
  the "modems" at an ISP that these 56k modems connect to.  The ISP must
  be connected directly to the digital system of the telephone company,
  otherwise the customers modem can't be used as a 56k modem.  The ISP
  likely has banks of many modems multiplexed onto a high capacity
  telephone cable that transports a large number of phone calls
  simultaneously (such as a T1, E1, ISDN PRI, etc.).  This requires a
  concentrator or "remote access server" (RAS).  This has previously
  been done by stand-alone units (like PC's but they cost much more and
  have proprietary OSs).  Now there are some cards one may insert into a
  PC's PCI bus to do this.  See ``Digital Modems'')


  25.  Appendix F: Leased Line Modems

  See the mini-howto: Leased-Line which covers leased lines where there
  is no dialtone.  Leased line modems are analog and not digital.  These
  special modems are used on lines leased from the telephone company or
  sometimes on just a long direct wire hookup.  They often will also
  work as ordinary modems but go into leased-line mode when the AT
  command &L1 is given.

  Ordinary modems for a telephone line will not normally work on such a
  leased line.  An ordinary telephone line has about 40-50 volts (known
  as the "battery) on it when not in use and the conventional modem uses
  this voltage for transmission.  Furthermore, the telephone company has
  special signals indicating a ring, line busy, etc.  Conventional
  modems expect and respond to these signals.  Connecting two such
  modems by a long cable will not provide the telephone signals on the
  cable and thus the modems will not work.

  Leased-line modems often use a "dumb mode" where they ignore AT
  commands, disable result reporting, etc.  One type of leased line used
  two pairs of wires (one for each direction) using V.29 modulation at
  9600 baud.  Some brands of leased line modems are incompatible with
  other brands.


  26.  Appendix G: Fax pixels (dots)

  Here's some info on the bloated bandwidth required for standard fax
  including the dot density.  You can of course send a fax via your
  modem if you dial the real telephone number of the recipient.



       A4 paper:    216mm (horizontal) * 297mm (vertical)
       normal mode       8dots/mm      * 3.85dots/mm
       fine   mode                     * 7.7dots/mm
       extra fine mode                 *15.4dots/mm



  Each dot is either white or black and thus 1 bit.  One sheet of A4
  paper using fine mode is (216*8) * (297*7.7) = about 4 million dots.
  With a compression ratio of 8:1 it takes about 50 seconds at 9600bps
  for transmission.


  27.  Appendix H: Antique Modems

  27.1.  Introduction

  By "antique" I mean modems with speeds of 14.4 kbps or less.  Many of
  them were made in the 1980s.  Faster modems are also included if they
  use a proprietary protocol.  This appendix compares the antique modems
  with the modern ones.  You should read it if you are interested in
  modem history or are intending to actually use an antique modem.
  Also, many current modems and software still support the old protocols
  and you might find that these have been configured by mistake.


  27.2.  Old CCITT (ITU) and Bell Protocols


  ·  Bell 103   300 bps; frequency shift keying = FSK (1962)

  ·  V.21       300 bps; frequency shift keying (used a different
     frequency than Bell 103) (1964)

  ·  V.23      1200/75 bps and 600/75 bps asymmetric; 75 bps is the
     reverse channel; frequency shift keying = FSK (1964)

  ·  Bell 212A 1200 bps; quadrature differential phase shift keying =
     QDPSK = DPSK

  ·  V.22      1200 bps; fallback to 600 bps ; QDPSK = DPSK (1980)

  ·  V.22bis   2400 bps; QAM (1984)

  ·  V.32      9600 bps; QAM (1984 but not widely used until years
     later)

  ·  V.32bis  14400 bps; QAM (1991)

     QAM= Quadrature Amplitude Modulation.  The word "Quadrature" is
     short for "quadrature differential phase shift keying" =QDPSK


  27.3.  Historical Overview

  27.3.1.  Teletypes and dumb terminals

  Prior to 1960, 110 bps ( 0.11) modems were used for teletype machines
  (like an electric typewriter only much more noisy).  Then in 1960
  AT$amp;T came out with a 300 bps modem (for use on it's phone system).
  Such slow (and expensive modems) were later mainly used for
  transmitting data between mainframe computers or for connecting a dumb
  terminal to a mainframe computer over phone lines.  Many dumb
  terminals didn't even have a screen display, but printed on paper what
  you typed at the keyboard along with responses from the computer.


  27.3.2.  PCs and BBSs

  With the advent of the personal computer (PCs) in the early 1980s, the
  PC was used like a dumb terminal for connecting to mainframes.  But
  now files could be transferred and one PC could connect to another.

  The 1980s saw the rise of the Bulletin Board System (BBS).  This was
  just a computer with a modem listening for incoming calls.  The public
  could dial up a BBS with a modem and then download free software,
  participate in discussions on various topics, play on-line games, etc.
  Dialing in to a BBS was something like going to an Internet site.
  Except that to go to another BBS site, you would need to dial another
  number (and possible pay long distance telephone charges).  Many BBSs
  would have a monthly charge but some were run by volunteers and were
  free.  Many companies established BBSs for customers that contained
  support information, catalogs, etc.  In the early 1990s, BBSs were
  booming.  By the mid 1990s some even offered Internet connections.
  For some history of BBSs see Sysops' Corner: History of BBSing
  <http://sysopscorner.thebbs.org/bbshist.html>


  27.3.3.  The Internet

  Then came the advance of Internet in the mid 1990s which resulted in
  the demise of the BBSs near the end of the 1990s.  Some BBSs became
  websites, but when BBSs were dying in droves, websites were quite
  expensive so most BBSs just disappeared.  Also, with the Internet
  free, the public was unwilling to pay for using websites like they
  previously paid for the use of BBSs.  There were such a huge number of
  free websites to visit for free, that subscription BBSs were no longer
  competitive.

  Modems permitted the public to connect to the Internet.  In the 1990s
  Modems became fast, cheap and widely used.  Then in the late 1990s,
  faster non-analog "modems" appeared: ISDN, DSL, and cable.  The
  history of these isn't in this HOWTO.
  27.3.4.  Speeds

  Before V.32 (9600 bps),  modems typically had speeds of 300 to 2400
  bps.  Some super fast ones had much higher speeds (such as 19.2k bps)
  and used non-standard protocols.  To utilize these "fast" ones, both
  modems for a connection needed to support the same proprietary
  protocol which often meant that they must be the same brand.

  Prior to the V.42 standard for error correction and the V.42bis (1990)
  standard for data compression, the MNP standards were usually used for
  both error correction and data compression.  An X.PC error correction
  standard was used on some commercial data networks.  Compression and
  error correction were available on some 2400 bps modems.

  >From 1960 to 1980 most modems only had a speed of 300 bps (which was
  also 300 baud).  This is only 0.3kbps.  Modern modems are over 100
  times faster.  Some old-slow modems are still in use so they are not
  really "antique" quite yet.


  27.4.  Proprietary protocols, etc.

  These were used in order to obtain higher speeds before more
  standardized higher speeds became established.  The modem at the other
  end needs to support the same protocol for this to work.  The dates
  shown below may be only approximate.


  ·  PEP (Packetized Ensemble Protocol 1985): 18k (at best).

  ·  Turbo PEP: 23k 1994?

  ·  Hayes Express 96: 9.6k (Hayes 1987)

  ·  HST: 9.6k  (US Robotics 1986)

  ·  HST: 14.4k (US Robotics 1989)

  ·  HST: 16.8k (US Robotics 1992)

  ·  V.32 terbo: 19.2k  (AT$amp;T 1993)

  ·  V.FastClass: 28.8k (Rockwell 1993)

  ·  X2 :57.3k (US Robotics 1997)

  ·  K56: Flex 57.3k (Rockwell 1997)

     The PEP used as much bandwidth as feasible by splitting the
     spectrum into as many as 512 sub-bands.  It was supported by Ven-
     Tel's Pathfinder and Telebit's Trailblazer.


  27.5.  Autobauding

  This term has a few different meanings.  In general it means either
  the automatic adjustment of modem-to-modem speed or of modem-to-
  serial_port speed.


  27.6.  Modem-to-modem Speed

  Modern modems negotiate the modem-to-modem speed and protocol when
  they first connect to each other and normally connect to each other at
  the highest possible speed.  If one side can't negotiate, the other
  side should accept whatever speed and protocol that the fixed side has
  available.  Except that some modern modems may no longer support some
  of the antique protocols.  During negotiations, one modem often must
  use a lower speed than its maximum in order to connect with the other
  modem.  This is sometimes called "fallback" since one modem falls back
  to a lower speed (although it never really used its higher speed
  modem-to-modem).  This is also called "autobauding" or "automode".
  Sometimes fallback also happens when both modems automatically lower
  their speed due to a noisy line.  Register S37 in a modem is normally
  set to enable autobauding but may also be set for a fixed modem-to-
  modem speed in some modems.

  Early modems didn't have such autobauding or fallback.  If you have
  such a modem, it will likely work OK if the other modem you connect to
  is a modern one that can adjust it's speed and protocol to yours.  But
  a problem arises if both modems which want to communicate with each
  other are both antique and don't support automode.  In this case they
  need to be manually set to the same speed and protocol.

  Even when this automode existed, there was sometimes a limited choices
  of speed (like only 1200/300 bps).  In olden days (and even today), a
  computer dial-in site might have groups of phone lines, where each
  group which had a specific type modem on it which supported specific
  speeds and protocols.  For example, if you had a 2400 bps Bell 212A
  modem then you simply only dialed in to certain telephone numbers that
  supported that speed and protocol.  Once a site obtained modems that
  could support a wider variety of speeds and protocols, then there was
  no need to have different groups of phone lines.


  27.7.  Modem-to-serial_port Speed

  27.7.1.  Same speed required

  For old modems (mostly under 9600 bps) the modem-to-serial_port speed
  had to be the same as the modem-to-modem speed.  This was because data
  flowed straight thru the modem without "speed buffering" (storing
  bytes) inside the modem.  This meant that both the modem's serial port
  and the computer's serial port had to be set to this speed.  That is,
  both ends of the serial cable had to be set for this speed.

  One might erroneously reason that if the serial port speed was higher
  than the modem-to-modem speed, all would work OK since then there
  would be no bottleneck in the serial line.  This works OK in the
  direction from the modem to the PC since a higher speed line can have
  a lower thruput speed due to time spacing between bytes.  But disaster
  strikes for the flow from the PC to the modem since it would flow at a
  speed faster than the modem could transmit the data.  Data would be
  lost since there is no speed buffering.


  27.7.2.  Equalizing speed

  If a modem had only one modem-to-modem speed (or was set by software
  or physical switches to only operate at one speed), then this wasn't a
  problem since one would just set the PCs serial port for this speed.
  Even if the modem had various modem-to-modem speeds which were set by
  negotiations with the other modem, there was no problem in setting the
  modem's serial port speed correctly.  It would simply set this port
  speed to it's current modem-to-modem speed.  Another way make the
  speeds equal is for the modem to detect the PC's serial port speed and
  then set it's modem-to-modem speed the same.  This will be explained
  later.

  But setting the computer's serial port to the modem-to-modem speed was
  a problem since the modem has no way to directly give commands to the
  PCs serial port to change it's speed.  Only system software can do
  that.  The modem finds out what speed to use based on negotiations
  with the other modem and thus the change of this serial port speed
  can't be done in advance.  How does the modem communicate its chosen
  modem-to-modem speed to the system software?


  27.7.3.  Use "CONNECT" message to set speed

  Here's one way to do it.  Consider the case of a dial-in modem that
  others dial into.  A getty program will be used to send login prompt
  thru the modems to users.  Getty will also be the system software that
  changes the speed of the serial port and the modem will tell getty
  what modem-to-modem speed it's using.  The modem will do this by
  sending getty a "CONNECT" message giving the modem-to-modem speed.

  But there's one problem here.  How does one insure that the "CONNECT"
  message, which the modem sends to getty via the serial port, is sent
  at the same speed as the PC's serial port?  Here's how it's done.

  When the modem is first sent an init string, the modem detects the
  speed of the computer's serial port and sets it's modem-to-serial_port
  speed to this value.  Now it can communicate with getty.  The modem
  senses the serial speed by examining the "AT" at the beginning of the
  string.  This is sometimes also called autobauding.  For modern
  modems, this same modem-to-serial port speed is always retained, even
  after the modem connects to another modem and regardless of what the
  modem-to-modem speed is.  But for our old modem this serial speed
  needs to be equal the modem-to-modem speed.

  So now, for example, getty gets a CONNECT 2400 from the modem and
  switches the PC's serial port speed to 2400.  The modem also switches
  its serial port speed to 2400.  Now both ends of the modem serial
  cable are at 2400 and there is no speed mismatch.  Then getty sends a
  login prompt out over the phone line thru the modems.  The 2400 bps is
  now both the modem-to-modem speed and the modem-to-serial_port speed.
  Problem solved.  Mgetty can do this by configuring it for
  "autobauding".

  For dialing out, the same method is used, but now the communication
  software must handle it instead of getty.


  27.7.4.  Setting modem-to-modem speeds by the serial speed

  Another way to switch modem-to-modem speed was by using a modem
  feature where the modem would set its modem-to-modem speed to be the
  same as the modem-to-serial port speed it detected.  The Bn AT
  commands would enable this and determine what protocol to use for each
  speed.  So with this enabled, setting the serial speed by the computer
  would also set the modem-to-modem speed to be the same.  This should
  result in this modem being inflexible in any speed negotiations
  between the modems.


  27.7.5.  Manual bauding

  Another (but cruder) way to solve the serial speed problem when
  dialing in to a site was to get the remote site to change it's modem-
  to-modem speed to match your serial port speed.  It works like this:
  The person trying to login over a modem connection doesn't see any
  login prompt because of a speed mismatch.  So the person trying to
  login hits a "break" key to send a break signal over the phone line
  (via modem) to getty on the remote machine.  A break signal will
  always get through even if there is a speed mismatch in a serial line.


  The remote getty gets this break signal and switches the remote's
  serial port to the next speed as specified in its getty configuration
  file.  This new remote serial port speed causes the remote modem to
  switch to the same modem-to-modem speed as previously configured by
  the ATB command.  Then the local modem would transmit the login prompt
  over the local's serial line at this speed.  If one doesn't see a
  login prompt, then they hit the break key again and a new speed is
  tried.  This continues until the remote getty finally gets the speed
  correct (equal to the serial speed set on the local PC) and a login
  prompt finally displays.  Note that PC keyboards have no "break" key
  but dumb terminal keyboards did.  Mgetty, agetty, and uugetty can do
  this obsolete break method and it's called "manual bauding".


  27.7.6.  Unsupported speeds

  In Linux, there's a problem if the speed is set to a speed not
  supported by Linux's serial port (for example 7200 bps).  You may dial
  out and connect at 7200 bps (both modem-to-modem and modem-to-
  serial_port speed) but you only see garbage since Linux doesn't
  support 7200 on the serial port.  Once you connect there is no simple
  way to hang up because even the +++ escape sequence can't be sent to
  the modem over a 7200 baud interface.


  27.7.7.  Modern modems, speed buffering

  To dial out by the antique method using some modern modems set &Q0 N0
  and S17=5 (if you want 1200 bps).  Some of the S17 settings vary with
  the make of modem.  S17=0 is the default that connects the modern way
  at the highest speed supported.

  Modern modems can use almost any serial port speed and it doesn't
  depend at all on modem-to-modem speed.  To do this they employ speed
  buffering and flow control.  Speed buffering means that modems have
  buffers so that there can be a difference between the modem-to-modem
  speed and the modem-to-serial_port speed.  If the flow entering the
  modem is faster than the flow exiting it, the excess flow is simply
  stored in a buffer in the modem.  Then to prevent the buffer from
  overflowing, the modem sends a flow control signal to stop the input
  flow to the modem.  This is true for either direction of flow.  See
  ``Flow Control'' for more details.


  27.8.  Before AT Commands

  Hayes introduced the AT command set and other modem manufacturers
  adopted it as a standard.  Before the AT commands, many modems used
  dip switches to configure the modem.  Another command set is the CCITT
  V.25bis command set.  Some modems supported both CCITT and AT
  commands.  The CCITT V.25bis also specifies how Synchronous modem-to-
  serial_port communication is to take place using either the ASCII or
  8-bit EBCDIC character sets.


  27.9.  Acoustic-Coupling

  This is where one connects a modem to a telephone using audio tones
  that one can hear.  The modem contains a microphone and speaker which
  "talks" directly into the telephone handset without using any wires.
  It's "wireless" in a sense but uses sound waves instead of radio
  waves.  The modem speaker is placed in contact with the telephone
  microphone (on the handset) so that the tones from the modem go into
  the telephone.  The modem microphone, picks up tones from the
  telephone handset speaker.  This scheme is called "acoustic coupling".

  A major problem is that outside noises can interfere and cause errors.
  The advantage is convenience: There are no cables to plug in.  Most
  modems that could do this were only 300 baud, but higher speeds were
  used too.  It's said that 9600 bps didn't work very well using this
  scheme.


  27.10.  Data Compression and Error Correction

  MNP 2, 3, or 4 were used for error correction.  MNP 5 was compression.
  Modern modems generally use V42 (error correction) and V42bis
  (compression).  Many modems support both MNP and V42.

  END OF Modem-HOWTO
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