Chip 1998 September/CHIP Eylül 1998.iso/Slackwar/docs/Serial-HOWTO

Text File | 1997-07-06 | 64.1 KB | 1,513 lines

  The Linux Serial HOWTO
  by Greg Hankins, [e-mail]
  v1.10, 6 July 1997

  This document describes how to set up serial communications devices
  on a Linux box.

  1.  Introduction

  This is the Linux Serial HOWTO.  All about how to set up modems and
  terminals under Linux, some serial tips, and troubleshooting.  This
  HOWTO addresses Linux running on x86 hardware, although it might work
  for other architectures.

  1.1.  Copyright

  The Linux Serial HOWTO is copyright (C) 1993 - 1997 by Greg Hankins.
  Linux HOWTO documents may be reproduced and distributed in whole or in
  part, in any medium physical or electronic, as long as this copyright
  notice is retained on all copies. Commercial redistribution is allowed
  and encouraged; however, the author would like to be notified of any
  such distributions.

  All translations, derivative works, or aggregate works incorporating
  any Linux HOWTO documents must be covered under this copyright notice.
  That is, you may not produce a derivative work from a HOWTO and impose
  additional restrictions on its distribution. Exceptions to these rules
  may be granted under certain conditions; please contact the Linux
  HOWTO coordinator at the address given below.

  In short, we wish to promote dissemination of this information through
  as many channels as possible. However, we do wish to retain copyright
  on the HOWTO documents, and would like to be notified of any plans
  to redistribute the HOWTOs.

  If you have questions, please contact Greg Hankins, the Linux HOWTO
  coordinator, at [e-mail] via email.

  1.2.  New Versions Of This Document

  New versions of the Serial HOWTO will be available at
  ftp://sunsite.unc.edu:/pub/Linux/docs/HOWTO/Serial-HOWTO and mirror
  sites.  There are other formats, such as PostScript and DVI versions
  in the other-formats directory.  The Serial HOWTO is also available at
  http://sunsite.unc.edu/LDP/HOWTO/Serial-HOWTO.html and will be posted
  to comp.os.linux.answers monthly.

  1.3.  Feedback

  Please send me any questions, comments, suggestions, or additional
  material.  I'm always eager to hear about what you think about the
  HOWTO.  I'm also always on the lookout for improvements!  Tell me
  exactly what you don't understand, or what could be clearer.  You can
  reach me at [e-mail] via email.  All email will be
  answered, although it may take me a week or so depending on how busy I
  am.  I get a lot of email from root and misconfigured sites.  Please
  send me email as a real person, and make sure that your email system
  is working so that I will be able to reply to you.  If you don't hear
  back from me within two weeks, please try sending your email again.

  I can also be reached at:
  Greg Hankins
  College of Computing
  801 Atlantic Drive
  Atlanta, GA 30332-0280
  via snail mail, and at http://www.cc.gatech.edu/staff/h/Greg.Hankins/.

  Please include the version number of the Serial HOWTO when writing,
  this is version 1.10.

  1.4.  Disclaimer

  Your mileage may vary.  The answers given may not work for all systems
  and all setup combinations.

  2.  Supported Serial Hardware

  Linux is known to work with the following serial hardware.

  2.1.  Standard PC Serial Boards

  ╖  standard PC serial boards (COM1 - COM4), to which external serial
     devices (modems, serial mice, etc...) can be connected

  ╖  standard PC internal modems (COM1 - COM4)

  ╖  Quickpath Systems Port-Folio 550e (allows IRQs of 3, 4, 5, 9, 10,
     11, 12, and 15)

  Note: due to address conflicts, you cannot use COM4 and IBM8514 video
  board simultaneously.  This is due to a bug in the IBM8514 board.

  2.2.  Plug And Play (PnP) Modems

  PnP modems will work with Linux.  However, I recommend against using
  them because they are difficult.  There are a couple things you can
  try;

  ╖  Try to disable the PnP features in the BIOS on your computer and
     modem.

  ╖  Boot Win95, configure the modem, and see if the settings are
     preserved when you boot Linux.  If not, then:

  ╖  Boot Win95, configure the modem, and then warm boot Linux so
     that the settings are preserved.  You can do this with the loadlin
     boot loader.

  ╖  Get the PnP tools from http://www.redhat.com/linux-info/pnp.

  ╖  And of course you can get a real modem.

  2.3.  Dumb Multiport Serial Boards (with 8250/16450/16550A UARTs)

  ╖  AST FourPort and clones (4 ports)

  ╖  Accent Async-4 (4 ports)

  ╖  Arnet Multiport-8 (8 ports)

  ╖  Bell Technologies HUB6 (6 ports)

  ╖  Boca BB-1004 (4 ports), BB-1008 (8 ports), BB-2016 (16 ports)

  ╖  Boca IOAT66 (6 ports)

  ╖  Boca 2by4 (4 serial ports, 2 parallel ports)

  ╖  Computone ValuePort V4-ISA (AST FourPort compatible)

  ╖  Digi PC/8 (8 ports)

  ╖  GTEK BBS-550 (8 ports)

  ╖  Longshine LCS-8880, Longshine LCS-8880+ (AST FourPort compatible)

  ╖  Moxa C104, Moxa C104+ (AST FourPort compatible)

  ╖  PC-COMM (4 ports)

  ╖  Sealevel Systems <http://www.sealevel.com> COMM-2 (2 ports), COMM-4
     (4 ports) and COMM-8 (8 ports)

  ╖  SIIG I/O Expander 2S IO1812 (4 ports)

  ╖  STB-4COM (4 ports)

  ╖  Twincom ACI/550

  ╖  Usenet Serial Board II (4 ports)

  In general, Linux will support any serial board which uses a 8250,
  16450, 16550, 16550A (or compatible) UART, or an internal modem which
  emulates one of the above UARTs.

  Note: the BB-1004 and BB-1008 do not support DCD and RI lines, and
  thus are not usable for dialin modems.  They will work fine for all
  other purposes.

  2.4.  Intelligent Multiport Serial Boards

  ╖  Comtrol RocketPort (36MHz ASIC; 4, 8, 16 or 32 ports)
     contact: [e-mail] or http://www.comtrol.com
     driver status: supported by Comtrol
     driver location: ftp://tsx-11.mit.edu/pub/linux/packages/comtrol

  ╖  Computone IntelliPort II (16MHz 80186; 4, 8, or 16 ports),
     IntelliPort II EXpandable (20MHz 80186; 16 - 64 ports)
     contact: Michael H. Warfield, [e-mail]
     driver status: pre-ALPHA

  ╖  Cyclades Cyclom-Y (Cirrus Logic CD1400 UARTs; 8 - 32 ports),
     Cyclom-Z (25MHz MIPS R3000; 8 - 128 ports)
     contact: [e-mail] or http://www.cyclades.com
     driver status: supported by Cyclades
     driver location: ftp://ftp.cyclades.com/pub/cyclades and included
     in Linux kernel since version 1.1.75

  ╖  Decision PCCOM8 (8 ports)
     contact: [e-mail]
     driver location: ftp://ftp.signum.se/pub/pccom8

  ╖  Digi PC/Xi (12.5MHz 80186; 4, 8, or 16 ports),
     PC/Xe (12.5/16MHz 80186; 2, 4, or 8 ports),
     PC/Xr (16MHz IDT3041; 4 or 8 ports),
     PC/Xem (20MHz IDT3051; 8 - 64 ports)
     contact: [e-mail] or http://www.dgii.com
     driver status: supported by Digi
     driver location: ftp://ftp.dgii.com/drivers/linux and included in
     Linux kernel since version 2.0

  ╖  Digi COM/Xi (10MHz 80188; 4 or 8 ports)
     contact: Simon Park, [e-mail]
     driver status: ALPHA
     note: Simon is often away from email for months at a time due to
     his job.  Mark Hatle, [e-mail] has graciously
     volunteered to make the driver available if you need it.  Mark is
     not maintaining or supporting the driver.

  ╖  Equinox SuperSerial Technology (30MHz ASIC; 2 - 128 ports)
     contact: [e-mail] or http://www.equinox.com
     driver status: supported by Equinox
     driver location: ftp://ftp.equinox.com/library/sst

  ╖  GTEK Cyclone (16C654 UARTs; 6, 16 and 32 ports),
     SmartCard (24MHz Dallas DS80C320; 8 ports),
     BlackBoard-8A (16C654 UARTs; 8 ports),
     PCSS (15/24MHz 8032; 8 ports)
     contact: [e-mail] or http://www.gtek.com
     driver status: supported by GTEK
     driver location: ftp://ftp.gtek.com/pub

  ╖  Hayes ESP (COM-bic; 1 - 8 ports)
     contact: Andrew J. Robinson, [e-mail] or
     http://www.nyx.net/~arobinso
     driver status: supported by author
     driver location: http://www.nyx.net/~arobinso and included in Linux
     kernel since version 2.1.15

  ╖  Maxpeed SS (Toshiba; 4, 8 and 16 ports)
     contact: [e-mail] or http://www.maxpeed.com
     driver status: supported by Maxpeed
     driver location: ftp://maxpeed.com/pub/ss

  ╖  Moxa C218 (12MHz 80286; 8 ports),
     Moxa C320 (40MHz TMS320; 8 - 32 ports)
     contact: [e-mail] or http://www.moxa.com.tw
     driver status: supported by Moxa
     driver location: ftp://ftp.moxa.com.tw/drivers/c218-320/linux

  ╖  SDL RISCom/8 (Cirrus Logic CD180; 8 ports)
     contact: [e-mail] or http://www.sdlcomm.com
     driver status: supported by SDL
     driver location: ftp://ftp.sdlcomm.com/pub/drivers

  ╖  Specialix SIO (20MHz Zilog Z280; 4 - 32 ports),
     XIO (25MHz Zilog Z280; 4 - 32 ports)
     contact: Simon Allen, [e-mail]
     driver status: BETA
     driver location:
     ftp://sunsite.unc.edu/pub/Linux/kernel/patches/serial

  ╖  Stallion EasyIO-4 (4 ports), EasyIO-8 (8 ports), and
     EasyConnection (8 - 32 ports) - each with Cirrus Logic CD1400
     UARTs,
     Stallion (8MHz 80186 CPU; 8 or 16 ports),
     Brumby (10/12 MHz 80186 CPU; 4, 8 or 16 ports),
     ONboard (16MHz 80186 CPU; 4, 8, 12, 16 or 32 ports),
     EasyConnection 8/64 (25MHz 80186 CPU; 8 - 64 ports)
     contact: [e-mail] or http://www.stallion.com
     driver status: supported by Stallion
     driver location: ftp://ftp.stallion.com/drivers/ata5/Linux and
     included in linux kernel since 1.3.27

  A review of Comtrol, Cyclades, Digi, and Stallion products was printed
  in the June 1995 issue of the Linux Journal.  The article is available
  at http://www.ssc.com/lj/issue14.

  2.5.  Unsupported Serial Hardware

  Modems that claim to be ``Winmodems'', for example the USR Sportster
  Winmodem and IBM Aptiva MWAVE, are not supported under Linux.  These
  modems use proprietary designs that require special Windows drivers.
  Avoid these types of modems.  In addition, stay away from modems that
  require Rockwell RPI drivers for the same reason.

  Intelligent serial boards that require drivers not available for Linux
  won't work either.

  3.  What Are The Names Of The Serial Ports?

  An I/O port is a way to get data into and out of a computer.  There
  are many types of I/O ports such as serial ports, parallel ports, disk
  drive controllers, ethernet boards, etc.  We will be dealing with
  serial ports since modems and terminals are serial devices.  Each
  serial port must have an I/O address, and an interrupt (IRQ).  There
  are the four serial ports corresponding to COM1 - COM4:

       ttyS0 (COM1) address 0x3f8 IRQ 4
       ttyS1 (COM2) address 0x2f8 IRQ 3
       ttyS2 (COM3) address 0x3e8 IRQ 4
       ttyS3 (COM4) address 0x2e8 IRQ 3

  If Linux does not detect any serial ports when it boots, then make
  sure that serial support is enabled and compiled into the kernel. In
  this document, I refer to COM1 as ttyS0, COM2 as ttyS1, COM3 as ttyS2,
  and COM4 as ttyS3.  Notice that by default these devices have overlap¡
  ping IRQs.  You cannot use all of the ports in this default configura¡
  tion, and you must reassign different IRQs.  See section ``Can I Use
  More Than Two Serial Devices?'' on setting IRQs.

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

  There has been some discussion on the merits of /dev/mouse and
  /dev/modem.  I strongly discourage the use of these links.  In
  particular, if you are planning on using your modem for dialin you may
  run into problems because the lock files may not work correctly if you
  use /dev/modem. Use them if you like, but be sure they point to the
  right device.  However, if you change or remove this link, some
  applications (minicom for example) might need reconfiguration.

  3.1.  Serial Port Devices and Numbers In /dev

       /dev/ttyS0 major 4, minor 64    /dev/cua0 major 5, minor 64
       /dev/ttyS1 major 4, minor 65    /dev/cua1 major 5, minor 65
       /dev/ttyS2 major 4, minor 66    /dev/cua2 major 5, minor 66
       /dev/ttyS3 major 4, minor 67    /dev/cua3 major 5, minor 67

  Note that all distributions should come with these devices already
  made correctly.  You can verify this by typing:

       linux% ls -l /dev/cua*
       linux% ls -l /dev/ttyS*

  3.1.1.  Creating Devices In /dev

  If you don't have a device, you will have to create it with the mknod
  command.  Example, suppose you needed to create devices for ttyS0:

       linux# mknod -m 666 /dev/cua0 c 5 64
       linux# mknod -m 666 /dev/ttyS0 c 4 64

  You can use the MAKEDEV script, which lives in /dev.  This simplifies
  the making of devices.  For example, if you needed to make the devices
  for ttyS0 you would type:

       linux# cd /dev
       linux# ./MAKEDEV ttyS0

  This handles the devices creation for the incoming and outgoing
  devices, and should set the correct permissions.

  3.2.  Notes For Dumb Multiport Boards

  The devices your multiport board uses depends on what kind of board
  you have.  These are listed in detail in rc.serial which comes with
  the setserial program.  I highly recommend getting the latest version
  of setserial if you are trying to use multiport boards.  You will
  probably need to create these devices.  Either use the mknod command,
  or the MAKEDEV script.  Devices for multiport boards are made by
  adding ``64 + port number''.  So, if you wanted to create devices for
  ttyS17, you would type:

       linux# mknod -m 666 /dev/cua17 c 5 81
       linux# mknod -m 666 /dev/ttyS17 c 4 81

  Note that ``64 + 17 = 81''.  Using the MAKEDEV script, you would type:

       linux# cd /dev
       linux# ./MAKEDEV ttyS17

  Note: the SIIG manual for the IO1812 listing for COM5-COM8 is wrong.
  They should be COM5=0x250, COM6=0x258, COM7=0x260, and COM8=0x268.

  Note: the Digi PC/8 Interrupt Status Register is at 0x140.

  Note: for an AST Fourport, you might need to specify skip_test in
  rc.serial.

  3.3.  Notes For Intelligent Multiport Boards

  Read the information that comes with the driver.  These boards use
  special devices, and not the standard ones.  This information varies
  depending on your hardware.

  4.  Interesting Programs You Should Know About

  4.1.  What is getty?

  getty is a program that handles the login process when you log onto a
  Unix box.  You will need to use getty if you want to be able to dial
  in to your Linux machine with a modem.  You do not need to use getty
  if you only want to dial out with your modem.  There are three
  versions that are commonly used with Linux: getty_ps, mgetty and
  agetty.  The syntax for these programs differs, so be sure to check
  and make sure that you are using the correct syntax         for
  whatever getty you use.

  4.1.1.  About getty_ps

  Most distributions come with the getty_ps package installed.  It
  contains two programs:  getty is used for console and terminal
  devices, and uugetty for modems.  I use this version of getty, so that
  is what I will focus on.

  4.1.2.  About mgetty

  mgetty is a version of getty for use with modems.  In addition to
  allowing dialup logins, mgetty also provides FAX support and auto PPP
  detection. The mgetty documentation is very good, and does not need
  supplementing.  Please refer to it for installation instructions.  You
  can find the latest information on mgetty at
  http://www.leo.org/~doering/mgetty/.

  4.1.3.  About agetty

  agetty is the third variation of getty. It's a simple, completely
  functional implementation of getty which is best suited for virtual
  consoles or terminals rather than modems.

  4.2.  What is setserial?

  setserial is a program which allows you to look at and change various
  attributes of a serial device, including its port address, its
  interrupt, and other serial port options.  You can find out what
  version you have by running setserial with no arguments.

  When your Linux system boots, only ttyS{0-3} are configured, using the
  default IRQs of 4 and 3.  So, if you have any other serial ports
  provided by other boards or if ttyS{0-3} have a non-standard IRQ, you
  must use this program in order to configure those serial ports.  For
  the full listing of options, consult the man page.

  5.  How Do I Dial Out With My Modem?

  5.1.  Hardware Requirements

  First, make sure you have the right cable.  Your modem requires a
  straight through cable, with no pins crossed.  Any computer store
  should have these.  Make sure you get the correct gender.  If you are
  using the DB25 serial port, it will always be the male DB25.  Do not
  confuse it with the parallel port, which is the female DB25.  Hook up
  your modem to one of your serial ports.  Consult your modem manual on
  how to do this if you need help.

  5.1.1.  Notes On Internal Modems

  For an internal modem, you will not need a cable.  An internal modem
  does not need a serial port, it has one built in.  All you need to do
  is configure it to use an interrupt that is not being used, and
  configure the port I/O address.  Consult your modem manual if you get
  stuck.  Also, see section ``Can I Use More Than   Two Serial
  Devices?'' if you need help on choosing interrupts or addresses.

  On some motherboards you will have to disable the serial port that the
  modem is replacing in order to avoid conflicts.  This may be done with
  jumpers or in the BIOS settings, depending on your motherboard.
  Consult your motherboard manual.

  Due to a bug in IBM8514 video boards, you may encounter problems if
  you want your internal modem to be on ttyS3.  If Linux does not detect
  your internal modem on ttyS3, you can use setserial and the modem will
  work fine.  Internal modems on ttyS{0-2} should not have any problems
  being detected.  Linux does not do any autoconfiguration on ttyS3 due
  to this video board bug.

  5.2.  Talking To Your Modem

  Use kermit or some other communications program to test the setup,
  before you go jumping into complex things SLIP or PPP.  You can find
  the latest version of kermit at http://www.columbia.edu/kermit/.  For
  example, say your modem was on ttyS3, and it's 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:

  See section ``Communications Programs'' about communications programs
  if you need some pointers.

  When you dial out with your modem, set the speed to the highest bps
  rate that your modem supports.  Versions of Linux with a libc version
  greater then 5.x have support for speeds up to 115200 bps.  libc
  usually lives in /lib, so look there for what version you have.  If
  Linux does not recognize a speed of 57600 or 115200 bps, then you must
  use the setserial program to set your serial port to a higher speed.
  See section ``How Do I Set Up My Serial Ports For Higher Speeds?'' for
  how to do this.  Then, set the speed to 38400 bps in your comm
  program.

  5.3.  Dial Out Modem Configuration

  For dial out use only, you can configure your modem however you want.
  If you intend to use your modem for dialin, you must configure your
  modem at the same speed that you intend to run getty at.  So, if you
  want to run getty at 38400 bps, set your speed to 38400 bps when you
  configure your modem.  This is done to prevent speed mismatches
  between your computer and modem.  In general, factory defaults that
  enable error correction and hardware flow control are the best setting
  for dial out modems, consult your modem manual for these settings.

  5.4.  Hardware Flow Control

  If your modem supports hardware flow control (RTS/CTS), I highly
  recommend you use it. This is particularly important for modems that
  support data compression.  First, you have to enable RTS/CTS flow
  control on the serial port itself.  This is best done on startup, like
  in /etc/rc.d/rc.local or /etc/rc.d/rc.serial.  Make sure that these
  files are being run from the main rc.sysinit file!  You need to do the
  following for each serial port you want to enable hardware flow
  control on:

       stty crtscts < /dev/ttyS3

  You must also enable RTS/CTS flow control on your modem.  Consult your
  modem manual on how to do this, as it varies between modem manufactur¡
  ers.  Be sure to save your modem configuration if your modem supports
  stored profiles.

  6.  How Do I Dial In And Out With My Modem Using getty_ps?

  Get your modem to dial out correctly.  If you haven't read section
  ``How Do I Dial Out With My Modem'' go read it now!  It contains very
  important setup information.  You do not need to read this section if
  you only want to dial out with your modem.

  6.1.  Dial In And Out Modem Configuration

  For dialin and dialout use, you have to set up your modem a certain
  way (again, using AT commands on your modem):

       E1       command echo ON
       Q0       result codes are reported
       V1       verbose ON
       S0=0     never answer (uugetty handles this with the WAITFOR option)

  If you don't set these correctly, your INIT string in your config file
  may fail, hosing the whole process.  But, more on config files
  below...

       &C1     DCD is on after connect only
       &S0     DSR is always on
               DTR on/off resets modem (depends on manufacturer - RTFM)

  These affect what your modem does when calls start and end.

  If your modem does not support a stored profile, you can set these
  through the INIT string in your config file.  See below.  Some modems
  come with DIP switches that affect register settings.  Be sure these
  are set correctly, too.

  I have started a collection of modem setups for different types of
  modems.  So far, I only have a few of them, if you would like to send
  me your working configuration, please do so!  You can get them at
  ftp://ftp.cc.gatech.edu/pub/people/gregh/modem-configs.

  Note: to get my USR Courier V.34 modem to reset correctly when DTR
  drops, I 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 Supra modems treat DCD 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.

  6.2.  Installing getty_ps

  Get the latest version from sunsite.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.

  From this point on, all references to getty will refer to getty_ps.
  References to uugetty will refer to the uugetty that comes with the
  getty_ps package.  These instructions will not work for mgetty or
  agetty.

  6.3.  Setting up uugetty

  For dialing into, and out from your modem, we want to use uugetty.
  uugetty does important lock file checking.  Update /etc/gettydefs to
  include entries for modems if they aren't already there (note that the
  entries point to each other, these are not         for fixed speed -
  blank lines are needed between each entry):

       # 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: #2400

       2400# B2400 CS8 # B2400 SANE -ISTRIP HUPCL #@S @L @B login: #115200

  If you have a 9600 bps or faster modem with data compression, you can
  lock your serial port speed and let the modem handle the transitions
  to other speeds. Then, instead of the step down series of lines listed
  above /etc/gettydefs only needs to contain one line for the modem, 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

  If you want, you can make uugetty print interesting things in the
  login banner.  In my examples, I have 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 getty 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 '@@'.

  Next, make sure that you have an outgoing and incoming device for the
  serial port your modem is on.  If you have your modem on ttyS3 you
  will need the /dev/cua3, and /dev/ttyS3 devices.  If you don't have
  the correct devices, see section ``Creating Devices In <TT>/dev</TT>''
  on how to create devices, and create the devices.  If you want to be
  able to dial out with your modem while uugetty is watching the port
  for logins, use the /dev/cuaN device instead of the /dev/ttySN device.

  When you are done editing /etc/gettydefs, you can verify that the
  syntax is correct by doing:

       linux# getty -c /etc/gettydefs

  6.4.  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
  getty (older than 2.0.7e), or aren't using FSSTND, then the default
  file will be /etc/default/uugetty.ttySN.  My /etc/conf.uugetty.ttyS3
  looks like this:

       # sample uugetty configuration file for a Hayes compatible modem to allow
       # incoming modem connections
       #
       # alternate lock file to check... if this lock file exists, then uugetty is
       # restarted so that the modem is re-initialized
       ALTLOCK=cua3
       ALTLINE=cua3
       # line to initialize
       INITLINE=cua3
       # 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 - config file location, port, speed, and default terminal
  type):

       S3:456: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 getty(1m) for a
  full description.  Among other things there is a scheduling feature,
  and a ringback feature.

  7.  How Do I Set Up A Terminal Connected To My PC?

  The instructions in this section will work for connecting terminals,
  as well as other computers to the serial port on your Linux box.

  7.1.  Hardware Requirements

  Make sure you have the right kind of cable.  A null modem cable bought
  at a computer store will do it.  But it must be a null modem cable!
  Many computer stores advertise this kind of cable as a serial printer
  cable.  Make sure you are using your serial port, the male DB25 or the
  DB9, and not your parallel port (female DB25 or centronics).

  For a DB25 connector, you need a minimum of:

          PC male DB25                            Terminal DB25
          TxD   Transmit Data         2 --> 3     RxD   Receive Data
          RxD   Receive Data          3 <-- 2     TxD   Transmit Data
          SG    Signal Ground         7 --- 7     SG    Signal Ground

  If you want to have hardware handshaking signals, you must have a full
  null modem cable:

          PC male DB25                            Terminal DB25
          TxD   Transmit Data         2 --> 3     RxD   Receive Data
          RxD   Receive Data          3 <-- 2     TxD   Transmit Data
          RTS   Request To Send       4 --> 5     CTS   Clear To Send
          CTS   Clear To Send         5 <-- 4     RTS   Request To Send
          DSR   Data Set Ready        6
                                      |
          DCD   Carrier Detect        8 <-- 20    DTR   Data Terminal Ready
          SG    Signal Ground         7 --- 7     SG    Signal Ground
                                            6     DSR   Data Set Ready
                                            |
          DTR   Data Terminal Ready  20 --> 8     DCD   Carrier Detect

  If you have a DB9 connector on your serial port, try the following:

          PC DB9                                  Terminal DB25
          RxD   Receive Data          2 <-- 2     TxD   Transmit Data
          TxD   Transmit Data         3 --> 3     RxD   Receive Data
          SG    Signal Ground         5 --- 7     SG    Signal Ground

  Alternatively, a full DB9-DB25 null modem cable:

          PC DB9                                  Terminal DB25
          RxD   Receive Data          2 <-- 2     TxD   Transmit Data
          TxD   Transmit Data         3 --> 3     RxD   Receive Data
                                            6     DSR   Data Set Ready
                                            |
          DTR   Data Terminal Ready   4 --> 8     DCD   Carrier Detect
          GND   Signal Ground         5 --- 7     GND   Signal Ground
          DCD   Carrier Detect        1
                                      |
          DSR   Data Set Ready        6 <-- 20    DTR   Data Terminal Ready
          RTS   Request To Send       7 --> 5     CTS   Clear To Send
          CTS   Clear To Send         8 <-- 4     RTS   Request To Send
          (RI   Ring Indicator        9 not needed)

  (Yes, the pins 2 and 3 really do have the opposite meanings in DB9
  connectors than in DB25 connectors!)

  If you are not using a full null modem cable, you might have to do the
  following trick: on your computer side of the connector, connect RTS
  and CTS together, and also connect DSR, DCD and DTR together.  This
  way, when the computer wants a certain handshaking signal, it will get
  it (from itself).

  Now that you have the right kind of cable, connect your terminal to
  your computer.  If you can, tell the terminal to ignore modem control
  signals.  Try using 9600 bps, 8 data bits, 1 stop bit, no parity bits
  for the terminal's setup.

  7.2.  Setting up getty

  Add entries for getty to use for your terminal in /etc/gettydefs if
  there they aren't already there:

       # 38400 bps Dumb Terminal entry
       DT38400# B38400 CS8 CLOCAL # B38400 SANE -ISTRIP CLOCAL #@S @L login: #DT38400

       # 19200 bps Dumb Terminal entry
       DT19200# B19200 CS8 CLOCAL # B19200 SANE -ISTRIP CLOCAL #@S @L login: #DT19200

       # 9600 bps Dumb Terminal entry
       DT9600# B9600 CS8 CLOCAL # B9600 SANE -ISTRIP CLOCAL #@S @L login: #DT9600

  If you want, you can make getty print interesting things in the login
  banner.  In my examples, I have the system name and the serial line
  printed.  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 getty 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 '@@'.

  When you are done editing /etc/gettydefs, you can verify that the
  syntax is correct by doing:

       linux# getty -c /etc/gettydefs

  Make sure there is no getty or uugetty config file for the serial port
  that your terminal is attached to (/etc/default/{uu}getty.ttySN or
  /etc/conf.{uu}getty.ttySN), as this will probably interfere with
  running getty on a terminal.  Remove the file if it exits.

  Edit your /etc/inittab file to run getty on the serial port
  (substituting in the correct information for your environment - port,
  speed, and default terminal type):

       S1:456:respawn:/sbin/getty ttyS1 DT9600 vt100

  Restart init:

       linux# init q

  At this point, you should see a login prompt on your terminal.  You
  may have to hit return to get the terminal's attention.

  7.3.  Notes On Setting Up A PC As A Terminal

  Many people set up other PCs as terminals connected to Linux boxen.
  For example, old 8088 or 80286 PCs are perfect for this purpose.  All
  you need is a DOS boot disk containing a version of DOS suitable for
  your terminal-PC, and a communications program for your terminal-PC to
  run.  kermit works very well for this purpose.  You can find
  precompiled versions of kermit for every OS in existence at
  http://www.columbia.edu/kermit/.  Other popular DOS comm programs such
  as telix and procomm will work equally well.  Be sure to input correct
  serial port information into your terminal-PC's communications setup.

  8.  Can I Use More Than Two Serial Devices?

  You don't need to read this section, unless you want to use three or
  more serial devices... (assuming you don't have a multiport board).

  Providing you have another spare serial port, yes you can.  The number
  of serial ports you can use is limited by the number of interrupts
  (IRQs) and port I/O addresses we have to use.  This is not a Linux
  limitation, but a limitation of the PC bus.  Each serial devices must
  be assigned it's own interrupt and address.  A serial device can be a
  serial port, an internal modem, or a multiport serial board.

  Multiport serial boards are specially designed to have multiple serial
  ports that share the same IRQ for all serial ports on the board.
  Linux gets data from them by using a different I/O address for each
  port on the board.

  8.1.  Choosing Serial Device Interrupts

  Your PC will normally come with ttyS0 and ttyS2 at IRQ 4, and ttyS1
  and ttyS3 at IRQ 3.  You can see what IRQs are in use by looking at
  /proc/interrupts.  To use more than two serial devices, you will have
  to reassign an interrupt.  A good choice is to reassign an interrupt
  from your parallel port.  Your PC normally comes with IRQ 5 and IRQ 7
  set up as interrupts for your parallel ports, but few people use two
  parallel ports.  You can reassign one of the interrupts to a serial
  device, and still happily use a parallel port.  You will need the
  setserial program to do this.  In addition, you have to play with the
  jumpers on your boards, check the docs for your board.  Set the
  jumpers to the IRQ you want for each port.

  You will need to set things up so that there is one, and only one
  interrupt for each serial device.  Here is how I set mine up in
  /etc/rc.d/rc.local - you should do it upon startup somewhere:

               /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
                IRQ  1    Keyboard
                IRQ  2    Cascade for controller 2
                IRQ  3    Serial port 2
                IRQ  4    Serial port 1
                IRQ  5    Parallel port 2
                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.  Just
  make sure it isn't being used by the motherboard, or any other boards.
  2, 3, 4, 5, or 7  is a good choice.  ``not assigned'' means that
  currently nothing standard uses these IRQs.  Also 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 serial board with a 16-bit bus
  connector, you can also use IRQ 10, 11, 12 or 15.

  Just make sure you don't use IRQ 0, 1, 6, 8, 13 or 14!  These are used
  by your mother board.  You will make her very unhappy by taking her
  IRQs.  When you are done, double-check /proc/interrupts and make sure
  there are no conflicts.

  8.2.  Setting Serial Device Addresses

  Next, you must set the port address.  Check the manual on your board
  for the jumper settings.  Like interrupts, there can only be one
  serial device at each address.  Your ports will usually come
  configured as follows:

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

  Choose which address you want each serial device to have and set the
  jumpers accordingly.  I have my modem on ttyS3, my mouse on ttyS0, and
  my terminal on ttyS2.

  When you reboot, Linux should see your serial ports at the address you
  set them.  The IRQ Linux sees may not correspond to the IRQ you set
  with the jumpers.  Don't worry about this.  Linux does not do any IRQ
  detection when it boots, because IRQ detection is dicy and can be
  fooled.  Use setserial to tell Linux what IRQ the port is using.  You
  can check /proc/ioports to see what I/O port addresses are in use
  after Linux boots.

  9.  How Do I Set Up My Serial Ports For Higher Speeds?  What Speed
  Should I Use With My Modem?

  This section should help you figure out what speed to use when using
  your modem with a communications program, or with a getty program.

  ╖  If you have something slower than a 9600 bps (V.32) modem, set your
     speed to the highest speed your modem supports.  For example 300
     bps (V.21/Bell 103), 1200 bps (V.22/Bell 212A), or 2400 bps
     (V.22bis).

  ╖  If you have a 9600 bps (V.32) modem, with V.42bis data compression,
     use 38400 bps as your speed.  V.42bis compression has a theoretical
     rate of 4:1, thus ``4 * 9600 = 38400''.

  ╖  If you have a 14400 bps (V.32bis) modem, with V.42bis data
     compression, use setserial with the spd_hi flag to configure your
     serial port to use 57600 bps (4 * 14400 = 57600).

     Use the spd_vhi flag if you have a 28800 or 33600 bps (V.FC or
     V.34) modem (4 * 28800 = 115200).

     Then, use 38400 bps as the speed in your communication program, or
     /etc/inittab.  This is now the high speed you have set.  Make sure
     you have 16550A UARTs.

     If your libc version is at least 5.x, there will be speeds named
     57600 and 115200.  libc lives in /lib, so look there for what
     version you have.  You can use these directly (without using
     setserial), if your applications have been compiled to take
     advantage of this.  There are many distributions out there, so the
     best thing to do would be to try using these higher speeds if you
     have a recent version of a Linux distribution.

  Test your setserial setting on the command line first, and then when
  you have them working, put them into /etc/rc.d/rc.serial or
  /etc/rc.d/rc.local so that they are done at startup.  In my
  /etc/rc.d/rc.local, I set ttyS3 to 115200 bps by doing:

       /sbin/setserial /dev/ttyS3 spd_vhi

  Make sure that you are using a valid path for setserial, and a valid
  device name.  You can check the settings of a serial port by running:

       setserial -a /dev/ttyS3

  10.  Communications Programs And Utilities

  Here is a list of some communication software you can choose from,
  available via FTP, if they didn't come with your distribution.

  ╖  ecu - a communications program

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

  ╖  minicom - telix-like communications program

  ╖  procomm - procomm-like communications program with zmodem

  ╖  seyon - X based communication program

  ╖  xc - xcomm communication package

  ╖  Other useful programs are term and SLiRP.  They offer TCP/IP
     functionality using a shell account.

  ╖  screen is another multi-session program.  This one behaves like the
     virtual consoles.

  ╖  callback is a program that will have your modem call you back.

  ╖  mgetty+fax handles FAX stuff, and provides an alternate getty.

  ╖  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://sunsite.unc.edu/pub/Linux/system/network/serial.

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

  11.  Serial Tips And Miscellany

  Here are some serial tips you might find helpful...

  11.1.  kermit and zmodem

  To use zmodem with kermit, add the following to your .kermrc:

       define rz !rz < /dev/ttyS3 > /dev/ttyS3
       define sz !sz \%0 > /dev/ttyS3 < /dev/ttyS3

  Be sure to put in the correct port your modem is on.  Then, to use it,
  just type rz or sz <filename> at the kermit prompt.

  11.2.  Setting Terminal Types Automagically

  To set your terminal type automagically when you log in, add the
  terminal type to the entry in /etc/inittab.  If I have a vt100
  terminal on ttyS1, I would add ``vt100'' to the getty command:

       S1:456:respawn:/sbin/getty ttyS1 DT9600 vt100

  You can also use tset, which can establish terminal characteristics
  when you log in, and doesn't depend on any defaults.

  11.3.  Color ls On Serial Connections

  If ls is screwing up your terminal emulation with the color feature,
  turn it off.  ls --color, and ls --colour all use the color feature.
  Some installations have ls set to use color by default.  Check
  /etc/profile and /etc/csh.cshrc for ls aliases.  You can also alias ls
  to ls --no-color, if you don't want to change the system defaults.

  11.4.  Printing To A Printer Connected To A Terminal

  There is a program called vtprint, available from
  ftp://ftp.sdsu.edu/pub/vtprint, and from http://www.sdsu.edu/~garrett.

  Another program that will do this is called xprt.  It can be found at
  ftp://sunsite.unc.edu/pub/Linux/system/printing.

  11.5.  Can Linux Configure The Serial Devices Automagically?

  Yes.  To get Linux to detect and set up the serial devices
  automatically on startup, add the line:

       /sbin/setserial /dev/ttyS3 auto_irq skip_test autoconfig

  to your /etc/rc.d/rc.local or /etc/rc.d/rc.serial file.  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.

  11.5.1.  Notes For Multiport Boards

  For board addresses, and IRQs, look at the rc.serial that comes with
  the setserial program.  It has a lot of detail on multiport boards,
  including I/O addresses and device names.

  11.6.  Using A Serial Console

  There is an article in issue 36 of the Linux Journal,
  http://www.ssc.com/lj/issue36/index.html that explains how to use a
  serial console.  Unfortunately, email to the author bounces.  I hope
  that the article will be on the Linux Journal web site at the above
  URL soon.
  11.7.  Higher Serial Throughput

  If you are seeing slow throughput and serial port overruns on a system
  with (E)IDE disk drives, you can get hdparm.  This is a utility that
  can modify (E)IDE parameters, including unmasking other IRQs during a
  disk IRQ.  This will improve repsonsiveness and will help eliminate
  overruns.  Be sure to read the man page very carefully, since some
  drive/controller combinations don't like this and may corrupt the
  filesystem.

  Also have a look at a utility called irqtune that will change the IRQ
  priority of a device, for example the serial port that your modem is
  on.  This may improve the serial throughput on your system.  The
  irqtune FAQ is at http://www.best.com/~cae/irqtune.

  12.  One Step Further...

  This section is not required reading, but may give you some further
  insight into Unix, and the world of telecommunications.

  12.1.  What Are Lock Files?

  Lock file are simply a file saying that a particular device is in use.
  They are kept in /usr/spool/uucp, or /var/lock.  Linux lock files are
  named LCK..name, where name is either a device name, or a UUCP site
  name.  Certain processes create these locks so that they can have
  exclusive access to devices, for instance if you dial out on your
  modem, a lock will appear telling other processes that someone is
  using the modem already.  Locks mainly contain the PID of the process
  that has locked the device.  Most programs look at the lock, and try
  to determine if that lock is still valid by checking the process table
  for the process that has locked the device.  If the lock is found to
  be valid, the program (should) exit.  If not, some programs remove the
  stale lock, and use the device, creating their own lock in the
  process.  Other programs just exit and tell you that the device is in
  use.

  12.2.  ``baud'' Vs. ``bps''

  ``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!

     baud
        The baud rate is a measure of how many times per second a
        signal, for instance one sent by a modem (modulator-demodulator)
        changes. For example, a baud rate of 1200 implies one signal
        change every 833 microseconds. Common modem baud rates are 50,
        75, 110, 300, 600, 1200, and 2400. Most high speed modems run at
        2400 baud. Because of the bandwidth limitations on voice-grade
        phone lines, baud rates greater than 2400 are harder to achieve,
        and only work under very pristine phone line quality.  Multiple
        bits can be encoded per baud, to get bit rates that exceed the
        baud rate. ``baud'' is named after Emile Baudot, the inventor of
        the asynchronous telegraph printer.

     bps
        The bps rate is a measure of how many bits per second are
        transmitted.  Common modem bps rates are 50, 75, 110, 300, 1200,
        2400, 9600, ...  115200. Using modems with V.42bis compression
        (max 4:1 compression), theoretical rates up to 115200 bps are
        possible.  This is what most people mean when they misuse the
        word ``baud''.

  So, if high speed modems are running at 2400 baud, how can they send
  14400 bps?  The modems achive a bps rate greater than baud rate by
  encoding many bits in each signal change, or phase change. Thus, when
  2 or more bits are encoded per baud, the bps rate exceeds the baud
  rate. If your modem connects at 14400 bps, it's going to be sending 6
  bits per phase change, at 2400 baud.

  How did this confusion start? Well, back when 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.

  12.3.  What Are UARTs?  How Do They Affect Performance?

  UARTs (Universal Asyncronous Receiver Transmitter) are chips on your
  PC serial board.  Their purpose is to convert data to bits, send the
  bits down the serial line, and then rebuild the data again on the
  other end.  UARTs deal with data in byte sized pieces, which is
  conveniently also the size of ASCII characters.

  Say you have a terminal hooked up to your PC.  When you type a
  character, the terminal gives that character to it's transmitter (also
  a UART).  The transmitter sends that byte out onto the serial line,
  one bit at a time, at a specific rate.  On the PC end, the receiving
  UART takes all the bits and rebuilds the byte and puts it in a buffer.

  There are two different types of UARTs.  You have probably heard of
  dumb UARTs - the 8250 and 16450, and FIFO UARTs - the 16550A.  To
  understand their differences, first let's examine what happens when a
  UART has sent or received a byte.

  The UART itself can't do anything with the data, it just sends and
  receives it.  The CPU gets an interrupt from the serial device every
  time a byte has been sent or received.  The CPU then moves the
  received byte out of the UART's buffer and into memory somewhere, or
  gives the UART another byte to send. The 8250 and 16450 UARTs only
  have a 1 byte buffer.  That means, that every time 1 byte is sent or
  received, the CPU is interrupted.  At low rates, this is OK.  But, at
  high transfer rates, the CPU gets so busy dealing with the UART, that
  is doesn't have time to tend to other tasks.  In some cases, the CPU
  does not get around to servicing the interrupt in time, and the byte
  is overwritten, because they are coming in so fast.

  That's where the 16550A UARTs are useful.  These chips come with 16
  byte FIFOs.  This means that it can receive or transmit up to 16 bytes
  before it has to interrupt the CPU.  Not only can it wait, but the CPU
  then can transfer all 16 bytes at a time.  Although the interrupt
  threshold is seldom set at 16, this is still a significant advantage
  over the other UARTs, which only have the 1 byte buffer.  The CPU
  receives less interrupts, and is free to do other things.  Data is not
  lost, and everyone is happy.  (There is also a 16550 UART, but it is
  treated as a 16450 since it is broken.)

  In general, the 8250 and 16450 UARTs should be fine for speeds up to
  38400 bps.  At speeds greater than 38400 bps, you might start seeing
  data loss.  Other PC operating systems (definition used loosely here),
  like DOS aren't multitasking, so they might be able to cope better
  with 8250 or 16450s.  That's why some people don't see data loss,
  until they switch to Linux.
  Non-UART, and intelligent multiport boards use DSP chips to do
  additional buffering and control, thus relieving the CPU even more.
  For example, the Cyclades Cyclom, and Stallion EasyIO boards use a
  Cirrus Logic CD1400 RISC UART, and many boards use 80186 CPUs or even
  special RISC CPUs, to handle the serial I/O.

  Keep in mind that these dumb UART types are not bad, they just aren't
  good for high speeds.  You should have no problem connecting a
  terminal, or a mouse to these UARTs.  But, for a high speed modem, the
  16550A is definitely a must.

  You can buy serial boards with the 16550A UARTs for a little more
  money, just ask your dealer what type of UART is on the board.  Or if
  you want to upgrade your existing board, you can simply purchase
  16550A chips and replace your existing 16450 UARTs.  They are pin-to-
  pin compatible.  Some boards come with socketed UARTs for this
  purpose, if not you can solder.  Note that you'll probably save
  yourself a lot of trouble by just getting a new board, if you've got
  the money, they are under US$50.

  13.  Troubleshooting

  13.1.  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.

  13.2.  When I Try To Dial Out, It Says ``/dev/cuaN: Device or resource
  busy''

  This problem can arise when DCD or DTR are not set correctly.  DCD
  should only be set when there is an actual connection (ie someone is
  dialed in), not when getty is watching the port.  Check to make sure
  that your modem is configured to only set DCD when there is a
  connection.  DTR should be set whenever something is using, or
  watching the line, like getty, kermit, or some other comm program.

  Another common cause of ``device busy'' errors, is that you set up
  your serial port with an interrupt already taken by something else.
  As each device initializes, it asks Linux for permission to use its
  hardware interrupt.  Linux keeps track of which interrupt is assigned
  to whom, and if your interrupt is already taken, your device won't be
  able to initialize properly.  The device really doesn't have much of
  any way to tell you that this happened, except that when you try to
  use it, it will return a ``device-busy'' error.  Check the interrupts
  on all of your boards (serial, ethernet, SCSI, etc.).  Look for IRQ
  conflicts.

  13.3.  I Keep Getting ``Id SN respawning too fast: disabled for 5 min¡
  utes''

  Make sure your modem is configured correctly.  Look at registers E and
  Q.  This can occur when your modem is chatting with getty.

  Make sure you are calling getty correctly from your /etc/inittab.
  Using the wrong syntax or device names will cause serious problems.

  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 ``getty Or uugetty Still Doesn't Work''.

  13.4.  Serial Devices Are Slow, Or Serial Devices Can Only Send In One
  Direction

  You probably have an IRQ conflict.  Make sure there are no IRQs being
  shared.  Check all your boards (serial, ethernet, SCSI, etc...).  Make
  sure the jumper settings, and the setserial parameters are correct for
  all your serial devices.  Also check /proc/ioports and /proc/interrups
  for conflicts.

  13.5.  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.  I
  saw my RD and SD LEDs go crazy when this happened to me.  You need to
  have your modem reset.  Most Hayes compatible modems do this with &D3,
  but on my USR Courier, I had to set &D2 and S13=1.  Check your modem
  manual.

  13.6.  I Have My Terminal Connected To My PC, But After I Type In A
  Login Name, It Just Locks Up

  ╖  If you are using getty: You probably don't have CLOCAL in your
     /etc/gettydefs entry for the terminal, and you're probably not
     using a full null modem cable.  You need CLOCAL, which tells Linux
     to ignore modem control signals.  Here is what it should look like:

       # 38400 bps Dumb Terminal entry
       DT38400# B38400 CS8 CLOCAL # B38400 SANE -ISTRIP CLOCAL #@S @L login: #DT38400

       # 19200 bps Dumb Terminal entry
       DT19200# B19200 CS8 CLOCAL # B19200 SANE -ISTRIP CLOCAL #@S @L login: #DT19200

       # 9600 bps Dumb Terminal entry
       DT9600# B9600 CS8 CLOCAL # B9600 SANE -ISTRIP CLOCAL #@S @L login: #DT9600

  Next, kill the getty process so a new one will be spawned with the new
  entry.

  ╖  If you are using agetty: Add the -L flag to the agetty line in your
     /etc/initab.  This will make it ignore modem control signals.  Then
     restart init by typing init q.  The entry should look like this:

       s1:345:respawn:/sbin/agetty -L 9600 ttyS1 vt100

  13.7.  At High Speeds, My Modem Looses Data

  If you are trying to run your modem faster than 38400 bps, and you
  don't have 16550A UARTs, you should upgrade them.  See section ``What
  Are UARTs?'' about UARTs.

  13.8.  On Startup, Linux Doesn't Report The Serial Devices The Way I
  Have Them Configured

  This is true.  Linux does not do any IRQ detection on startup, 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.

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

       Jan 23 22:25:28 misfits vmunix: tty02 at 0x03e8 (irq = 4) is a 16550A

  when Linux boots.  You have to use setserial to tell Linux the IRQ you
  are using.  After Linux boots, you can look at the /proc/interrupts
  file, to see what IRQs are really configured.

  13.9.  rz And/Or sz Don't Work When I Call My Linux Box On A Modem

  If Linux looks for /dev/modem when you try to transfer files, look at
  /etc/profile, and /etc/csh.cshrc.  There are a bunch of aliases
  defined there on some distributions, most notably Slackware.  These
  aliases mess up the zmodem programs.  Take them out, or correct them.

  13.10.  My Screen Is Printing Funny Looking Characters

  This happens on virtual consoles when you send binary data to your
  screen, or sometimes on serial connections.  The way to fix this is to
  type echo ^v^[c.  For the control-character-impaired, thats:

       linux% echo <ctrl>v<esc>c

  13.11.  getty Or 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.

  14.  Other Sources Of Information

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

  ╖  Your modem manual

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

  ╖  PPP HOWTO: help with PPP

  ╖  Printing HOWTO: for setting up a serial printer

  ╖  Term HOWTO: everything you wanted to know about the term program

  ╖  UPS HOWTO: setting up UPS boxen connected to your serial port

  ╖  UUCP HOWTO: for information on setting up UUCP

  ╖  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.networking
               Networking and communications under Linux.
       comp.os.linux.setup
               Linux installation and system administration.

  ╖  The Linux serial mailing list.  To join, send email to
     [e-mail], 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.

  ╖  A white paper discussing serial communications and multiport serial
     boards is available from Cyclades at http://www.cyclades.com.

  ╖  Modem FAQs:
     Navas 28800 Modem FAQ
     <http://web.aimnet.com/~jnavas/modem/faq.html>
     Curt's High Speed Modem Page
     <http://www.teleport.com/~curt/modems.html>

  ╖  Serial programming: Advanced Programming in the UNIX Environment
     <http://heg-
     school.aw.com/cseng/authors/stevens/advanced/advanced.nclk>, by W.
     Richard Stevens (ISBN 0-201-56317-7; Addison-Wesley)
     <http://www.ora.com/catalog/posix/> NAME="POSIX Programmer's
     Guide">, by Donald Lewine (ISBN 0-937175-73-0; O'Reilly)

  15.  Contributions

  There was no possible way to write this HOWTO alone.  Although a lot
  of the HOWTO is my writing, I have rewritten many contributions to
  maintain continuity in the writing style and flow.  Thanks to everyone
  who has contributed or commented, the list of people has gotten too
  long to list (somewhere over one hundred).  Special thanks to Ted T'so
  for answering questions about the serial drivers, Kris Gleason who
  used to maintain getty_ps, and Gert D÷ring who maintains mgetty.

  END OF SERIAL HOWTO