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modem                 Technical Information                 modem




The word  _m_o_d_e_m is an  abbreviation for ``modulation/demodulation
device''.  With  the COHERENT system,  you can attach  a modem to
your computer either to dial out for remote communication, to let
others dial into your COHERENT system, or both.  With your modem,
too, you  can use COHERENT's  UUCP commands to  exchange mail and
files with  remote sites automatically, and  to download news and
files from the USENET.

This article gives a summary of how to connect your modem to your
computer  and  describe  it  to  the  COHERENT system.   It  also
discusses some problems that may  crop up when you attempt to use
your modem.

***** Internal vs. External Modems *****

You  can use  internal  and external  modems  with COHERENT.   An
internal modem  is, however, more difficult  to diagnose problems
because  there  are   typically  no  status  lights  to  indicate
operation.

***** Hooking up a Modem *****

A modem must be hooked up  to a serial port on your computer.  To
plug your  modem into the computer, simply  take a normal serial-
port cable, one with an  RS-232 plug of the appropriate gender at
each end,  plug one end  into your modem  and the other  into the
serial  port  you  wish  to  use.   The  Lexicon  article  RRSS-223322
describes the wiring of the RS-232 plug in detail; but if you are
not skilled with a soldering iron, you are well advised simply to
purchase a  cable from your  local electronics store  and be done
with it.

***** Serial Ports *****

The COHERENT system supports up to four serial ports; the devices
for these are named /ddeevv/ccoomm11rr through /ddeevv/ccoomm44rr. If you are not
sure which  port you  have plugged  your modem into,  perform the
following  test:  First,  turn  on  the  modem.  Then,  type  the
following command:


        echo FOO >/dev/com1l


If the ``TX'' light on the  modem blinks, then you know the modem
is plugged into  CCOOMM11. If it does not, try  the command again for
/ddeevv/ccoomm22ll,  and   so  on  through  ccoomm44ll   until  you  find  the
appropriate port.  If no  command works, check the wiring on your
cable and make sure that the plugs are securely inserted.

Once you have established  which port your modem is plugged into,
links the device /ddeevv/mmooddeemm to it, using the following command:




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modem                 Technical Information                 modem




        ln -f /dev/com?l /dev/modem


where ? gives the number of the port, 1 through 4.

***** Edit /etc/ttys *****

The next step is to edit  the file /eettcc/ttttyyss to tell COHERENT how
you want  the port handled.   You must know (1)  whether you want
the port enabled  or disabled; (2) the baud rate  of the port (as
set by your modem); and (3)  the name of the port (which you just
determined).

If  a port  is enabled,  remote  users can  log into  the system,
either via  a terminal  directly plugged into  the port or  via a
modem.  COHERENT sends a login prompt to every enabled port.  The
COHERENT system also  restricts permissions on all enabled serial
ports, so that only the superuser  rroooott can read and write to the
port.  This prevents other users who may be using the system from
accessing the  serial port.  If a port is  disabled, you can dial
out  or use  a direct-connect UUCP  connection via  that disabled
port.

To dial  out on an enabled  port, you must first  use the command
ddiissaabbllee to disable the port.  When you have finished dialing out,
run the command eennaabbllee to re-enable the port.  Before you can use
these commands  with a port, the port must  first be described in
the file /eettcc/ttttyyss.

See the Lexicon  article on ttttyyss for details on  how to edit this
file.   Note that  a modem  is a  remote device,  and must  be so
described in /eettcc/ttttyyss, or it will not work correctly.

After you have made your changes, type the command


        kill quit 1


to make COHERENT re-read /eettcc/ttttyyss and implement your changes.

***** Remote-Access Password *****

If you intend to let people dial into your computer, you are well
advised  to set  the remote-access  password.  This  will require
that people  who dial in  know a special password  in addition to
whatever password  their personal account may  have.  To set this
password, log in as the superuser rroooott; then enter the command








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modem                 Technical Information                 modem



        passwd remacc


This will  walk you through  setting a password  for user rreemmaacccc,
which is the remote-access account.

***** Edit /etc/modemcap *****

Next, you must check the  file /eettcc/mmooddeemmccaapp and see if that file
holds a description that matches your modem.  mmooddeemmccaapp is used by
a number  of programs to control access to  modems, and this file
comes with descriptions for many commonly used modems.  You find,
however,  that you  must  edit an  existing entry  to match  your
modem's  features exactly;  for example,  the existing  entry may
assume that you  have a Touch-Tone telephone whereas you actually
have a pulse telephone.   The Lexicon entry on mmooddeemmccaapp will walk
you through  this process.  When you  have completed editing this
entry, note it down, for you will need to insert it elsewhere.

***** Edit /usr/bin/modeminit *****

Next, check the  file /uussrr/bbiinn/mmooddeemmiinniitt. This program is read by
programs  that manipulate  the modem,  to re-initialize  it after
use.   You  must  decide  how  you  want  the  modem  to  be  re-
initialized.   Basically, if  you wish to  have people  dial into
your system, you turn on the modem's auto-answer feature; and you
must  turn off  echoing and  the printing  of result  codes.  The
commands  to  use  will   vary  from  modem  to  modem;  see  the
documentation that comes  with your modem for details.  See below
for details on modifying this script.

***** Edit L-devices *****

If you  intend to use  your modem with  UUCP, you must  insert an
entry for it  into your the file /uussrr/lliibb/uuuuccpp/LL-ddeevviicceess. See the
Lexicon entry LL-ddeevviicceess for details.

***** Modem Maladies *****

The rest of this article discusses problems that have arisen with
remote  login via  modem, as diagnosed  by the  technical support
staff of Mark Williams Company.

Difficulty in logging in from a  remote site via modem can be the
result  of problems  in one  or more  of the  following: cabling;
enabling/disabling  the  port;  flaws  in  the contents  of  file
/eettcc/ttttyyss; incorrect configuration  of the modem; and setting the
port to  an incorrect state.   See Lexicon articles  tteerrmmiinnaall and
UUUUCCPP  for  additional   information.   The  following  paragraphs
discuss the above-named items in detail.

***** RS-232 Cabling *****

When  attaching  an  external  modem  to  your  computer,  it  is
important  to  use  a  modem  cable  that supports  ``full  modem


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modem                 Technical Information                 modem



control''.   COHERENT   relies  on  modem-control   signals  when
operating a  modem for remote access  purposes.  When attaching a
terminal directly  to a serial port, a  ``null modem'' cable must
be used.   When attaching a  modem, a ``straight  through'' cable
must  be used.   See  Lexicon articles  RRSS-223322  and tteerrmmiinnaall  for
further details on cabling.

***** Enabled vs. Disabled Ports *****

A  serial  port can  be  either enabled  or  disabled for  remote
access.  Enabling  a port allows  a user on a  remote terminal or
modem to log into your COHERENT system.  Disabling a port permits
a user  to dial out or  use a direct connect  UUCP connection via
that disabled port.

If a  port is enabled  for remote logins  and you will  use it to
call out,  you must use  the command ddiissaabbllee to  disable the port
before  you  access   the  port.   The  commands  uuuuccpp  and  mmaaiill
automatically disable and re-enable a port.

The  port name  supplied  to an  eennaabbllee or  ddiissaabbllee command  must
_e_x_a_c_t_l_y match the last part of  a line in the /eettcc/ttttyyss file (see
below).   For example,  for the  command  eennaabbllee ccoomm22pprr  to work,
there  must be  an entry  in the file  /eettcc/ttttyyss which  ends with
ccoomm22pprr.

When a port  is enabled, (1) the first character  for the port in
file /eettcc/ttttyyss is set to a `1' (one), (2) the permissions for the
port are  changed so  that only the  superuser rroooott can  read and
write  to the  port, preventing  other users  on the  system from
accessing the port while a remote session is in progress, and (c)
a login prompt is sent to the port.

***** /etc/ttys *****

This file should  have permissions of 644 (-rw-r--r--) and belong
to owner  and group  rroooott. Review the  Lexicon entry for  ttttyyss to
ensure that the format of your version of /eettcc/ttttyyss is correct.

Leaving blanks at the end  of a line in /eettcc/ttttyyss usually results
in error messages stating that a device could not be found.

You do  not need  to edit  the initial `0'  or `1' in  entries in
/eettcc/ttttyyss;  this digit  is  updated by  the  commands eennaabbllee  and
ddiissaabbllee. See the Lexicon  entries for eennaabbllee and ddiissaabbllee for more
information.

Another problem  can arise when using  a variable-speed modem and
specifying  the  baud rate  as  `3' in  file  /eettcc/ttttyyss. The  `3'
setting represents a 2400-1200-300 baud variable-speed port.  The
problem appears  to be that while a modem  defaults to 2400 baud,
COHERENT may  actually be  set at one  of the other  speeds.  The
answer to this probem is to run the command mmooddeemmiinniitt.




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modem                 Technical Information                 modem



***** Constant Flickering *****

Another problem is a constant flickering of send/receive LEDs and
an unexplained  continual access of the  hard drive.  This occurs
when  the port  is enabled  and the  modem is  set in  echo mode:
COHERENT sends the login prompt to the modem, the modem echoes it
back to  COHERENT, COHERENT  then thinks  the modem is  trying to
talk to it and sends the password prompt, and so on _a_d _i_n_f_i_n_i_t_u_m.
To fix this problem, place  the modem into no-echo mode, and turn
off the display of result codes.  The following section discusses
this in more detail.

***** Modem Configuration *****

A modem  that fails to  answer an incoming call,  hangs up before
locking onto the remote  carrier, becomes stuck in a loop echoing
characters sent  to it from the computer, or  fails to operate at
the  expected baud  rate probably  is configured  improperly.  To
remedy this situation, send the appropriate control string to the
modem.

We  offer some  guidelines here for  modem settings.   Be warned,
however, that modems  from different manufacturers usually behave
differently,  regardless of  claims of  Hayes  compatibility, and
that you will need to check the manual for your particular modem.

-> Echo should be OFF (usually by setting ``E0'').

-> Result codes should be OFF (usually by setting ``Q1'').

-> Modem status ``DCD'' should follow true carrier detect status,
   rather than being always on (usually by setting ``&C1'').

-> Auto answer should be ON (usually obtained by setting register
   S0  to a  nonzero value  equal to the  number of  rings before
   answer).

-> The delay  value for  ``Wait for Carrier/Dial  Tone'' (usually
   register S7) should not be too short.

The  scripts  below  show  typical  initialization  for  ``Hayes-
compatible''  and Trailblazer  modems.  They  are  only examples;
your modem  may need something  different.  Please note  that the
commands sslleeeepp  and ssttttyy  are necessary  in the first  example so
that the command  string will be sent to the  modem at 2400 baud;
otherwise, the string is sent at the default port speed, which is
9600 baud.


        # initialize 2400-baud Hayes-compatible modem
        disable com3r
        sleep 3 > /dev/com3l &
        stty 2400 > /dev/com3l
        echo 'AT E0 Q1 V0 S0=1 &C1 M3' > /dev/com3l
        sleep 3


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modem                 Technical Information                 modem



        enable com3r



        # initialize 9600 baud internal Trailblazer on com2
        /etc/disable com2r
        cat > /dev/com2l << EOF
        at
        at e0 t v0 x3 h0
        at s0=1 s7=60 s48=1 s51=252 s52=0 s54=3 s58=2
        at
        EOF
        /etc/enable com2r


You can edit the  file /uussrr/bbiinn/mmooddeemmiinniitt to suit your modem.  To
ensure  that  your  modem is  initialized  every  time you  start
COHERENT, you should add a line saying


        /usr/bin/modeminit


to your copy of the file /eettcc/rrcc.

***** See Also *****

mmooddeemmccaapp, RRSS-223322, tteecchhnniiccaall iinnffoorrmmaattiioonn, tteerrmmiinnaall

***** Notes *****

One  final  bit  of  hard-won  wisdom:  once you  have  something
working, write down  what you did, and store it  in a place where
you won't lose it.  It makes life easier just knowing that you're
looking for a female-to-female cable instead of male-to-female or
male-to-male.





















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