News Group Posts

Modem enabled on non-modem port

Back in the bad old days, modems were pretty darn important. While there are still a lot of modems in use for dial-out (to connect to credit card processors etc.), dial-in modems on servers are vanishing quickly.

Three issues represented most of the modem configuration problems on SCO systems:

• Confusing the modem control and non-modem control ports
• Not setting the modem and the system to fixed DTE speeds
• Not understanding the relationship of getty and /usr/lib/uucp/Devices

A lot of this is covered in detail at Configuring high speed modems.

SCO Modem control ports are designated with an upper case letter: /dev/tty1A is COM1 with modem control, and /dev/tty1a is COM1 for direct connections like terminals. People would sometimes screw up and have getty's running on both of these, which caused confusion and unhappy results.

See Answer light on modem- ats0, which discusses how this works.

Setting the modem and the inittab entry to a fixed DTE speed was another area of misunderstanding. Since the modem could talk to another modem at any speed the two agreed upon, people thought they needed cycling speeds for getty. Cycling speeds are supported, but are not necessary or desired. Finally, after a hangup, getty resets the line speed to the speed indicated by anything it finds in /usr/lib/uucp/Devices that matches the port it is on, so that speed needs to match.

This was in reference to:

Date: 16 Mar 1999 23:10:35 -0500
Message-ID: http://groups.google.com/groups?selm=36F06F00.6DD46F3E%40mindspring.com&output=gplain

The newsgroup article referenced suggests modem misconfiguration, specifically guessing

.. echo enabled on a non-modem controlled port or with CD
wired up (or configured up in the modem) and login is engaged
in a shouting match with the modem
 

Modem configuration, whether dial-in or dial-out, used to be a very big deal, but it got easier as modems matured. Out of the box, more modern modems would "just work" for dial-out, and all that was necessary for dial-in was to enable auto-answer, which you could usually do with external dip switches. DTE speeds were preset to 57600; all you needed to do was set inittab and /usr/lib/uucp/Devices to match.

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