Motivation
One of the key features of LINX data terminals is the ability to use a modem to dial into
remote terminals and collect stored data. Because the RS232 serial communications
standard limits the length of a serial cable to 100 feet, the use of modems to dial into
more remote terminals greatly increases the physical area of the network. Obviously, it is
a tremendous advantage to have this option available to network planners. Unfortunately,
some people have been having problems obtaining good connections with recent
modems. This article will attempt to alleviate some of the confusion surrounding this
feature.
In principle, the concept is simple. Rather than have a terminal attached to a computer by
way of a serial cable, we introduce two modems into the system. When it comes time for
Topsail to poll the terminal, it uses the PC’s modem to call the modem of the terminal.
Once the connection is established, the communication channel exists just as it had for
the system with only the serial cable.
The problem many developers face is that the connection between the two modems is not
easily established. To make matters worse, sometimes the modems appear to connect, but
the expected communication is still not present. In most cases, the difficulty is due to an
improper or inconsistent baud rate set on the modem. However, the larger issue is
configuring the modem in general, which will be addressed here.
Theory and Background
As stated before, the principle problem resides setting the modem baud rates, or rather
setting the baud rates and keeping them set. LINX recommends a maximum RS232
communication rate of 9600 baud using the Request/Response protocol. This should be
the maximum speed between the terminal and the modem connected to it.
While it is possible to have the rest of the communication lines operating at a faster
speed, it is not recommended since the buffering required can throw the timing of higher
protocols out of sync. For this reason, the best course of action is to set all of the
communication segments to the same value.
The problems arise with the creation of modern modems. The current versions of Topsail
and LINX Terminals have been designed to comply with the Hayes Compatible AT
command set for modem configuration. However, due to a shift in the technical
standards many modern modems are not fully Hayes compatible anymore. Rather, they
support some small subset of the Hayes command set, and the Microsoft Unimodem
command set. (Even Hayes has reportedly abandoned portions of their protocol.) Some
modem manufacturers have taken liberties with modem auto-configuration, that are not
available with fully Hayes-Compatibility. Consequently, modern modem behavior is
sometimes difficult to anticipate.
(155 pages)
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