In the case of baud rate fixing, this has been a particularly sticky problem. Some
modems will automatically negotiate to a lower baud rate if a line is deemed to noisy for
9600 baud. Normally this would only be a problem for a single session, except that the
modem may decide that only the lower baud rate is now acceptable, and will never again
revert to the original setting of 9600.
Dual purpose modems, used for Topsail communications as well as other
communications (such as dialing into the local ISP), have also expressed symptoms
related to this problem. When the modem is used for ISP communications, the Windows
modem drivers generally don’t respect any saved settings that Topsail has used.
Consequently, the modem must be fully reinitialized by Topsail. (Something that recent
versions of Topsail can be configured to do.)
The solution that we have for these and other related problems is fairly simple: Never
assume the modem is configured correctly before you dial. Always issue the modem its
initialization string before attempting to use it. On the PC side, simply provide Topsail
with the appropriate communication string. On the terminal side, it is trickier; as the
program running on the terminal must be “aware” of the modem, and send the
initialization string itself.
Hardware Requirements
The modem that LINX recommends is the Multi-Tech MT2834BL 33.6Kbps Intelligent
Data/Fax Modem. Although any modem that is fully Hayes compatible should work,
these are difficult to find. It is reasonable to assume that any modem constructed after
2001 is not Hayes compatible, regardless of the marketing claims. If unsure, contact your
modem manufacturer.
For the purposes of this document the modem commands used have been tested on the
MT2834BL. If you want to attempt to use a different brand, consult its documentation to
determine the necessary configuration commands.
The initialization string used on the MT2834BL is “AT$SB9600$MB9600Q2&W0”.
This breaks down into the following commands:
AT - Attention command prefix
$SB9600 - Set the serial port to use 9600 baud
$MB9600 - Set the modem connection to use 9600 baud
Q2 - Use Dumb Answer Mode (don’t negotiate baud rate)
&W0 - Store the current settings in static ram (in case, of reboot)
Other modems may require other commands to be added to this string as necessary. For
instance, if your modem has replaced dipswitches with software configuration, it may be
necessary to insure other parameters remain constant as well. These parameters may
include, but are not limited to: Parity, Data Bits, Stop Bits, RTS/CTS, and Auto-Answer
mode.
(155 pages)
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