Need help with 3b1 modem control registers

[Jonathan Clark]

In article <631 at oswego.UUCP> ostroff at oswego.UUCP (Boyd Ostroff) writes:
>Specifically, some of my users at the University of Rochester are unable to
>establish a good 1200 baud connection - typically they get "bracket" 
>characters and after a few minutes lose the carrier completely.  Evidently
>the source of the problem lies in poor-quality long distance lines, since
>I frequently call the system at 1200 baud without any problem.  Part of the
>problem may also be related to their phone system which consists of an
>analog phone line from the user's terminal connected to a Concord series 2
>modem (part of a modem-pool).

This sounds very similar to a problem which we found the hard way.
The SCM chip used in the unix pc has a very low tolerance for shaved
stop bits - it insists on having at least one full stop bit. The symptoms we
saw were that a uucp transfer would start up fine, then time out completely
and die. Typically no recovery was possible, and if a transfer did succeed
then the throughput would be atrocious (6 bps was typical). For complex
reasons terminal emulation worked perfectly, it was just uucp transfers
which died (and ?MODEM transfers too).

We eventually fixed the problem by putting another modem pool which was
specially tweaked to provide a full stop bit.

However, the business with the "bracket" characters (typically '{' and '}')
sounds like extreme line noise, possibly coupled with the notorious T-1
sync slip probem (if the users are seeing '}i' pairs). There is nothing I
can think of which will correct this problem.

>Can the OBM really work at 2400 baud?

No. Or rather, not without help. Some other AT&T products use the SCM chip
and work at 2400 Baud, but not the unix pc. Sorry.
-- 
Jonathan Clark		jonathan.clark at mtune.att.com, attmail!jonathan
Any affiliation is given for identification purposes only.

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