Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10 beta 3/9/83; site sdcrdcf.UUCP
Path: utzoo!watmath!clyde!floyd!harpo!ihnp4!zehntel!hplabs!sdcrdcf!darrelj
From: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Newsgroups: net.micro
Subject: Re: Hayes 1200 Problems
Message-ID: <984@sdcrdcf.UUCP>
Date: Tue, 10-Apr-84 11:34:35 EST
Article-I.D.: sdcrdcf.984
Posted: Tue Apr 10 11:34:35 1984
Date-Received: Fri, 13-Apr-84 06:08:21 EST
References: <301@iuvax.UUCP>
Reply-To: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Organization: System Development Corporation, Santa Monica
Lines: 25

First note that the bit pattern in 'U' is 01010101.  What can happen is that
since the modem actually sends of four states for PAIRS of bits, noise can 
occasionally get one of the modems out of proper sync with the data stream.
Another thing which can happen is that the Bell 212 protocol includes a way
to put the far end modem into a loop-back self-test mode (and occasionally
noise will mimic the sequence).  In fact this just
happened to me while reading about other 1200 bps problems (symptom: first,
about 20 Us, then remote end started loopback, echoing exactly what I typed,
notable in that RETURN comes back without the LINEFEED that unix adds), and
I fixed it on my old Prentice 212 by turning the Remote-Digital-Loop control
on and off, taking the remote modem out of loopback mode.  Or course most of
the new cheap 212 compatibles save some of the money by taking out all the
switches (the prentice has 5 on front panel, ~40 internal, plus 10 LEDs and
output level programmable with an external (TELCO provieded) resistor).
The cheap modems will work in 95% of applications and on 95% of phone lines
instead of all applications and 99% of lines.  Generally, an acceptible
tradeoff.
-- 
Darrel J. Van Buer, PhD
System Development Corp.
2500 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,sdccsu3,trw-unix}!sdcrdcf!darrelj
VANBUER@USC-ECL.ARPA