Xref: utzoo comp.dcom.modems:5015 comp.mail.uucp:3885
Newsgroups: comp.dcom.modems,comp.mail.uucp
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: an echo on the line
Message-ID: <1990Jan2.235724.7287@utzoo.uucp>
Organization: U of Toronto Zoology
Date: Tue, 2 Jan 90 23:57:24 GMT

A little while ago, we were having trouble talking to one of our uucp
neighbors.  Some investigation turned up a bizarre problem:  when talking
to their dial-in modem, everything we sent got echoed!  It wasn't the
host doing it, because during the login sequence, at times when echoing
should be on, *double* echos occurred.  It happened when signed on from
a terminal, too, so it was definitely at their end.  Otherwise, everything
was fine; characters *were* getting through.

I couldn't think of any reason why this should be happening.  Many modems
have an option to echo on the RS232 line, for use with dumb terminals, but
not on the phone line.  Some have a remote command mode, but this was
happening in normal data mode.  There were good reasons for suspecting
that it wasn't the host or the interface.  Clutching at straws, I suggested
a close inspection of the RS232 cable.  Turned out the cable was half-
unplugged at the modem end, and this was the problem.  My guess is that
it was far enough out for the ground wire, pin 7, to be disconnected.
I don't entirely understand how that would give the observed symptoms,
but I do know that a broken ground line can do bizarre things.

Uucp, incidentally, responded very badly to this.  We run a fairly-recent
HDB (aka BNU) version.  Despite the unexpected echoing, it successfully
fought its way through login and protocol startup!  When it asked to
send a file, it saw the echo of the request as the response to it, noted
that this was a request and not a response, and promptly had a cardiac
arrest.  It fell over with a very uninformative assertion-blown message.
-- 
1972: Saturn V #15 flight-ready|     Henry Spencer at U of Toronto Zoology
1990: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu