Date   : Mon, 02 Dec 1985 18:57:00 EST (Mon)
From   : "Leonard N. Foner" <FONER%MIT-OZ@MIT-MC.ARPA>
Subject: Hayes Test Pattern

I hesitate to mention this, because I'm sure someone's already said it
and I haven't seen it yet.  But...

That UUUUU pattern you see is what happens when a modem which
communicates in dibits (as does the Hayes) loses carrier.  [Thus your
1200 bits-per-second modem is actually 600 baud, since each change of
signal corresponds to a cartesian product of two one-bit quantities,
(generally either phase and frequency or phase and amplitude---I don't
recall which---each have two possible states, so you can combine them
four ways to get a dibit) which means each change of signal can
transmit two bits simultaneously.  Anyway, 'nuff on that.]

ASCII U is alternating ones and zeros, and the "no-carrier" state
looks like the dibit for 01.  Thus, you get a few UUU's and then the
modem recognizes it has lost carrier.  My VA3451 does that whenever
the foreign system drops carrier on logout, for example.

As for *why* one modem sees carrier dropped, I can't help you.

                                               <LNF>