T.MOORE%MIT-EECS@MIT-EDDIE.ARPA (Andrew Moore) (12/01/85)
I think there was a discussion on this before, but I have noticed the problem for the first time on my Hayes recently. When connected to a system, the modem will (at random) send out "UUUUUUUUUU"'s and go offline. I can resolve this by quickly unplugging the phone line and plugging it in fast enough to keep the connection -- the modem is back to normal then and I am returned to the system. I can only suppose that this is some sort of test pattern for the Hayes. I am using MEX usually when this happens, but it has occurred once or twice in another terminal program as well. Any explanations and/or solutions would be appreciated; mail me directly, I am not on info-cpm. Thanks. -drew -------
Wilkinson@HI-MULTICS.ARPA (12/02/85)
I have experienced the same phenomenon on my Pop-Comm X100 external
modem which is Hayes compatible (I suspect that is extraneous) when
connected over local city lines to our main-frame host (I am on an IBM
PC.XT running QMODEM. f you get an expanation I would apprecite hearing
it. How about a bug in a modem chip that they both use?
Richard Wilkinson {Wilkinson@HI-MULTICS.ARPA}FONER%MIT-OZ@MIT-MC.ARPA (Leonard N. Foner) (12/03/85)
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>