autobauding

[Dave Buck]

If nothing went to the getty, it would not be able to see a framing
error and correct the baud rate. So, something must therefore be
received by getty. You don't believe this idea that the terminal
x-off's or x-on's after receiving 2-3 characters, do you?
After all, the terminal is receiving a few framing errors and characters
at 300 baud, when getty starts talking at 1200 or 2400 (or so), so
getty's "login:" string is less than 2 characters (add more for your
favorite greeting line preceding login, but divide the character count
by the speed ratio). I don't know of any terminals that x-off at
300 baud upon receipt of 1-2 characters, unless those characters scroll,
clear the screen, insert a line, etc. Even then, at 300 baud I've not
seen it!

Back to my original point, in case some people were confused ... my
message was not totally clear.
In many cases, the modem to which the getty is attached sends
a connected message, not to the partner modem, but to the DTE to
which it is connected. Wanna see it? Well, then, connect a data
link analyzer between the getty port and the modem, and watch what
happens when the connection is established. Getty, upon completing
the open (DCD becomes asserted, for most tty drivers), sends out the
greeting and "login:" lines. Getty wasn't watching the line, but
something like "CONNECTED 300" was being received from the modem
as it was talking. It then tries to read a response, and finds some
framing errors in the receive queue since it was expecting 1200 (or
whatever it started with). It cycles the speeds until it gets the
right one.

To prove to myself that this is true in practice in addition to theory,
I just connected my ancient large-pocket-sized terminal (300 baud) to
my computer system, with a comm analyzer set to 300 baud, and had getty
set to cycle from 2400 to 1200 to 300. I have a Racal Maxwell modem,
but I've seen similar stuff on other modems. The modem sent on the
RxData line to getty "CONNECTED 300 (CR and LF, I think)", and the
analyzer saw getty's 2400 baud transmission as a couple of characters
at 300. I had to hit return once on the terminal to get the speed changed
to 1200, another time for 300, so I wasn't as luck as Henry.
But the behavior explains Henry's original question.
--
Dave Buck (408)972-2825 da...@dlb.BUCK.COM, {amdahl,sun}!dlb.UUCP!dave
D.L.Buck&Assoc.,Inc. 6920 Santa Teresa Blvd. San Jose, Calif.95119