[TUHS] Teletype

Noel Chiappa jnc at mercury.lcs.mit.edu
Sun Aug 17 00:35:05 AEST 2014

    > From: John Cowan <cowan at mercury.ccil.org>

    >> if you're dialing up, you need to find a dial-up port that supports
    >> 110 baud.

    > I dialed up The World's local dialup line for my area, and heard a
    > large variety of tones including Bell 103-compatible FSK, which is 300
    > baud. I suspect that anything that can do Bell 103 can fall back to
    > Bell 101, which was 110 baud.

There are two more things one needs to have for the port to support 110: i)
the serial interface needs to support 110 (even if the modem is integrated
with the serial hardware on one board, the serial hardware might not do 110),
and ii) the software needs to be willing to go 110.

I don't know anything about how contemporary dial-up ports work, so maybe
there's some side-channel from the modem to the interface which allows the
software to find out directly what speed the modem is using. However, 'back in
the day' with multi-speed ports, there was no such mechanism (the RS-232
interface spec didn't provide for speed indication), and one had to hit BREAK
and the serial line device driver would see that, and try the next speed in a
list. You can still see this in the big table of terminal types in getty.c,

  /* table '0'-1-2 300,150,110 */

which tried 300, 150, 110. So if the software isn't looking for 110...


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