break on pdp11 serial ports

[Noel Chiappa]

    > From: Sytse van Slooten

    > digging through the documentation of KL-11 and DL-11 I did find
    > references to generating a break (bit 0 in the XCSR). But not on how it
    > would be received. ... How did a DL-11 like interface signal the
    > reception of a break?

As JohnW says, framing error. FWIW, the UART chips used back then actually
produce a 'framing error' output, which is sent straight into that bit in the
RCSR.

    > And how did the operating systems and software deal with it? Was it
    > actually used at all?

Different systems used it for different things.

Unix V6 used 'break' on dial-up lines as the signal to switch speeds when you
first connected up - it would try 110, then 150, then 300. (Later this got
extended, I expect - too lazy to check.)

I see the hacked PWB1 Unix at MIT used it to send an interrupt:

       if (c & FRERR) {
                signal(tp->t_pgrp, SIGINT);
                return;
                }

That's as far as my knowledge extends, others may know of more uses.

	Noel