break on pdp11 serial ports
Noel Chiappa
jnc at mercury.lcs.mit.edu
Fri Mar 30 15:09:03 CDT 2018
> 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
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