ASR-33 Data Format?

From: Bill Yakowenko <yakowenk_at_cs.unc.edu>
Date: Wed Mar 31 17:31:11 1999

] Having secured a copy of Tom Pittman's Tiny BASIC on paper tape
] (autographed by the man himself!), I'm in the process of adding a
] teletype simulator to the COSMAC Elf simulator. It's a bit strange

Without knowing anything much about ASR-33's in particular, but
having done bit-banging serial stuff before, here are my best
guesses.

Do you have any delay on the recieving end that could make it
miss the beginning of the next byte? The reason for the stop
bit(s) was to give the reciever time to do something with the
byte that came in, before the next byte started arriving. If
the reciever needs more processing time, then the sender should
include more stop bits. (In the real world, I've never heard
of needing more than two stop bits.)

Also, bits typically don't have any space between them. So,
for instance, you should not return to '0' between sending two
'1' bits. Between bytes, a reciever will typically watch for
a falling edge to know when a start bit has begun. So if you
return to zero even for an instant between two stop bits, the
reciever may interpret that as the start bit of the next byte.

And of course, between bytes, the line has to remain at '1',
or risk looking like a start bit.

        Good luck!
        Bill.
Received on Wed Mar 31 1999 - 17:31:11 BST