7 Oct
2020
7 Oct
'20
11:08 a.m.
On Oct 7, 2020, at 11:00 AM, Ethan Dicks
<ethan.dicks at gmail.com> wrote:
...
Not flags, that's an HDLC concept. Bisync uses sync characters (as DDCMP does) but
instead of doing framing by byte counts it does it by a frame terminator, and for
transparency if that occurs inside the data it has to be escaped.
Bisync is usually associated with older IBM protocols like 2780, but it's occasionally
found elsewhere. One of my nightmare memories is debugging the communication between a
PDP-11/70 running Typeset-11 (on IAS) and a Harris 2200 display advertising graphics
editing workstation. That runs Bisync, half duplex, multipoint, with modem control, on an
async comm link -- DL11-E devices at the PDP-11 end. Yikes. At our customer site in
downtown Philadelphia, it tended to lock up, but only during the "lobster shift"
-- midnight to 8 am.
I don't really know anything about that particular protocol beyond what I just
mentioned, but I'm fairly sure it didn't have anything to do with IBM products.
paul
At least in the 2780 emulation case, where BISYNC
protocol handling is in software and the serial ports just are raw byte pipes.
Yes, Bisync is just a stream of raw bytes, but ISTR the COM5025 (the
common 1970s USART for this purpose) might throw in flag bytes or
something minor that the application doesn't know about or see, but
the other end sees in the received byte stream.