Remote job submission from PDP-11

Paul Koning paulkoning at
Wed Oct 7 13:08:59 CDT 2020

> On Oct 7, 2020, at 11:00 AM, Ethan Dicks <ethan.dicks at> wrote:
> ...
>> 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. 

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.


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