Remote job submission from PDP-11

Mattis Lind mattislind at
Sat Oct 10 07:35:16 CDT 2020

Den lör 10 okt. 2020 kl 13:23 skrev Stefan Skoglund via cctalk <
cctalk at>:

> ons 2020-10-07 klockan 14:08 -0400 skrev Paul Koning via cctalk:
> >
> > 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.
> Bit stuffing ? ie if the payload contains a sequence which is reserved
> add a an escape for example an A after three consecutive spaces.

 Bitstuffing is used in HDLC and SDLC and is where you insert a 0 after
five consecutive ones. That is to differentiate it from the flag which has
a zero followed by six ones and then yet another zero.

> ON the receiving end remove the A, if it came after three spaces.
> Four consecutive spaces in the wire stream, that is a frame marker...

BSC has the concept of escape character. They use the DLE which is 0x10 in

BSC can operate in transparent and non-transparent mode. Transparent mode
text is initiated with DLE STX while  non-transparent text is just STX.

> >
> > Bisync is usually associated with older IBM protocols like 2780, but
> > it's occasionally found elsewhere.

BSC was widely used to connect various IBM terminals like 2260 and 3270.
BSC has a polling concept with the ENQ character which is used to poll
terminals on a shared line for data to send.

> 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
> >
> >


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