10 Oct
2020
10 Oct
'20
4:11 a.m.
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.
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...
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