10 Oct
2020
10 Oct
'20
5:35 a.m.
Den l?r 10 okt. 2020 kl 13:23 skrev Stefan Skoglund via cctalk <
cctalk at classiccmp.org>:
ons 2020-10-07 klockan 14:08 -0400 skrev Paul Koning
via cctalk:
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.
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...
BSC has the concept of escape character. They use the DLE which is 0x10 in
EBCDIC.
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
/Mattis
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