🚀 go-pugleaf

I have an application talking IBM 2780 BSC using a Zilog Z8530 SCC
chip, and I can't seem to work out which bytes to include in the
CRC calculation.

I am using transparent mode so a packet looks like:

	<SYN><SYN><DLE><STX>[ data ... ]<DLE><ETX><BCC1><BCC2>

where data has any actual <DLE> characters escaped as <DLE><DLE>
and may also contain some <DLE><SYN> time spacing characters which
drop out. My problem is that I can't find any references on exactly
which of these bytes contribute to the CRC.

On the Z8530 CRC calculation can be enabled or disabled on a
character by character basis and my reading of what documentation
I have indicates that CRC calculation should start after the <STX>
is seen (but not including it) and should not include any <DLE>
characters and should finish after the <ETX> (inclusive).

Using an ARC Interview 7500 Protocol Analyzer I can interact with
my target system and both agree on the CRC value, but I can't seem
to make my system generate (or recognise as valid) packets from
either. I constructed a minimal packet <SYN><SYN><DLE><STX><SOH><DLE><ETX>
which in hex is <32321002011003> for which the correct CRC-16 (as
generated by the Interview) is <4191> or 0x9141. Try as I might
including and excluding bytes from the above message I can't generate
this CRC manually or via the z8530 chip! This is despite the fact
that the remote end is using exactly the same chip (but I can't
read the software).

What am I missing?

I have tried both CRC-16 and the CCITT CRC (CRC-SDLC) (even though
the Interview stolidly claims it is using CRC-16). I have tried
presetting the CRC value to all ones (even though the Interview is
certain it is using all zeroes). Nothing seems to work.

I am going out of my mind!

Has anyone got a clear idea of how this ought to work? Which message
bytes to include? Reference material with this kind of detail?
Anything?
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