[rescue] Trying to get Cadmus 9200 running (serial console trouble)

Mouse mouse at Rodents-Montreal.ORG
Mon Feb 4 11:01:12 CST 2019

>> With one unknown black cable and no null modem adapter, I got some
>> strangeness: there is partial echo back of what I type, but that is
>> garbled (about 1 out of 3 characters is the same, 2 are garbage that
>> is different for each character).

Is the garbage consistent?  That is, if you type (say) x, do you always
get the same garbage back?  (It's not clear whether your "different for
each character" means "differs from one typed character code to a
different code" or "differs from one instance of a given character to
another instance of the same character".)

If so, or even if just _usually_ so, this sounds a lot like a parity
mismatch to me (eg, the terminal using 8N and the machine using 7E).
That will garble half the character codes, not 1/3 or 2/3, but,
depending on what you're typing, it could hit a fraction far off 1/2 of
the _typed_ characters.  Do all the characters that get echoed cleanly
have the same parity and the characters that get garbled have the other
parity?  Then it's almost certainly got something to do with parity.

>> Interesting part is that I only get echo for 4 last serial ports
>> (including console on 8) but no echo on first 4 serial ports.  Might
>> that be some strange pinout of the black cable that is somehow
>> activated by remote port handshake?

This could very well be hardware flow control of some sort (DCD,
DSR/DTR, CTS/RTS).  However, that's unlikely to be what's behind the
garbled echo.  Lower-numbered ports are more likely to support fancy
features like hardware flow control, I'd say.

> I'd be strongly tempted to probe that black cable to see *exactly*
> how it is wired.  Assuming DB25 plugs, probe each PC-end pin 1-8+20
> against *every* pin 1-8+20 at the other end.

I'd actually be tempted to buzz out all 25 pins, since it's only an
assumption that it's standard serial port pinout.  Unless someone knows
Cadmus used standard pinout...?

> Which raises the possibility that swapping the cable round the other
> way is worth a shot, too!

Yes.  Reversing the cable end-for-end is definitely something to try.

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