Serial card handshaking

[paul.parkhurst@uttsbbs.uucp (Paul Parkhurst)]

-> Well, I've got a bit of a problem.  I've got an Apple SSC that seems
-> to not want to do hardware handshaking.  Does anyone know if there's
-> something beyond the minor notes in the manual that tell of a magic
-> switch setting that I'm missing?
->
-> I think it's just dead.
->
->-> No, it's not dead. It's a problem with the 6551 on the SSC. I believe
->-> someone said that it's possible to buy a new 6551 with the problem fixed,
->-> but I'm not sure...

I can tell you exactly what the problem is.  The old ACIA chips didn't
implement CTS handshaking properly.  What it would do if CTS dropped low is
stop all transmissions (as it should).  The problem is that if you were in the
middle of transmitting a character, it would stop, and then transmit the rest
of the character when CTS went back on.  This garbaged out the first character
sent after CTS turns back on.

As Brandon said, there's a replacement for the old 6551 ACIA chip (not sure
what the number is) that fixes this.  I've gotten handshaking to work on the
SSC by doing some cable magic.  Basically, I jumpered DTR to CTS so CTS is
always on, then use the DCD line for handshaking.  This won't work in most
modem drivers, since the only bit thats being checked is the tramsmit buffer
empty bit (you also have to check for DCD on or off).  The downside of this is
that, when DCD is turned off, the SSC won't receive any characters
(incidentally, this is why the DSR line is normally used for carrier detect on
the SSC).  So, when DCD is off, you're basically in half-duplex mode and you
can't receive any characters.  This might be a problem if you're uploading with
zmodem and there's an error; the other side will send its attention string, but
you won't receive it if your CTS (ie DCD) line is off.

In fact, because of this problem, the SSC drivers in ANSITerm specifically
check for both transmit empty and DCD on.  Since the DCD line is normally
always on when you're connected, it works with both the normal cable design,
and the handshake cable thats in the manual.


Paul Parkhurst

uttsbbs!paul.parkhurst@PacBell.COM
- OR - pmp@Delphi.COM

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