Date   : Wed, 02 Feb 1983 0832-:00CST
From   : DSDC-TECH@gunter-adam.arpa
Subject: MODEM problem..

The MODEM programs we use down here fall into the routine GETACK after
a block is sent to get the response from the reciever.  In ours, anything
but an ACK is taken as a NAK.  This causes a problem with the new version
(5?) of TOPS-20, which tries to enforce flow control on the comm link,
causing the sending micro to recieve 'random' DC1 and DC3 characters.  This
forces the link into a continuous retransmit sequence.

Our solution:  after the check for ACK in GETACK, throw in a check for
a NAK with these two lines:

       CPI  ACK     ; Already in
       RZ           ; the GETACK routine
       CPI  NAK     ; Added to
       JZ   GETACK  ; routine

This forces MODEM to accept only ACK or NAK as valid block control messages.
The only problem I see is when a true ACK or NAK is perverted on the line,
which this mod will ignore.  In this case, a timeout will have to occur
and the block be retransmitted.  If anyone sees any other problems,
please let me know.  Thanks,

Doug Huneycutt