Date   : Sat, 28 May 1983 18:02:00 EDT
From   : Keith Petersen <W8SDZ@mit-mc>
Subject: MODEM overlay problem solved

Some users of the Kaypro, Xerox-820 and Televideo overlays for
MDM70x and other MODEM7 programs have reported problems getting
it to work.  This has finally been tracked down to the serial
port initialization routine used in the user customization
overlay.  It's not a bug - rather it's a matter of how the
hardware itself is used by the user.  The Z80 SIO chip has
a thing called "auto-enables" which can be turned on or off
at initialization time.  When it's turned on, the Z80 SIO
looks for several signals from the modem, including Data
Carrier Detect.  That's fine IF you hooked up the wires in
your RS-232 cable and IF your modem supplies these additional
signals (or you add jumpers to fake them).

It turns out that the people who had the problems has used only
pins 2, 3 and 7 of the RS-232 which is all you really need for
send, receive and ground.  However, the auto-enables must be
TURNED OFF in order for the Z80 SIO to function in this
environment.

In the initialization routine look for the code that does
MVI  A,0E1H  and then outputs that to the SIO chip.  That is
the proper byte for auto-enables ON, but it should be changed
to  MVI  A,0C1H  for auto-enables OFF.

--Keith