-=> /* Quoting Derek Oldfather to James Mckenzie */ <=-
 JM> I wonder what type of video card you are using, since
 JM> you are using COM4.
 DO> Why would that make a difference?
Certain video cards use a hex address that ends with
"02e8". That happens to be the same as the address for
COM4.  Since your serial port only interprets these ending
hex numbers your serial card thinks certain video related
commands are for it and your serial card will respond to a
video command (bad).  Diamond Cards are notorious for this
but so are a number of others.  Cirrus cards don't do it.
Don't know about Matrox.
For instance, one of the most common problems is if you
cause your system to do a video "mode change" it will cause
extra characters to be sent out of your COM4 serial port
sometimes causing CRC errors during transfer and sometimes
causing a carrier loss or a lock up of COM4.  An example of
a "mode change" might be a {ctrl-esc) or a change from
windowed to full screen mode.
Since the only "standard" comm ports that are defined by
the official AT standard are COM1 and COM2, the video card
manufacturers don't care about COM3 and COM4 addresses.
They figure they're free game.  The addresses for COM3 and
COM4 are "de facto" standards in the industry but are not
hardcoded into the official AT specs.
-Frank
(fsexton@xpert.net - http://www.concentric.net/~fsexton)
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