[comp.sys.ibm.pc.misc] A perplexing problem with com ports

dlukas@mydland (David Lukas;133B Barker;x2-6145) (12/03/90)

I am posting this to see if anyone can help to fix (or at
least diagnose) the following problem I am having.  As I
will describe, there are workarounds, but I am curious to 
know what is going on...

I own a DTK Tech-1000A, a PC-AT (286) clone.  I have been
using it for over a year with a US Robotics 2400 baud internal
modem and a Hercules-compatible Mono graphics and printer
card.  When I recently received a serial mouse (Genius brand)
I decided to install a multi-serial port (and printer port,
and game port) card in the system.  Following the jumpering
instructions I set the modem for com2, the printer port on the
new card to lpt2, and the serial ports on the add-on card to
com1 and com3.  The modem made a screeching sound and didn't work.
After trying various other port configurations on the two cards
(modem and multi-port) I tried a different brand internal modem and
a different I/O card.  In every combination, the modem would not work
when the multi-port card was in place.

Since I tries so many combinations and configurations, I am left to
suspect a problem with the motherboard, although I am not nearly
knowledgeable enough to figure out how such a problem would arise.
(The system has 640K memory and a 40 MB Everex hard drive with a
Winchester controller, if that makes any difference.)

For now, I plan to try to make the system work by putting the mouse
on one serial port of the multi-port card, and an external modem on
the other.  Also, I could try the system out with an internal modem
and either a single-port serial card or a bus-mouse card, but I am
reluctant to do so because I would have to invest more money and I
have a hunch it would't work.

Anyone have any ideas, or know what components on the motherboard
might relate to the COM ports or deal with peripherals in slots?

Thanks,

David Lukas
dlukas@mydland.berkeley.edu
dlukas@geoworks.com
uunet!bsw!dlukas