Hello All
  Not all serial cards are alike. We have found out that there are some
cards that want to support IRQ3 and IRQ4 so badly that even the gold
fingers for the other IRQ lines are MISSING. To make it easy for you,
make sure if you can, that you buy a com board that has all the fingers
on both sides.
  If you have one that doesnt, all is not lost. After talking to technicians
and Mike WA8BXN, there is another way.
  Here it is:

You can use same IRQ lines (share them amoungst devices) as long as:
1) They have different addresses
2) You modify the com board by placing a diode in series with the IRQ
   line.

Theory:
   For example, if you run com1 on 3F8 IRQ4 and want to run com3
on 3e8 IRQ4, this is what happens.
   Under no Interrupt requests, the com board keeps the IRQ4 line
of the pc buss socket LOW.
   If com1 gets the interrupt, it tries to raise the IRQ4 line high,
but since you have another com board that is on com3 with the same
IRQ line (which it is holding low since it has no interrupt), the
IRQ from com1 will never be seen.
   You must isolate both IRQ lines via cutting the traces leading
to the IRQ4 line on BOTH com boards and inserting a diode in series
with the foilcut in both boards. The positive end of the diode points
in towards the com board and the cathode points towards the pc buss.

Here is a list of pin connections. Pick what ever pin is applicable to
your computer as per free IRQ lines you may wish to use.


Pin: IRQ:
---- ----
B4 2
B25 3
B24 4
B23 5
B22     6
B21 7

The B side of the connector is the FOIL or BOTTOM of the 2port com board.
The A side has the components on it.

Other notes:
-----------
AT class computers have more IRQ lines than XT class computers.
IRQ 2 has been said to be very important in an AT computer so you might
want to avoid using this one until the very last.

What I have in my com board is that there are traces running to B24 and
B25 but none to B4,B23,B22 and B21. This is standard for most com boards
. I already had these IRQ lines used by the multifunction card of my
XT, so on my com board, I taped over B24 and B25; set dip switches on
com board for an address of 3e8; soldered a wire from B24 to B2;
configured the msys.opt file for 3E8 IRQ4, tnctype... (0 for a KPC2)
fired MSYS up and it ran!
  The previous note on using debug to poke memory was not needed. I'm
sure this setup will work with phone modem accesses and extra ports
that you might want to add. Also, the cost of a com board is relatively
cheap. Here in Victoria, they run $25 with 1 port of 2 installed.
  The second com port (future com4) is socketed for an extra 8250
1488, and 1489 chip set. If you get the buffered 8250, you can run
higher speed packet if the unbuffered 8250 boggs your system down.
  The theory behind this according to the technician is that a regular
8250 waits for 2 bytes before flagging an interrupt, while a buffered
8250 will wait for 16 bytes before doing same, thus interrupting your
processor less often - more efficient.
    73 - Larry, VE7DIE @ VE7DIE.#vic.bc.ca