> I have a need to operate 3 comm ports on my pc, so bought a
> standard card that has FDD, HDD, game, LPT and 2 COMs. I disabled
> the FDD, HDD and game ports and configured the com ports to
> 3 and 4 using jumpers on the board.

This is a 16 bit ISA card then I guess.

> Having installed the card, Win95 detected the new ports. All well
> and good, but in use, com3 stops com1 working and com4 stops com2.

Most of these multi I/O cards only allow you to select interrupts 3
and 4 for the serial ports.  This in effect only allows you to swap
which interrupt you use for each port.  Sharing interrupts for COM1/3
or COM2/4 (as typically configured) is fine until you want to use one
or both of these pairs of ports together.  If you want to do that,
you will need separate interrupts.

> It must (surely) be possible to configure the system so that I can
> use at least three of the ports at the same time.

Well, if you can select for example interrupts 10 and 11 for COM's 3
and 4 respectively, then that is all you need to do.  If you can't do
that, the only other option is to modify the card.

I have done this on 8 bit cards, all you need is a couple of bits of
wire and some Veroboard.  I suspect you may run into a bit of a
problem modifying your card if it is 16 bit.  Because it is common
practice to only put plated tracks in place on pins which are
actually used by the card, you may find that there aren't any for the
pins where other spare interrupts are located.  This will make it a
bit more of a pain to do the modification.  You can get around this
by plugging your Veroboard into another 16 bit slot, if you have one
spare, or behind an existing 8 bit card which is occupying a 16 bit
slot.

In the case of the 8 bit cards, all you do is cut the tracks to the
pins for interrupts 3 and 4, and then link them to the tracks on a
bit of Veroboard which you plug in behind the the card in a 16 bit
slot.

If you want/need to do this, I can post a pin out of the 16 bit ISA
connector if you don't have any information on it already.

Hope this helps.

Cheers, Ian.