Rather than replacing the motherboard, I have seen surplus adapter
daughterboards which plug into the microchannel and provide three
ISA/EISA sockets.  They are just traces and connectors, so they are cheap
and simple.  I've never had occasion to use one, but they are one more
connection to go wrong, and the ISA cards should be supported parallel
to the motherboard - with space and potential overheating problems.
I'm guessing that there might be addressing problems, too.
(e.g. POST card uses 280 H instead of 080 H).

If you have room in the box, are otherwise satisfied with the system,
and want to add an ISA card, this might avoid the hassle of retrofitting
a new motherboard - incidentally, make sure you can adapt the drives.
If I recall correctly, some PS/2's used 40-pin connectors on the floppies.
The "extra" 6 edge-card connections on their own little tab were for +12 V
+5 V and Ground, taken two by two.  Check with IBM for pinouts.

If you know someone who could use a 486 at a good price (I do.),   :-)
given the cost of a new case and power supply which is guaranteed to
fit the motherboard of your choice, that might be a better choice -
even if you only assess yourself ten cents an hour!

If you are doing this for the fun of it - have fun!

Boyd Ramsay

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