Hello

I recently acquired a Compaq Portable, and have finished restoring it to full working order.
In the process of doing that, I upgraded the onboard RAM from 256K to 640K.
This is an offically supported upgrade by Compaq, but requires a replacement Bipolar PROM for memory decoding (U35 on motherboard).
Bipolar PROMs are rare and programming equipment rare/expensive.
I used an SPLD to replicate with what the 640K PROM would have done, and made a little adapter PCB.

If you have a Compaq Portable, and would like to upgrade the onboard RAM to 640K, ensure you have at least BIOS Rev C first (if you have later than Rev C, let us know, as that hasn't been archived).

You can build the adapter PCB and program the SPLD yourself using the files here.

Or you can buy a ready-made one here, as I have spares.

Cheers
JD