Originally posted by TheDrip View Post
I have been putting together a replacement ISA riser for my Model 25-286 and decided I had enough room to fit a small SRAM to provide UMBs.

In theory it will work so I sent the gerbers out for fab. I got in a mood yesterday and decided to mock up the UMB circuit. A floppy controller card from Sergey was sitting nearby and has most of the signals in mostly the right places.

What you see is 128K of memory, mapped by a GAL in 4K blocks. C800-DF00 for my Model 25 (with XT-IDE at DE00). It also works in my Sergey 8088 providing UMBs from C800-F000, lasting through 30 minutes of continuous ram tests.

Don't judge my soldering/wiring, this is a quick throw together to prove a schematic, and I was trying to make use of the solder that came with my new soldering iron. I know better, but I tried anyway!

The important part to this is that it works.