full writeup here: http://www.glitchwrks.com/2019/04/01/gw-sdk80-ram1

With my SDK-80 cleaned up and working, I wanted to expand the RAM to do some experiments and possibly later design some extra hardware for it. I didn't want to put the expansion RAM in the prototype area quite yet, since I wanted something that wouldn't result in permanent modifications to the SDK-80. I ended up with a little mezzanine board that contains enhanced memory decode and a full 64K of static RAM:



Here's a closeup of the board:



The two testpoints on the bottom are used for controlling ROM. With nothing connected to them, ROM is located at 0x0000 and enabled. The rest of memory space, all remaining 60K, is RAM! Grounding TP1 moves ROM up to 0xF000. Grounding TP2 disables ROM entirely, leaving all 64K of memory space as RAM.

The board completely replaces all 2111/8111 type SRAMs, as well as the 8212 address bus buffers and 8205/3205 decoders. It's a drop-in upgrade (as long as your board has sockets) and is completely reversible by just unplugging the board and putting the original ICs back in.

I'll be running more of these boards and making them available to other folks. I know a lot of hobbyists won't want to attempt the surface mount stuff, so I'll probably provide them pre-assembled too. I can provide them tested, but really it's best to plug the male:male headers into your board's sockets, then place the mezzanine on top and solder the headers in -- there's enough tolerance in the header mounting holes on the GW-SDK80-RAM1 to handle differences in IC socket placement.