I designed this printed circuit board (PCB) to allow regular 2732 through 27256 EPROM chips to be plugged into the cartridge expansion port of a TRS-80 Color Computer. My main motivation was to crank out a simple design to try out a prototype PCB vendor called OSH Park. Unlike most small-run, online-ordered PCB vendors, they provide boards with ENIG (gold plating) surface finish, which is much better than HASL (solder plating) for edge connectors.
Here is the schematic diagram:
I have released all of the Eagle PCB design files under GPLv3, and they are available on GitLab:
Here are some pictures of the bare boards:
When the bare PCBs arrived from OSH Park, they still had remnants of the panelization tabs present. Those easily snap off with pliers, leaving “rat bites” behind. The rat bites on the edge with the card edge contact fingers should be filed off for proper insertion into the card edge connector. The rat bites on the other edges can be left alone or filed off as desired.
The edge with the card edge contact fingers should also be lightly beveled on both sides for smooth insertion into the connector.
The PCB fits properly in the housing from an old Color Scripsit program pak.
Here are two of them assembled; one each with the two different socket options, so that I can test all of the EPROM sizes that the board is supposed to support.
I have tested the boards with 2732 (4k), 2764 (8k), 27128 (16k) and 27256 (two banks of 16k each) EPROMs. The board should also support 27512 (four banks of 16k each) EPROMs, but I don’t have any handy to test at this time.
The OSH Park site has a feature called Shared Projects, which is ideal for sharing open-source PCB designs. It lets anybody order bare PCBs for the shared projects from OSH Park directly. I have made CoCoEPROMpak boards available here:
I don’t make any money off of this. If you order PCBs from OSH Park, I won’t be involved and I won’t make a penny. But if you do order them, please feel free to comment below. Please follow that link for updated pricing. When I ordered my samples, I got three boards for just under $20 including shipping.
To use the boards, you’ll need to solder on a DIP socket, a 0.1uF capacitor, and a few jumpers. You might like to wire the bank select jumpers to switches mounted in the edge of an old Program Pak housing so that you can easily switch banks when using larger EPROMs. You’ll need to decide whether you want to use an 2732 EPROM or one of the larger ones, so that you can install the correct socket. Only one socket or the other may be installed, depending on whether you will use a 2732 (24 pins) or a 2764-27512 (28 pins).
Ok, time for some disclaimers: This project is pretty much unsupported, and I’ve made it available in hope that it may be useful to moderately experienced electronics hobbyists. Please study the schematic diagram carefully and understand it fully before you try this out.