I previously designed a small adapter board to allow a regular 2764 EPROM to be plugged into a TRS-80 Model 4 (Gate Array version only) in place of ROM chip U4, in order to use the new auto-booting feature of the FreHD hard disk emulator. As I posted earlier, the assembled board was a bit too tall to fit under the aluminum shield that is present on US Model 4 computers, and it also needed a couple of pull-up resistors added. I changed my board design to address these problems.
TRS-80 Model 4 (Gate Array Version) ROM B/C EPROM Adapter
I’ve received my M4-GA-ROM-BC v1 PCBs from OSH Park and assembled them. They work, with the following errata:
Fred Vecoven has designed a neat hard disk drive emulator for the TRS-80 model I, III, 4, 4P and 4D computers, called "FreHD". That is pronounced “Fred”; the "H" is silent. Ian Mavric manufactures and sells them, and I ordered one from him recently via his eBay store. It works quite well.
Now they’ve come up with a cool new innovation: Modified ROMs for those computers which allow them to boot directly from the FreHD without needing a floppy drive, and even present an on-screen menu to allow the operator to select which image to mount! Different ROMs are needed for each model and production variant of the 8-bit TRS-80 machines. My Model 4 happens to be the so-called “gate array” version. It uses a 24-pin mask-programmed 8k x 8 ROM which is not pin-compatible with common 28-pin 2764 EPROMs. It’s not too hard to adapt a 2764 with a few soldered wires, but I’m really allergic to blue wires. So, I’ve designed a little board which should plug in to the ROM socket in place of the original ROM, and then accept both the original ROM and a 2764 EPROM, with a jumper to select one or the other.