Oct 272013
 

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.

I’ve just sent the board file off to manufacture some prototypes. I’ve already made the board orderable from OSH Park for anybody who feels adventurous, but please note: This board has NOT been tested yet, so there is NO guarantee that it actually works or even fits inside the computer! Also, please be aware that this board is only intended for use with the gate array version of the TRS-80 Model 4, which can be recognized by its RS-232 port which faces the rear of the computer instead of downwards.

Top Side of Board (Rendering)

Top Side of Board (Rendering)

Bottom Side of Board (Rendering)

Bottom Side of Board (Rendering)

I’ll announce whether the boards work or not once I receive them and test them out. All board files are available under the Gnu Public License from my GitLab repository:

https://gitlab.com/NF6X_Retrocomputing/M4-GA-ROM-BC

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