TRS-80 Model 1 - Character ROM

I recently acquired a TRS-80 Model 1. Amazingly, the unit functioned perfectly well after possibly 30+ years of sitting around in a basement, although I did have to construct a cassette cable, video adapter, and power supply (the hard way, by disassembling and rewinding an existing laminated transformer). These other modifications will be detailed in another article.

This particular is a stock 1978 model, and like all Model 1's, displayed capital letters only. Tandy left off one of the video RAM chips, which reportedly saved about 5 dollars off the cost of the machine. The character generator IC did actually contain the lowercase letters, so all that is needed is to add an additional 2102 RAM chip, and make a slight change to the circuitry. This does require modifying the original board, but it's a period accurate modification that was done all the time, and was eventually a kit offered by RadioShack.

The kit also came with a new character generator rom. An early machine like mine contained lowercase characters that were a little off, with such defects as "the flying a" where the a is a little higher than the rest of the lowercase characters, and letters with decenders like g y q and p looking a bit off. The new character rom fixed these issues.

Character ROM editor

thumb|Screenshot of the editor While 2102 ram chips are still well available, the MCM6673 that the TRS-80 used as the character rom are non-existent these days. However, while on ebay, I did find someone selling TRS-80 lowercase kits using a programmable ROM. I figured that the same thing could be done with an EPROM, which I have obtained a reasonable quantity of, and even if I don't have the right one, they are still out there. I also ordered a cheap programmer which I can use for this and all sorts of projects in the future also.

The MCM6673 was a custom version of Motorola's MCM6674, a character rom. It holds 128 5x8 characters. The way you access it is by inputing 7 bits (0-127) with the character you want, then select which of the 8 lines of the character you need (with 3 more bits). The character line is output on 5 pins. So that's 10 inputs, and 5 outputs, plus a chip enable pin. This means I should be able to use a 1kb x 8 EPROM (which has 10 address pins to use as inputs) to replace the character rom (though 3 of the 8 data pin outputs will not be used).

What I could not find was a ROM image of any kind of TRS-80 character rom. That means that I would probably have to come up with the chip program myself. I could do this on paper, but thanks to modern computers, I can make a character editor program to help me.