Saturday, 24 April 2021

Microbee SuperPAK - An EPROM Expansion Board

Quite a while ago I developed hardware for a compact flash coreboard for my Microbee. This added IDE based compact flash storage, allowing massive disk storage under CP/M. Others helped out with the BIOS software. It never got to the point where I was completely happy with it. Compact Flash cards are finicky things, and the combination of a 5V system and very slow timing conspired to make things less than 100% reliable. Being a read/write thing, every time something crashed it would corrupt the directory structure and necessitate reformatting the card. This was (and is) frustrating.

I realised that the reason I wanted all the storage was just so I could have a straightforward way of loading software on the Bee that doesn't necessitate waiting for tapes to load. I've got a couple of dozen games and suchforth that I like to play every now and again, and frustrations in getting it going mean I don't play with the gear as much as I'd like.

So back in the day I had a "ROMPAK" for my bee, which was a little expansion board that could take eight 2764 EPROMS, for a whopping 64K of storage. By writing to port 0A, you could select one of the eight ROMs, and it'd appear at C000 in the memory map. We used it to allow us to have Wordbee, EDASM and the Mytek wordprocessor in the same bee without having to swap ROMs, and it was neat. I envisaged loading Emu Joust into a couple of EPROMs and writing a short routine to copy it into RAM at the corect location and then jump to it.

So this board allows you to do that, plus maybe store some other games and bits of software. I've allowed all 256 possible PAK ROM positions, using an 8-bit latch and four 27C040 EPROMs. These are current production parts that go for the princely sum of $8 or so ea, as long as you don't mind them in a OTP flavour. You can get Flash versions too.

The idea is that the first PAK location holds software with a menu, that allows you to choose what you want to run, then copy that into RAM (or leave it bee in EPROM if it's a PAK thing), and execute it. Sort of like the PC85 but on steroids. The software to do that should be a doddle, or at least much easier than writing stuff to run CP/M. Even better, there's no FPGAs and no CPLDs to worry about. all the decode is done with bog-basic 74 series parts.

Edit: I just found the "round the corners" and "teardrop" plugins for KiCad, which make boards look like they were layed out using bishop graphics and tape. I reckon it looks so nice that it's worth leaving the solder mask off and going for an ENIG finish, ala expensive Hewlett Packard boards of the 1980s.

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