Ok, it the 4th day of the Retro Challenge, and this is just a quick blog update about what I did yesterday, which was my 2nd day of doing Retro Challenge stuff. So, yeah, it’s Day Something.
Due to life getting in the way, I didn’t get an early start last night, but I did have time to do 3 schematic and 2 PCB layouts for my Z80 that I’m building.
The first board will be for a 2764 EPROM that’s programmed with Microsoft BASIC. Technically this is not part of RC for me, but I know MS BASIC works on my breadboard Z80 so it’ll be an easy and convenient way to test out the hardware.
The next board is for a 27C512 64k EEPROM (Electrically Erasable instead of UV light). This will be what I load my raw machine code stuff up from. Why such a big size? Simple answer is that I bought a bunch of them cheap from eBay! I’ve designed it with 3 of the address lines going to jumpers, so effectively segmenting it in to 8 different 8k ROMS. If I end up writing assembly language programs bigger than 8K I might have to revaluate!
Lastly is the RAM. I’ve got 62256 chips that I found at Nottingham Hackspace, which give 32k of memory to play with. Plenty!
The schematics for these 3 boards are all very very similar. In fact, the pinouts of the chips are almost identical. I pondered over designing one single board that could be jumper-configured to match any of these requirement, and as the PCB fab house I’m going to use (OSHPark) do boards in multiples of 3, it would save quite a bit of money. However, it will add a bit more complexity than I really want right now, so quickest thing is to do 3 individual boards.
Tonight I hope to get the clock circuit, Blinkenlights and serial I/O boards done, so I can send off all 7 to be manufactured this weekend.