I've started a little hardware project to build an expansion board for the Replica 1. It's a simple memory expansion card that can hold one or two 8K RAM, EEPROM, or EPROM chips. The two chips are addressed at $8000-$9FFF and $A000-$BFFF. This can conflict with devices on some other cards like the EEPROM on the Multi I/O board if you have it installed at the same time (You would need a slot expander to do that). This is about all the space there is available for addressing beyond what is on the Replica 1 board. There will be a write enable jumper for each chip so you can disable writing (which only really makes sense for EEPROMs). Basically, it uses Vince Briel's one-chip EEPROM circuit documented here and used on the Multi I/O board, with suitable address decoding (my implementation takes 2 chips).
It's a simple board but it will allow me, for example, to put programs larger than 8K (like Microsoft BASIC or my adventure game) in EEPROM or to have up to a total of 48K of contiguous RAM available. There will be room on the board for more circuitry later, if needed.
|Prototype on Breadboard|
So far I have prototyped the circuit on a breadboard. I didn't hook up all the data and address lines and only used one EEPROM. I've confirmed that it can read and write. I used the gschem schematic capture software, more just to make a nice printed schematic than for any other reason. I did several rough designs earlier on paper.
I ordered the remaining parts I needed, most notably a Vectorbord proto board. I got most parts from Unicorn Electronics. I bought the protoboard from Digikey as shipping was the most reasonable of several vendors.
There are several Vectorbords from Vector Electronics that fit the Replica 1 slot. Note that the protoboard sold by Achatz Electronics will not work on the Replica 1 as it uses a different connector.
I will post another installment reporting on my progress when I start assembling the real board. I'll also include the schematic.
If I get really ambitious I might lay out a circuit board. This would probably only be feasible if I made a few boards. Anyone interested in a board like this either as a bare board, kit of parts, or assembled? If so, drop me a line.