by John Tsiombikas
In preparation of receiving the first Amiga 500 for my retro-computer collection, which sadly didn't come with an A501 RAM expansion, I decided to make my own memory expansion board for it. Keeping with the "using whatever I happen to have in my hardware bin" theme, I decided to use 30pin SIMMs in this. I had to use at least two modules, since each SIMM is 8bit wide and we have a 16bit bus to populate.
The SIMMs I used are 512k each, bringing the theoretical total of this expansion to 1MB, but the OCS Agnus can't address any more than 512k. My amiga is actually a later model which comes with the ECS Agnus chip, and the A500+ main board. It still can't see more than 512k on the trapdoor expansion, but I'm pretty sure it's an easy modification to the mainboard to allow it to use the full 1MB. I'll update this page if I attempt that at some point.
The circuit is extremely simple. Pretty much all the signals from the trapdoor expansion connector go straight through to the SIMMs, except for the data lines, and the CASL/CASH (Column Address Strobe Low/High) signals. CASL goes to the CAS input of whichever module is connected to data lines D0-D7, and CASH goes to the other one which is connected to D8-D15.
Here's a schematic illustrating how everything is connected on my prototype board:
The resistors are not strictly necessary. They might help with signal integrity, but the main reason I added them was as a safety, to mitigate potential bus contention due to timing issues, or mistakes on my part in wiring up the circuit.
If signal integrity is an issue, ideally we would also need resistors on the WE, CAS, and RAS lines, all of which could potentially be fed to edge-triggered inputs in the DRAM chips, and cause problems if they have excessive ringing.
I have not included any capacitors, as SIMMs have local decoupling on them. But it wouldn't hurt to add some more if you encounter power issues.
Download kicad files (don't trust the PCB design yet)
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