Nicolas Welte: A570 2MB RAM

A570 2MB RAM expansion

The A570 has two expansion slots: one external slot for an optional SCSI adaptor and an internal one for a 2MB RAM expansion. Commodore didn't make expansions for either one, but there are some third-party SCSI adaptors to plug into the external slot. It's not sure if a RAM expansion was ever produced, so I decided to make my own one.

The biggest problem with designing the expansion was that the schematics for the A570 were never released by Commodore. The best I could get were schematics for the CDTV and the A590. The CDTV is very similar to the A570, but it is older and less integrated, and it does not have a port for a memory expansion at all. The A590 is a completely different device, but based on the same DMAC chip and it does have the RAM expansion. With this information and some measurements with an Ohm meter I could figure out all signals on the expansion connector except six, which are connected to the XC2064 programmable logic chip. With the help of an oscilloscope I could identify RAS and CAS, but not the other four signals. So I decided to make a prototype that has a patch field for these signals. I already learnt that the A570 supports 1MBit type DRAMs (not 256kbit like the A590!) and it has the multiplexing circuit already on the mainboard. So the memory board just needs to have the memory chips themselves.

Due to the very limited space inside the A570 I had to use 1M x 4 RAMs that I got from an old 4MB PS/2 SIMM. In the picture the patch field and the jumper for selecting the expansion are visible as well:

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