Back in the day, I had a Super Expander cartridge with my (first) VIC-20. That is a BASIC extension, which includes graphis and sound instructions plus a 3k RAM expansion.
Commodore VIC-1211A Super Expander Cartridge
I still have it, but it became of special interest, when I noticed, that the VIC-20 diagnostic cartridge is based on the same PCB, so I did a reverse engineering. It was not very complicated, since I already understood the VIC-20 cartridge science. Data bus is data bus and address bus is address bus, so the main interest were the chip select mechanisms.
VIC-1211A Super Expander Schematics (click to enlarge)
The RAM expansion consists of three pairs od 1kx4 Static RAM 2114 (low nibble (4 bit), high nibble), that are connected to /RAM1, /RAM2 and /RAM3. There are also two ROMs (2332=4kByte or 2364=8kByte), which are connected to /BLK2 or /BLK5 (IC1) and /BLK1 or /BLK3 (IC2). This makes the PCB to a very versatile cartridge board. The Super Expander does not make use of IC2.
Since we prefer using EPROMs instead of the ROMs, the Super Expander II is a pretty little modification of the original super expander cartridge. The RAMs are still available:
I have also ordered it successfully from AliExpress. The search term is "upd2114".
For the EPROMs, I have used two 27C512. The 8k memory slot can be selected by jumper. The chip select signals for each EPROM can also be jumpered (instead of a solderbridge, like in the original cartridge). A RESET button is also included.
The Super Expander Software is just 4kB and resides at the addresses $A000 - $AFFF, thus, it is connected to /BLK5.
Super Expander II PCB
The dimensions of the PCB are the same like the original Super Expander PCB. So, it fits the original cartridge case and the The Future Was 8 Bit VIC.-20 cartridge case. Installing the ICs on sockets and vertical jumpers do not interfere with neither cartridge case.
Super Expander II in the tfw8bit case
The Super Expander II was tested fully functional. The project data will be released on my github repositories as soon as the documentation is finished.