Unfortunately there are a number of electrical differences. As Daniel says, the ADP1 adaptor is an option, albeit expensive.
There does seem to be quite a bit more interest in the RAM4 type than the RAM1.
The original RAM4 uses static RAM devices with a battery. This may have been done because of costs associated with EEPROM at the time. The RAM1 uses EEPROM, but it is a quarter of the storage size. If I were to design a RAM4 compatible cartridge I'd consider using EEPROM as the battery backup would be unnecessary.
I don't have a RAM4 cartridge (or a DX7II/s) myself so it would be tricky to reverse engineer. I took a number of the measurements for my design from an original RAM1 cartridge.
The schematics I have seen for the RAM4 also seem incomplete, particularly concerning the chip enables. Having a real cartridge to compare, probe and measure would be useful but typically $100 for a second hand one would be a significant up-front cost.
Anyone fancy a collaboration?? :-)
