Recently I got a double-sized cartridge together with a used SQ80.
So I took my multi meter and got the following schematics:
There's not much magic inside the cartridge: The heart of any cartridge is an 8k EEPROM, providing 4k each to CART A and CART B. Address and Data Lines are directly connected to the EEPROM, only the write signal has some extra circuitry to guarantee the proper write pulse as needed by the EEPROM.
The latter is due to the fact that CWRT# (cartridge pin 28) is set up for use with a SEEQ 52B33 EEPROM which requires dedicated write-signal conditioning. Not sure why this signal is used and converted into a "classic" write pulse again rather than using the native BWR# signal on pin 25. With contemporary "behaves (almost) like SRAM" EEPROMs, you'd use the standard BWR# signal.
If, like me, you have a double-sized EEPROM (or even bigger) there's also an extra switch needed to route the /CS signal to the desired EEPROM -- each 2864 then holds two banks.
Now, what's a bit weird here is the Cartridge Present circuitry -- although it's included in my cartridge it's not used at all -- the connecting line to /CPRES is cut, which is just hardwired to GND, so the cartridge is always detected as present when inserted.
Finally, for stabilizing the voltage a 2.2uF tantalum capacitor is used and for the sake of proper termination each address, data and control signal is pulled up by a 4k7 resistor.