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 one (or in my case two) 2864. 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. If 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 holds 2 banks referred to
as CART A and CART B.
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 pin D is cut.
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.