This is the basic schematic of the cartridge. Some GPIO pins from ESP12E module controls the output enable of the input isolators (541) as well as the output enables of the shift registers (595). It also generates the write pulse to the SRAM. The shift registers receive serially each address and data to be recorded on the SRAM. The SCK and RCK pins are connected because there is no need to synchronize the changing of the bits as the /WE pulse will go down at the end of the transfer when the addresses and data are stable. It also saves one GPIO pin.