The switch has the function of choosing which ROM we want to have active. If it is in position 1, we activate the ROMCS signal and the EPROM of the interface is the one that works. If it is in position 2, the VCC signal on the OE pin causes the EPROM to be disabled and the internal ROM of the Spectrum is the one that works. Resistor R1 unlocks the 74LS32 output signal to the OE pin when the VCC signal is turned on.

    So far this would be the basic design to replace the internal ROM with an external one. For the memory dump functions that the modified ROM brings, a push button has been added to activate the MNI signal from the Spectrum. The mission of the resistor and the capacitor is to avoid bouncing when pressed.

    A push button has also been added to be able to RESET, which is useful on the Spectrum 48K model.