Joypad

Joypads have 4 directions and 6 buttons switches.

Registers used to read the button's states: see Memory mapped registers

The SYSTEM_IO ($C0044A) BIOS call can be used to read joypads more easily: it sets values in the BIOS RAM which gives currently pressed, state change, and repeat values for both joypads.

NeoGeo CD joypads

"G9" internal schematic. Pin 2 is ground, 1 and 3 are buttons, 4 and 5 are port inputs.

The NeoGeo CD joypads use membrane switches and springs for the 4 directions (replacement part ?) which directly set the corresponding inputs to ground.

The buttons are buffered with 3, dual NPN digital transistors marked "G9"