Regular NeoGeo joypads or joysticks have 4 directions and 6 button switches.
Only one output is used: OUT3, which is connected to the D button input. This loop is used for detection (D = 0 if OUT3 = 1).
NeoGeo CD joypads
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, providing good press detection even if the carbon contacts have high resistance.