How can it multiplex four controllers onto two controller ports? Would it be possible to extend it further with three devices?
How does the Nintendo NES Four Score work
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Best Answer
EveryNES and NESdev Wiki may help you sort out some of the details. The two joypads connect to the NES through a form of serial communication. The controller sends a synchronized 8-Bits across the #7 pin connector.
The value of the button that is pressed is encoded within 8 bits that are communicated and synchronized with a clock signal sent across the #2 pin.
When the NES Four Score or NES Satellite adapter is connected, the adapter sends a 24-bit stream across both controllers' #7 pin channels. The first bytes on each line correspond with controllers 1 & 2's buttons. The second byte across both lines corresponds to controllers 3 & 4 and the last byte is an "ID" byte used to detect the adapter's presence and verify that none of the preceding bits were deleted by a known glitch that occurs during sample playback.