Four player adapters
The NES Four Score and NES Satellite accessories allow four NES controllers to be connected to the NES's two controller ports. Controllers connected to this accessory are read using the same process as the standard controller. After the normal 8 reads from one port to get one controller data, 8 more are done to get another controller.
Note that the NES Four Score is not compatible with the Famicom 4-Players adapter, which connects the 3rd and 4th players to the 2s bit instead of the 1s bit; that functionality is described in greater detail on the standard controller article.
D3 and D4 are not connected on the Four Score, so accessories like the Zapper and Power Pad will not work.
Input ($4016 write)
7 bit 0
---- ----
xxxx xxxS
|
+- Controller shift register strobe
Writing 1 to this bit will record the state of each button on the controller. Writing 0 afterwards will allow the buttons to be read back, one at a time.
Output ($4016/$4017 read)
7 bit 0
---- ----
xxxx xxxD
|
+- Serial controller data
- Read $4016 8 times to get the 8 buttons for controller #1.
- Read $4016 8 more times to get the buttons for controller #3.
- Read $4016 8 more times to get the signature, $10.
- Read $4017 8 times to get the 8 buttons for controller #2.
- Read $4017 8 more times to get the buttons for controller #4.
- Read $4017 8 more times to get the signature, $20.
Button status for each controller is returned in the following order, as on the standard controller: A, B, Select, Start, Up, Down, Left, Right. 1 if pressed, 0 if not pressed.
The $10 and $20 signatures are sent most significant bit first. They can be used to detect 1. whether the Four Score switch is in the 4 player position and 2. whether a DMC sample fetch caused a bit deletion.
Controllers #3, #4, and the two signatures are only sent when the Four Score switch is in the 4 player position. When the switch is in the 2 player position those reads get the same result as a normal controller, either $00 or $FF depending on the controller.
