Controller port pinout

The NES controller pinout:

        .-
 GND -- |O\
 CLK <- |OO\ -- +5V
 OUT <- |OO| <- D3
  D0 -> |OO| <- D4
        '--'

(Directions are relative to the jack on the front of the NES)

The FC Twin NES/SNES combo clone uses Super NES controllers, whose pinout is as follows: (source: superfamicom.org)

 1 [oooo|ooo) 7  1:+5V  2:Clk  3:Out  4:D0  5:D1  6: I/O  7:Gnd

An adapter to use Super NES controllers on an NES (or NES controllers on an FC Twin) could be constructed as follows (leaving D3 and D4 unconnected):

        .-
 GND -- |7\
 CLK <- |21\ -- +5V
 OUT <- |3o| -- (D3)
  D0 -> |4o| -- (D4)
        '--'

1. Buy two controller extension cables, one for NES and one for Super NES, and cut them apart. Strip the cut ends to reveal a small amount of bare wire.
2. Using inline splice technique, wrap each wire from one cable with the corresponding wire from the other cable.
3. With solder and a soldering iron, glue each wrapped pair together.
4. With electrical tape, wrap each joint to insulate it from the other wires.
5. Apply heat shrink around the whole assembly.