Controller port pinout

The NES controller pinout:

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

NES/SNES combo clones use 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 such famiclone) could be constructed as follows (leave D3 and D4 unconnected):

      .-
GND - |7\
CLK - |21\ - +5V
OUT - |3O| - D3
 D0 - |4O| - D4
      '--'