C64/VIC-20/PET User Port -> SNES Adapter
Rough draft instructions based on early adapter from TexElec and David Murray

SNES Connector:
** See TexElec-SNES_Adapter.PDF **

/-\
!o! GROUND
!o! N/C
|o! N/C
! !
!o! DATA
!o! LATCH
!o! CLOCK
!o! +5V
---

Connect User Port Pins 1,12,A,N to GROUND on the SNES controller

Control pins:
User Port Pin F <-> CLOCK.
User Port Pin J <-> LATCH.
User Port Pin K <-> DATA.
The TexElec controller also had a 4.7k pull-up resistor to +5V on each of 
the above pins also, as well as a capacitor of unknown value on each, 
I'm guessing like .1uF.

Where to get +5V for the SNES connector and the pull-up resistors:
On the C64/VIC, you can get it from Pin 2 on the user port.  
On the PET, get it from the cassette/datasette port pin 2, or B.
