I originally obtained this information from the Bally/Astrocade FAQ, but they have the pins numbered incorrectly, and have made a few other errors.
9-pin female D-sub connector on the controller.
There's no proper ground or 5V supply. Directional inputs are connected to pin 8 with ordinary mechanical switches. The pot is 50k linear taper, and goes between pins 7 and 9. All four ports have pin 9 connected to each other.
That's all I know about it at the moment. *shrug*