Commodore 64 / 128 Game I/O
Two 9 pin D-sub males (D9M) at the computer.
Note: POT is a linear 470 kOhm (±10 %)
Note: In Commodore 128 the pin 7 (+5 V) is max. 50 mA
Note that when a 1350 (rolling joystick) mouse is used, pins 1 through 4 will have pulses on them instead of logic low levels as a joystick would provide. Also, a pressed right mouse button grounds pin 9. The POT lines are otherwise unused by a 1350. When a 1351 (proportional) mouse, a paddle, or graphics tablet is used, the two POT lines are used for direction control. A pressed right mouse button grounds pin 1.
Source: Commodore 64 User Manual, Commodore 128 Programmer's Reference Guide