C64 User Port is a 24-pin edge connector providing 8 bits of parallel I/O, RS-232 serial, and timing signals for modems, printers, and custom hardware interfaces.
| Pin | Name | Function | Type |
|---|---|---|---|
| 1 | GND | Ground | GND |
| 2 | +5V | 5V power | PWR |
| 3 | /RESET | Reset signal | I/O |
| 4 | CNT1 | CIA1 count input | I/O |
| 5 | SP1 | CIA1 serial port | BUS |
| 6 | CNT2 | CIA2 count input | I/O |
| 7 | SP2 | CIA2 serial port | BUS |
| 8 | PC2 | CIA2 handshake | I/O |
| 9 | SERIAL_ATN | Serial attention | I/O |
| 10 | 9V_AC | 9V AC power | PWR |
| 11 | 9V_AC | 9V AC power | PWR |
| 12 | GND | Ground | GND |