Commodore 64 user port pinout

The Commodore 64 user port exposes eight individually programmable parallel I/O lines and handshake signals from the 6526 CIA 2.

24-contact user port

24 contacts Current package

Commodore · 12 positions × 2 sides · keyed PCB edge

24-contact user port pinout

Diagram rotated to show both contact rows. Numbers are the upper side. Letters are the lower side. Match the keyed connector to the rear view in the manual.

Selected pin details

2 · +5 V

Supply output · Power

5 V from the computer. Leave it unused for a passive measurement.

Commodore 64 Programmer’s Reference Guide, Appendix I, User Port

Electrical limits · Link to this pin

All 24 pins

Pin	Signal	Direction	Function
1	GND Return	Return	Digital ground. Use this reference for a logic measurement.
2	+5 V Supply output	Supply output	5 V from the computer. Leave it unused for a passive measurement.
3	RESET Active-low reset	Active-low reset	Connected to the computer’s reset line. Leave it alone for ordinary parallel I/O.
4	CNT1 Bidirectional	Bidirectional	Serial shift clock for CIA 1.
5	SP1 Bidirectional	Bidirectional	Serial shift data for CIA 1.
6	CNT2 Bidirectional	Bidirectional	Serial shift clock for CIA 2.
7	SP2 Bidirectional	Bidirectional	Serial shift data for CIA 2.
8	PC2 Handshake output	Handshake output	CIA 2 port handshake signal.
9	SER ATN Serial bus signal	Serial bus signal	Serial attention signal. Consult the applicable board schematic for the interface circuit.
10	9 VAC AC supply contact	AC supply contact	One contact of the 9 VAC supply pair. Keep it isolated from signal pins.
11	9 VAC AC supply contact	AC supply contact	Second contact of the 9 VAC supply pair. Keep it isolated from signal pins.
12	GND Return	Return	Digital ground. Use this reference for a logic measurement.
A	GND Return	Return	Digital ground. Use this reference for a logic measurement.
B	FLAG2 Handshake input	Handshake input	Edge-sensitive input of the 6526 CIA 2.
C	PB0 Programmable I/O	Programmable I/O	6526 CIA 2 parallel data bit. Select its direction in the data direction register.
D	PB1 Programmable I/O	Programmable I/O	6526 CIA 2 parallel data bit. Select its direction in the data direction register.
E	PB2 Programmable I/O	Programmable I/O	6526 CIA 2 parallel data bit. Select its direction in the data direction register.
F	PB3 Programmable I/O	Programmable I/O	6526 CIA 2 parallel data bit. Select its direction in the data direction register.
H	PB4 Programmable I/O	Programmable I/O	6526 CIA 2 parallel data bit. Select its direction in the data direction register.
J	PB5 Programmable I/O	Programmable I/O	6526 CIA 2 parallel data bit. Select its direction in the data direction register.
K	PB6 Programmable I/O	Programmable I/O	6526 CIA 2 parallel data bit. Select its direction in the data direction register.
L	PB7 Programmable I/O	Programmable I/O	6526 CIA 2 parallel data bit. Select its direction in the data direction register.
M	PA2 Programmable I/O	Programmable I/O	Port A bit 2 of CIA 2.
N	GND Return	Return	Digital ground. Use this reference for a logic measurement.

Signal assignments

Eight-bit parallel port

Set each pin’s direction using the 6526 CIA 2 data direction register. Use an input-only instrument for the first measurement.

Electrical limits

Parameter	Limit	Conditions and source
Parallel port controller6526 CIA 2	6526 CIA 2	The measured signal is PB0 at contact C. Commodore 64 Programmer’s Reference Guide, Appendix I, User Port
Bench loadHigh impedance only	High impedance only	Use a 10× passive scope probe. The example draws no supply current from the connector. Measurement fixture design

Models and documents

The map covers the external 24-contact user port. The signal roles and supply contacts are specific to the C64 shown in the manufacturer guide.

Commodore 64: User-port connector in the cited manufacturer documentation.

Commodore 64 Programmer’s Reference Guide· Appendix I, User Port, p. 397.
Commodore 64 service manual· User port, CIA circuit and 9 VAC routing.