User Port

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User port of a C64

The user port connects external devices such as modems and printers to the Commodore 64. Commodore's Automodem, 1660, 1670; Taihaho, Westridge, and so on, plug into the user port directly, supplying 2400 baud. However, non-Commodore printers and modems will only connect via an RS-232 adapter. The user port also connects the C64 to other systems, such as PCs and Arduinos.

The 6526 IC manages the user port. It is accessed via PEEK and POKE commands from BASIC. Lines 0-7 are the Parallel Port (input / output).

The C64 and C128 share the same user port standard, but it differs from other Commodore computers. The Plus/4 manual refers to its communication port as the RS-232 port. The VIC-20 is the same, on 1-3, 9-12 A, C-L, and N. The PET shares the same connector (called J2), with a similar eight-bit port on the same pins (C-L).

The port edge connectors have 3.96mm pitch (distance between contacts) and accept cards (PCBs) 1.57mm thick. The orienting notches that prevent upside-down insertion are "polarizing keys."

Pin Assignment


Pin Name Description
1 GND Ground
2 +5V +5 VDC (100 mA max)
3 /RESET Reset, will force a cold start. Also a reset output for devices.
4 CNT1 Counter 1, from CIA #1
5 SP1 Serial Port 1, from CIA #1
6 CNT2 Counter 2, from CIA #2
7 SP2 Serial port 2, from CIA #2
8 /PC2 Handshaking line, from CIA #2
9 ATN Serial attention in
10 9VAC 9 VAC (+ phase) (100 mA max)
11 9VAC 9 VAC (- phase) (100 mA max)
12 GND Ground
A GND Ground
B /FLAG2 Flag 2
C PB0 Data 0
D PB1 Data 1
E PB2 Data 2
F PB3 Data 3
H PB4 Data 4
J PB5 Data 5
K PB6 Data 6
L PB7 Data 7
M PA2 PA2
N GND Ground

Programming the User Port[edit]

Links[edit]

References[edit]

  • Easy Intercomputer Connection. Simon Fodale. The Transactor. Vol. 6 Issue 2. p 40
  • Commodore 64 Users Manual
  • Commodore 64 Programmer's Reference Guide
  • RS-232 Converter. Warren Tustin. C Hacking, Vol. 1 Issue 3.
  • Hardware Corner series. Domenic Defrancisco, Chris Zamara. (The Transactor. Vol. 5 Issue 1. p 72, Vol. 5 Issue 3. p 27).