User Port

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User port of a C64

The user port is an interface built on a Commodore 64 and was originally designed for communication with devices such as modems and printers. Non-Commodore printers need an RS-232 adapter to correctly connect via this port. Modems can connect using the same method (Modems such as Commodore's Automodem, 1660, 1670; Taihaho, Westridge, etc., plug in directly). For modems, this port was originally limited to a 2400 baud communication rate.

The user port can also be used to connect to other computers.1

The same user port is built into the C128, whereas the VIC-20 and Plus/4 are different. The Plus/4 manual refers to its communication port as the RS-232 port. VIC-20 is the same on 1-3, 9-12 A, C-L and N. The PET has the same connector (called J2) with a similar eight bit port on the same pins (C-L).

The user port is connected to the 6526 and is accessed via PEEK and POKE commands from BASIC. Lines 0-7 are the Parallel Port. These lines can be used for either input or output.

The edge connectors have 3.96mm pitch (distance between contacts) and accept cards (PCBs) 1.57mm thick.

The parts that prevent insertion upside down are called "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]

1. 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).