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.

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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


[edit] Programming the User Port

[edit] Links

[edit] References

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).
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