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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).
|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)|
 Programming the User Port
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).