|This article is very short and not very detailed. Please help to improve it.|
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 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".
|2||+5V||+5 VDC (100 mA max)|
|3||/RESET||Reset, will force a cold start. Also a reset output for devices. (see note below!)|
|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)|
Note on the /RESET pin: the page mentions "forcing a reset", which may work but may damage chips inside the C64, as the line is an output only, driven by an inverter (U8, pin 12) and pulling this pin to ground will shorten that output pin.
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).