Greetings everyone,
Last year I found a deceptively easy ascii file describing how to construct an Userport RS232 interface for Commodore 8bits. It is based on the Max232 line driver IC and a 74LS00 IC (though with few alterations a 74LS04 inverter IC could be substituted). The parts can be found at many Electronics parts stores, the user port connector might be a bit tougher to find but not impossible. I found them at Electrosonic www.e-sonic.com for less than $5 each, the part number for that one is EDAC 307-024-500-202. Assembly is not too difficult for anyone who can handle a soldering iron and can even be done on perfboard and the whole project can be mounted in any electronics project box that is large enough for the board and connectors.
The plans can be located at:
ftp://ftp.giga.or.at/pub/c64/library/rs232FAQ