[Slothie]

The original "Basic 1" commodore PET (With the "*** Commodore Basic ***" banner rather than the "### Commodore Basic ###") had built in diagnostics that would activate by holding down a pin on the user port. Later versions aparrently removed this feature rendering the LED on the main board redundant (possibly not fitted?). Commodore also made a cable loom that connected to the keyboard, cassette ports, user port & IEEE port and tested the ports - I beleive the software came on cassette. I looked for a link to information on this, but ten or so minutes of searching yeilded nothing (sadly theres so much C64 information it tends to get lost) but I did find the attached manual which might be interesting or give some clues/useful info. Looking through the archive sites on zimmer.net or 6502.org might prove fruitful. Other than that testing the user port would mostly be a matter of POKEing to the port and measuring the levels on the pins, or connecting to CB2 to see if you can get tones etc. A good reference for this is "The PET Revealed" by Nick Hampshire, copies easily found online. The IEEE port being bidirectional would be more of a hassle without some kind of test fixture unless you can get a IEEE peripheral and cable and check it out that way!