One of the lesser known features of the PCjr was the manufacturing test mode. I think output was via the ports on the back. (Not the famous 'L' port though, which appears to have no use.) The manufacturing test mode required a lot less of the machine to be functional than the user level diagnostics or the advanced diagnostics. Even for machines that run diagnostics from disk, assuming that the memory or CPU is ok is a big leap. To do diagnostics correctly you really need an external processor or an onboard service processor like midrange machines do, and the appropriate debug ports on the parts you are going to test. Mike