Apologies if this is obvious/already covered, but along the same lines as Noel's "it's probably 'JKDBD0', not "JKDBDO" it's almost certainly "JKDBD0 DCF11 AA CPU DIAGNOSTIC" or maybe even "JKDBD0 DCF11-AA CPU DIAGNOSTIC" not "JKDBDO DCFlI AA CPU DIAGNOSTIC". Of itself, that may not matter much. However, it may lead elsewhere. The F11 was [obviously] the 11/23 (and /24) chipset. Perhaps not so obviously,11/23 boards were available in variants with or without floating point, and (iirc) with or without memory management unit (MMU) (correction welcome). Also, the relevant bits were socketed, hence a particular board today may not match the board (and part number) as originally manufactured. Relevant handbooks etc will contain further details. I can't remember what's been previously written in this thread, but is it vaguely conceivable that the diagnostic in question needs a piece of hardware that isn't on the board in question, or is seriously broken on the board in question? A search for DCF11-AA CPU Diagnostic gets a few hits, including http://www.pdp-11.nl/pdp11-24/cpu/diags/jkdb.html which contains the delightful line "For more infromation refer to your microfiche library." and some rather more useful stuff too - sounds like you've found this or an equivalent already? Hope this helps, best of luck. John Wallace