PDP8a CPU fixed! (was Re: DEC DELQA - seems not to work. Anyone got a spare?)

On Mon, Nov 7, 2016 at 11:44 AM, Ian S. King <isking at uw.edu> wrote: Assuming my notes and sketchy memory make any sense: One of my PDP 8a CPU boards (the M8315) passed all the self tests I could toggle in, EXCEPT, it would ignore HLT instructions. It'd just blow right through them. That'll get you questioning your sanity real quick. Several hours of squinting at schematics said I'm looking for the STOPL signal - I found three or four places, one of which involved a ROM (at least my notes say there is a ROM involved). Uh oh. Page 3-17 of the microprocessor user manual lists when STOPL is asserted. Basically, front panel or HLT. Page 4-39 has the logic for the front panel, and that was enough to narrow it down to one instance of STOPL in the schematics, in the middle of page H-9. E39 (an 8881 aka 7439) or E33 (a 7402). I swapped those two out, repaired the trace that I busted, and viola! HLT now works. I vaguely remember it was the 8881 at fault. BTW, I'm open to suggestions as to how to even begin debugging an HP 1000 E Series. There seems to be a case holding a power supply in the way of any access to the motherboard. The machine fails to exit the 'counting' self test right after power up. I plan on writing up a better description once it gets cold and snowy out, so don't feel bad if you don't see this plea for help. Cheers! b