I've just done a little dance of joy. Whilst looking at the code and trying to cross reference it against the timing signals I was seeing for RD operations it became obvious that there was an address mixup and a RET instruction was jumping back to the wrong part of the code - a return to 0x005C was actually going back to 0x0054 which implied bit 4 could be having a problem.
I piggybacked a 4116-2 onto bit 4's chip and got a burst of activity then what seemed like a crash, looking at the decoded address output I could follow the code through several loops to a IN instruction where it stopped and sure enough IO/M had gone high on the 8085 - first time I've seen that happen :D Replacing the chip and testing the old one in my 4116 tester showed it was properly dead. Onwards, ever onwards... Now I need to find out either what device should be at 0xE3 or which particular chip on the IO/M path has a stuck input (there's 3) - given this machine's failure rate at blown gates I'll not be surprised if this is the next fault. Cheers folks! On 04/02/2017 13:47, "Noel Chiappa" <j...@mercury.lcs.mit.edu> wrote: >> From: Jon Elson > >> Any time you see really narrow glitches, especially when they are one >> LA sample wide, you have no idea what they actually look like. The LA >> detects that the pulse was there at the instant it sampled it, but you >> don't know whether it was 5 ns wide, or 70 ns wide ... You also don't >> know whether they were full-amplitude pulses or runts that just barely >> crossed the logic threshold of the analyzer. > > Which is why I always prefer to work with an LA _and_ a 'scope: the 'scope > lets me see what the signals look like, how much noise/etc there is, etc, > etc, while the LA can do other things - better triggering, capture longer > time periods, etc. > > (Now they have those fancy new digitial 'scope with capture capability, and > you can get the best of both worlds with one box, but I guess they are still > kind of pricy.) > > But you can probably pick up an old 'scope for not much money on eBait. I > can't imagine working on anything without one. > > Noel -- Adrian/Witchy Binary Dinosaurs creator/curator Www.binarydinosaurs.co.uk - the UK's biggest private home computer collection?
