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
Replacing the chip and testing the old one in my 4116 tester showed it was
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.
On 04/02/2017 13:47, "Noel Chiappa" <jnc at mercury.lcs.mit.edu> wrote:
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.
Binary Dinosaurs creator/curator
- the UK's biggest private home computer