Small breakthrough (was: Re: Logic Analysers)

Adrian Graham witchy at
Sat Feb 4 08:32:28 CST 2017

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" <jnc at> 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

Binary Dinosaurs creator/curator - the UK's biggest private home computer

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