2 Jun
1999
2 Jun
'99
1:23 p.m.
The point about the "best piece of equipment" is perhaps the thing I like
best about using a logic analyzer for a difficult problem. It's such a pain
to set up, it forces you to think through the analysis before making any
measurements . . . or assumptions. Another nice feature is that you can
rearrange the displayed waveforms without rearranging the probes. If you
choose to see a signal set differently than you previously did, you don't
necessarily have to redefine your trigger equations or any such fun.
Dick
-----Original Message-----
From: Tony Duell <ard(a)p850ug1.demon.co.uk>
To: Discussion re-collecting of classic computers
<classiccmp(a)u.washington.edu>
Date: Wednesday, June 02, 1999 5:29 PM
Subject: Re: Re[6]: More Bringing up a CPM
independantly
On the
subject of LA/scope/logic probe/voltmeter. I personally
consider a LA as the shot gun approach. It will often find
problems when one has no idea where to look. The only problem
I can't agree with that. OK, so a logic analyser will monitor a lot of
points at once, so you can test unrelated sections in the hope of finding
the fault. But when you've got 64 or 128 or whatever traces, you still
have to think about the fault.
is that someone with an oscilloscope can often
have the
machine repaired by the time one gets the leads connected
I've found that a small logic analyser (say about 3-4 channels),
intellegently used, is very useful. It doesn't take long to connect it
up. And it will tell you a lot about the circuit you're investigating.
I rarely use a logic probe because I don't
have one. I
can use an oscilloscope just as well with the additional
features that an oscilloscope can provide when needed.
It is like a logic probe with more added.
You _need_ a LogicDart :-)
Still the best overall trouble shooting tool I
have is
an analog voltmeter. I can move quicker with one of these
I agree. I use my old analogue meter a lot more than a digital one. It's
a lot quicker to read an analogue meter for one thing.
than many can with other tools. As far as I'm
concerned,
DVM's are only good for setting power supply voltages.
Oh, they are also useful for setting voltage references on analogue
subsystems. And checking the exact setting of a variable resistor (when
you want to change it to (say) 3 time the value as an inital point in a
modification).
I have one ( an accurate 5 digit unit ) but the
analog meter
is still the first thing I reach for.
There are 4 instruments that get most of the use on my bench. In order :
An analogue VOM.
The LogicDart
A good bench PSU (30V, 10A). Useful for powering up subsystems A cheap handheld 'scope, audio band only. Useful
for checking for PSU
ripple, looking at motor drive waveforms, SMPSU waveforms, etc.
I find that most don't know how to trouble
shoot. The
sequence is simple but many still don't get it:
1. Observe symptoms
2. Make predictions
3. validate predictions
4. If predictions do not pan out add this
to the observed symptoms and go back to step 2 or 1.
5. Repair bad part.
I find that most don't understand the importance of steps
2 and 3. They try to go from 1 to 5 and usually it doesn't
work. Step 2 does require that you understand what to
expect from each kind of failure.
My method is related to that...
1) Look for silly faults (cables fallen out, etc). Check fuses, etc
2) Unless there's a very good reason, power up and observe the fault
3) Do some standard checks. Check PSU rails, master clock, etc
4) Think of possible causes for the observed symptoms
5) Do tests (either execute commands or make measurements) to check out
each possible cause.
6) If no possible cause is the cause of the fault, you've missed one of
the possible causes.
7) You now know the bad part.
The best piece of 'test equipment' you can have is a brain. _Always_
think about the fault before trying to cure it. Often I make some intial
measurements and then sit and think for maybe an hour before doing
anything else.
-tony