Are most chip problems catastrophic... ie- open or shorted?  If so,
then can't I troubleshoot typical problems with just a logic probe to
test for highs, lows, and pulsing?
Since the internal resistance of TTL chips is low, can't I just remove
power and test the input-to-output pins?
How about hooking up a 3volt battery in series with a buzzer, then
negative to ground/positive to input, and listen for the buzzer (ie-
current flowing)... or visa-versa?
Surely I don't have to understand all the AND/NAND/OR/NOR functions in
the practical world of troubleshooting... ie- it's either working or
it's not?

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Request for Question Clarification by markoft-ga on 15 Jun 2002 12:54 PDT

Are you looking to troubleshoot individual chips or fixing a large
scale electronic device such as PCs, VCRs, or a video game console?

To properly answer your question, a little more context is needed. 
Still, this might help.

At the silicon level, yes, most IC failures are opens or shorts. 
Failure analysis consists of removing part of the black casing and
inspecting the circuit under a microscope.

But that's rare.  It's only done when you're verifying that the fab
manufactured the chip in a way that corresponds to the layout files
you sent them.

So it depends what kind of chip you're talking about.  The above is
used for microprocessors, for example.  Smaller chips that are
well-understood and stable designs are tested differently.  There are
IC testers that will take (for example) a 14-pin TTL logic 74xx series
chip, test the ins and the outs, and tell you a) what kind of chip it
is (the xx part above) and b) whether all gates are working properly.

You *can* do this with a logic probe, and in fact the chip tester
works the same way.  But you do need to know the inputs and the
expected output to know whether you have a good chip.

Transistors are frequently tested with an ohmmeter.  A single
transistor's most common failure mode is shorted.  So 0 ohms across
the transistor (usually test the outermost two pins) is a bad
transistor.  A logic gate is made up of several transistors, so
although 0 ohms is still an indication of a bad gate, it's a much less
common failure mode (several transistors would have to have failed
identically for this to occur), so it's usually not a valuable test.

Lastly, you won't be able to drive a buzzer with a TTL chip.  The
current required to make the buzzer buzz is way above the chips
driving abilities.  At best it wouldn't work (with certain clamping
designs), at worst it would toast the transistors inside the chip.  A
small transistor amplifier could do it, but it wouldn't work any
better than a logic probe.  (some logic probes have piezoelectric
transducers in them, switchable to chirp on logic high or low.)  You'd
still need to know exactly what you were testing.

Hope this helps.

You should also keep in mind that some errors could be logic errors in
the logic programming, i.e. 1+1!=2. Usually these types of errors are
not that trivial, and can only really be found with actual use of the
chip (i.e. you don't find the problem until you sit down and start
programming it) Here is a real life example, at my last company we
developed a DSP, and when we got back 1st silicon we found a few
problems (like the ones talked about above) before we really started
using it, and so we fixed those problems, then produced new chips.
When we got 2nd silicon we started to really use it for complex
programs. When we did this we found that if an interrupt occurs in the
second cycle of a multiply complex (mpycx) the results of the mpycx
were lost! Something like that would be very hard to find without
actually sitting down and using the chip...

After reviewing the two most recent replies, one must also consider
problems that occur such as propagation delay and "edge" rise and fall
times.

It would be a little more helpful if there were some specifics to your
question. However, to answer one portion of your question;
 
No, logic chips can fail for other reasons than a simple open(s) or
short(s). That is to say, that your IC is configured as a trigger or
as a simple oscillator/timing circuit, timing errors caused by
propagation issues within the devices internal capacitances can
occur....

Exactly what type of circuit are you attempting to troubleshoot?

Hello ignoranttech,

I agree with most everything that other users have shared. Most chip
(assuming you are talking about microprocessors, flip-flop chips and
the like) failures are an open or short case, however like
pcnetworkctr stated - there can be numerous other causes, all
dependent on the circuit and chips in question.

Without knowing the quiescent state of the chip in circuit, trying to
troubleshoot it would be a waste of your time unless you could get
your hands on something like a Huntron trakker. (
http://www.hammondelec.com/tr200.html ) Essentially this injects a
signal, and you read the output of the signal after it has been
processed through the IC. You could take a known good chip, and do pin
to pin tests, and compare that to the same readings from the suspect
IC.


There are older industry practices that aren't in regular use anymore,
but may prove helpful since you stated you didn't want to get into
binary logic.

Here is what you *can* try, assuming an everyday cheap IC:

1) Buy some of the same chips you think may be defective.

2) Piggyback. This means place the new chip on top of the old chip
while it is still in circuit. Place it on top of the chip in the same
pin order...i.e pin 1 on 1, pin 2 on 2 ( Be sure to turn off power
though :-)

This does many things. If you have an internal open circuit situation,
the path of least resistance is through the 'working chip', as a
complete open is infinite resistance. The signal ,(logic level -
waveform etc..) then is in a completed circuit and *should* work as
normal. I say should, because you could be dealing with an IC that has
more problems than just the open. If this solves your problem, then
you can simply replace the IC in question.

This will not work with a short, because the path of least resistance
will be the short itself, and the signal in question will once again
take this path.

This is an old technique, and I found myself using it on rare
occassions years ago. I would not use this in a daily routine however.
There is definitely the possibility of destroying perfectly good chips
if the problem is elsewhere in the circuit.

Hope it helps -

sgtcory