Logic Analyzer Comparison test technique

uriahsky

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I was wondering if there was a way you could use a logic analyzer to test IC's while in circuit by storing a known good pattern and then comparing that to the pattern from a board under test?

I was thinking that if I could just store a pattern that is tied to the clock and somehow find a start time then if a chip is working correctly I should be able to see the same pattern on another board?

Do you think this could be done? Anyone do something like this or have some suggestions on how to implement this idea?
Thanks
Russ
 
As you say you would have to sync the boards exactly or its not going to work for the majority of chips on the board. It sounds like a method of troubleshooting that is about 1000 times harder than just learning how to troubleshoot things normally. Not least because you would always need to have an exact working copy of the board you are trying to fix, same software revision, same hardware revision etc etc.
 
Technically thats how a logic comparator works..it compares a known good chip to the one in circuit and displays any differences in output.
 
Technically thats how a logic comparator works..it compares a known good chip to the one in circuit and displays any differences in output.

It is - but he is asking about using a logic analyser to record the behaviour of chips and then comparing the recordings taken which sounds like a hell of a lot of work. Its likely that only a 100% match would be the useful result, any non-match is more likely to be a timing issue than a fault. Plus a fault on a board usually causes a cascade effect in the logic, finding a mismatch on a board with 100 or so ICs is more likely to be a symptom of the fault than the cause.

A comparator would be easier if you have a stock of known good chips, but ones that support chips over 16 pins are hard to find. Have heard of folk resorting to building their own for the larger chips.
 
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Logic Comparator

I was emailing with Joe Farr the designer of a IC tester in Everyday Practical Electronics.
This is a much cheaper way to build a IC tester that has the ability to add the library of testable IC's that is hard to find on many pocket IC testers.

Here is a video of the tester.

http://www.youtube.com/watch?v=FsvtkHIPlpE

That isn't me but it a good way to see what he did.

I was asking him if there was a way to make his tester work on chips in-circuit and the problem he told me would making it fast enough to work in-circuit, but one of the other ideas was a software logic comparator. Take the hard ware clip and then build all of the IC's in software then use that to clip on and test IC's.

Has anyone done this? Make a software controlled Logic Comparator? I am surprised someone hasn't already created this. We never reached a inexpensive conclusion.

Another ideas was a pattern generator and logic analyzer but that doesn't work in circuit when other things are driving the inputs.

ABI Electronics makes a tester that has a compare feature but their gear starts at around 10K.
Russ
 
Its a very impressive project but most USB eprom readers have the same function built in, at least for 74 series and SRAM/DRAM chips.

Never heard of a software based comparator, I suppose it is perfectly feasible tho, hardware comparators can be had on ebay for less than $50, but you need a known good chip to test against. Which is where a desoldering iron and a chip testing USB eprom reader comes in very handy.
 
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