Signatures

[alan.bain]

I'm curious to know how "stable" signatures are between analyser vendors. I have a flight (late flite) electronics 20MHz signature analyser which passes all its tests (and measures all the correct signatures on a similar flight analyser). Manual states signatures should match HP ones.

Using it on an HP3563 (with fault) I'm getting some signatures that match the HP manual and some that don't, but some are making me suspicious that this is not because of my fault e.g. a replaced 74LS164 shift register clocked at the signature analyser clock. The signature on the serial input matches that in HP manual and the signature at the first tap is identical to that at the input - whereas the manual suggests should be different and I see the signature which should be at the first tap at the second tap and ditto all the way down. This rather suggests to me that there is a discrepancy in the point of data capture relative to the clock edge between the Fligh and HP signature analysers.

Ideally I'd compare with an HP 500x. But I lack one and they seem to sell for lots more money that I'd expect and normally I prefer to fault find logically, but in this case that would need more knowledge of what the test routine does than HP provide!

[Stray Electron]

   I can't help you with those models but I was a Field Engineer for Burroughs Computers back in the day and they attempted to use Signature Analysis on their very large, all TTL logic, computer boards and IMO it was a huge failure. Every time that they made any modification to the circuit boards (and there were MANY), it would change some of signatures but the Change Notices never told us the new signatures and most of the people didn't keep very good records of the revision/modification levels of the boards in their computers. So we always ended up chasing our tails and ultimately everyone that I knew simply quit using the SAs.  Years later I bought both an HP 5004 and a 5005 SA and I tried using them for some of the HP equipment that I had but I never really found them useful.  In most cases it was impossible to tell what component had failed by using an SA and I ended up having to test the ICs using a logic probe or one of HP's 10529 Logic Comparators. Those methods were faster than trying to figure why I was getting a wrong code on a SA or perhaps it would be more correct to say IF I was getting a wrong code.

   IMO there is a GOOD reason why no one tries to use Signature Analysis any longer. 

[alan.bain]

Agreed about utility of the signature process - and of course these days built in microprocessor self-tests are the way forward.  I wonder why they didn't program the signatures into the end of one of the firmware EPROMS - at least that way they would stay in sync and could easily be read out (HP's required test equipment to fix one of these things usually includes half a lab full of other HP boxes, so one more eprom programmer wouldn't hurt surely?).
Normally I prefer logical faultfinding methods, but the block in question which intermittently malfunctions includes a statemachine, serial x parallel multiplier an EPROM used as a filter and various shift registers operated under microprocessor control.  Point checks with the scope show it multiplying correctly (twos-complement serial multiplication - last time I saw someone checking this with a scope was on an Elliot computer).
I did start into disassembling the firware to find out exactly what it should be doing and found the relevant code, but needed more information on the address space to make progress and next step was to get an STM32 to dump out the 82S100 address decoder and this hasn't happened quite yet.