Mysterious Problem?

Ok. Riddle me this one.

I have a machine that's been running for a couple of years. It is outfitted with a AB 1768-L43S Processor with a 1769-IQ16 digital input module in the first slot. Today one of the inputs from a 2-wire prox went out - no light on the prox and no signal on the input module. Once all of the field hardware was verified to be working (and indeed it all was), I identified that the PLC input, itself, had to be the culprit. I was able to do this by lifting the wire from the "bad" input and touching it to a known "good" input. Doing this, prox works again and a signal is received by the PLC. Putting it back on it's original input, however - nothing.

Bad input on the card you say? Seemingly not.

I replaced the card with a new one and the same input still does not work. What is going on? The only thing left, it seems, is the processor and something with it. At least that's all I can think of. Fortunately there was a spare input on the same module and by moving the wire and some minor reprogramming, I was able to get the machine going again.

Finally, I took the old module back to my office and connected it to a processor I have on the bench. The suspect input (12) works just fine on it.

Any thoughts?

Is common wire same for both inputs?

do a cross-reference on the input - see if anything shows up as "destructive = Y" for it ... rare - but not as rare as many would suspect ... (incidentally, that's what changed my life for the better about 20 years ago) ...

how about an HMI or similar which has recently had a programming change - and might be writing into that input bit/box over a network connection? ... rare - but possible ...

It's a stretch...but could the wiring arm in the field have a bad connection to the module on the faulty channel?

Lare said:

Is common wire same for both inputs?

Click to expand...

As a two-wire prox, there is no common beyond the backplane of the PLC card. The 24V power, however, is verified to be the same and referenced to the common of the PLC.

Ron Beaufort said:

do a cross-reference on the input - see if anything shows up as "destructive = Y" for it ... rare - but not as rare as many would suspect ...

Click to expand...

No, nothing interesting in the cross-reference that is destructive.

Ron Beaufort said:

how about an HMI or similar which has recently had a programming change - and might be writing into that input bit/box over a network connection? ... rare - but possible ...

Click to expand...

Now this is a possibility...The HMI program MIGHT have a TOGGLE type input of which I am unaware that references that input. If that's the case, it wouldn't show up in my PLC program and could be "held" low. I will have to look into that. Thanks.

Mispeld said:

It's a stretch...but could the wiring arm in the field have a bad connection to the module on the faulty channel?

Click to expand...

Nope. Checked that too. The wiring between cabinets and up to the PLC moduel all tested good.

When you changed the IQ16, did you change the removable terminal block also?

You say you didn't get a signal, were you looking in the software, or did you actually see the led not light up on the front of the IQ16?

There is a filter setting per group. Was the channel you swapped for in a different group? If so compare the filter settings.

Second what Lare said about the commons being different for groups 1 and 2.

Not an AB guy by any means, but aren't the LED's on the card hardware controlled, not software controlled. Seems like it is on every one I've looked at.

That said, I'm with Mispeld on a wiring issue, perhaps in the card rack / backplane or whatever they use to connect the i/o cards in that model...

I had a similar problem years ago.

tighten all terminals and see if the problem goes away.
then stress test the wiring by pulling on it. you may have a connection that is corroded and about to break.

james

Ron Beaufort said:

... (incidentally, that's what changed my life for the better about 20 years ago) ...

Click to expand...

Oh do tell.

Well you proved that the CARD is good.
What is left is wiring coming in and program.

or the processor ...

Steve Etter said:

Finally, I took the old module back to my office and connected it to a processor I have on the bench. The suspect input (12) works just fine on it.

Any thoughts?

Click to expand...

Processor, backplane, or IO terminal strip on problem system?

James Mcquade said:

I had a similar problem years ago.

tighten all terminals and see if the problem goes away.
then stress test the wiring by pulling on it. you may have a connection that is corroded and about to break.

james

Click to expand...

I second this. I have seen problems like this solved by following the wire all the way to the source and back and opening every splice along the way to check for corrosion or broken strands. Especially if this is aluminum wire.

Steve Etter said:

...I identified that the PLC input, itself, had to be the culprit. I was able to do this by lifting the wire from the "bad" input and touching it to a known "good" input. Doing this, prox works again and a signal is received by the PLC. Putting it back on it's original input, however - nothing.

Bad input on the card you say? Seemingly not.

I replaced the card with a new one and the same input still does not work. What is going on? The only thing left, it seems, is the processor and something with it...

Click to expand...

Fault finding hat on...

This is not to 100% rule out wiring here, but if I'm reading the above correctly...

If there was a wiring fault at the point in time when the sensor was not working and the input was not turning ON, then I would imagine that this fault would have transferred to the test input. But as soon as the assumed faulty wiring was moved to the test input, it turned ON. When returned to the original input it once again did not turn ON. This would suggest to me that the module wiring and field sensor are OK.

So I would agree that it is most likely something with the processor itself, or the backplane, or programmatical influences on the input.

Steve Etter said:

...Finally, I took the old module back to my office and connected it to a processor I have on the bench. The suspect input (12) works just fine on it...

Click to expand...

This would support the above as the program, if different, the processor, and the backplane have all been substituted.

If you cannot find anything obvious in software that may be influencing the input then I would continue to try and eliminate the processor and backplane (chassis), if possible, so as to rule them in out. This, of course, may be easier said than done if you do not have like for like spares.

Regards,
George

Some ideas,...

Have you changed any of the other I/O on that bank of 8? or the entire card?

Do you still have a solid 24V on the power supply?

Did you try to change out the prox?

If you take everything else off the card and put the original prox wire in input 12 will it work then?