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Old April 23rd, 2018, 02:31 PM   #1
asitec84
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[ControlLogix] Analog input values flicker between ZERO and 'expected value'

Hi everybody,
I inherited a redundant 1756-L71 ControlLogix system with 2 racks with some DI, DO, and AI redundant modules (1715 series). The racks are of course connected through 1715-AENTR adapters.

Now disclaimer, I'm not an expert in these kind of systems.

The question I wanted to ask you guys, as even the technical support couldn't help me is the following:
on the HMI and even by going online with RSLogix5000 and monitoring the tag values I constantly see ALL the analog readings "flickering" between 0.00 and the correct value.
By flickering I mean that for one second (for example) I read 0.00, after that for one second I read the expected value, then agaiun 0.00 and so on...
What can possibly cause this problem ?


FYI:
- the modules are 1715-IF16
- we're using the 1715-TADIF16 termination assemblies
- the modules are set in 'redundant mode' but not in 'SIL2 mode'
- the incoming current is correct (checked through a multimeter and also simulated through a current generator just on the termination assembly pins)


Thank you for your support
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Old April 23rd, 2018, 02:39 PM   #2
VAN
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Does the value go to zero in the PLC?

I would trend it and see if the values are actually changing in the PLC, I've seen something kind of like this when putting a zero value in to kind of clear a registry before dumping "clean" values in. (The solution was to not do that, or have values specifically for screen display)
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Old April 23rd, 2018, 02:45 PM   #3
asitec84
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Hi VAN,
yeah even in the PLC they go to zero... and no I'm not putting any value inside those input tags.
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Old April 24th, 2018, 02:36 AM   #4
chopin
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It could be the wiring or configuration on your analogue card.
Are your channels configured for single ended or differential.
Check your analogue instruments wiring schematics.
You can disconnect all channels and try connecting them one by one.
From what you saying your analogue signals might be sharing a common which might cause the problems.
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Old April 24th, 2018, 02:44 AM   #5
daba
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Try inhibiting the module, then put a value into the input data tags.

This will determine whether the zero is coming from the module, or from code in the controller. If they go to zero, something in the controller is writing to the tags....

Cross-reference your input data tags "By Logic" and look for "destructive" writes that shouldn't be there.

Cross-reference your input data tags "By Tag" and check for double-aliasing.
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Old April 28th, 2018, 12:05 PM   #6
asitec84
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Thank you guys,
the problem was that I was using a COP instruction in order to map AI to Modbus registers but SOURCE and DESTINATION were inverted

As I told you I do not have experience with RSLogix5000 and checking for "destructive" writes as daba suggested revealed the problem.

Much appreciated.
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Old May 1st, 2018, 07:02 PM   #7
daba
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Quote:
Originally Posted by asitec84 View Post
Thank you guys,
the problem was that I was using a COP instruction in order to map AI to Modbus registers but SOURCE and DESTINATION were inverted

As I told you I do not have experience with RSLogix5000 and checking for "destructive" writes as daba suggested revealed the problem.

Much appreciated.
That's what we are here for.... a true "bonkhead" moment, so glad you made it 3

If you are ever tracing logic problems, that "destructive" column in the x-ref is very valuable, and you can double click that column header (you may need to do it twice), to bring the Yes's to the top of the list (That equates to Sort Descending).

So now you have all of the instructions and locations neatly stacked at the top of the x-ref listing, so much easier to see if you've got a problem now.
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