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Old July 11th, 2018, 02:34 AM   #1
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1794-IE8 - strange readings

Hi All,

I've probably programmed hundreds of 1794-IE8's over the years. The panel builder I'm working with has installed thousands of these so I am reasonably sure that they wired them properly. Anyway, yesterday I was checking out a panel with 5 of these and all of them exhibited exactly the same problem.

I scaled the inputs programmatically to report 0 at 4ma and 30840 at 20ma. Every single input gave an offset of about 600 counts so instead of 0 input reported 600 at 4ma and 31440 at 20ma.

All of the modules are configured for 4-20ma and I DID NOT check out the wiring, perhaps I should have...

Anyone ever encounter this?

Thanks!

Yosi
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Old July 11th, 2018, 09:22 AM   #2
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Ground loops ?

See link...

https://www.google.com/search?q=grou...hrome&ie=UTF-8
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Old July 11th, 2018, 10:23 AM   #3
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I've seen two Flex I/O racks in noisy panels get flaky. Analog inputs and outputs would flatline and hold whatever value was happening at the time. Discrete I/O remained fine. A power cycle would clear that for a few hours or a few weeks. Turned out the Flex base ground tabs didn't pierce the oxide layer on the aluminum DIN rail. With a module pulled you can see one hole for panel mounting instead of rail mounting is grounding. None had continuity to the rail. A change to steel rail remedied the problem.

Last edited by 5618; July 11th, 2018 at 10:47 AM.
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Old July 12th, 2018, 01:14 AM   #4
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Thanks! Greatly appreciated. I'll look into that when I'm in the field next week.
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