All PLC I/O Common or Not

[polareng]

Where can I get some information on good plc I/O connection practice?

My question is basically whether I should be connecting my I/O's to an isolated common or should the modules be independently common of each other?

The reason I am asking is because I found that the (-) output 0-10Vdc signal from our VFD to our ControlLogix 5000 input module possesses an internal ground at the drive in which obviously is not an isolated common circuit.

In addition, the PanelView Plus, that I had on the same isolated common, I found out has an internal ground, in which, the manual specifically states that there should not be any other grounds in the system for safety reasons, in other words, other than the PanelView's ground.

So this led me to beleive that maybe I don't necessarily need to be connecting all my I/O modules to a common common, I could just leave them independent of each other. Is this true?

 

[Skogsgurra]

No, not as a general rule. Many manuals are a bit nebulous when it comes to grounds, signal grounds, earth, internal zero reference, chassis, screen ground - or whatever they are called.

Sometimes, there may be a need for a separate signal ground, but that signal ground is usually tied to common ground somewhere.

Keep it simple. And make sure you don't read more into the text than there is.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...