PLC5 drops comms and seems to reset some outputs? No errors logged..

Dravik · Jul 3, 2011

Has anyone ever seen a PLC5 just drop communication(controlnet, dh+, or serial, i tried em all) and kill some outputs and then a few seconds later come back like nothing ever happened? I don't get anyone errors at all when it happens inside the PLC itself.

PLC5 80c w/ 2 remote racks via RIO.
1 panelview plus and 1 panelview standard on ControlNet, though I disconnected both of them and it does w/ just my laptop as well.(The comms loss, symptom of the larger problem I think).

Several VFDs on the system, We were able to get this to happen consistently by jogging one while the carousel it was driving was slightly out of position.

I've never seen this before, little bit lost!

Dravik · Jul 3, 2011

Oh, and i've replaced the PLC processor as well.

Ron Beaufort · Jul 3, 2011

my distinguished colleague Ken Roach is your best bet on an answer for this ...

in the meantime, check that big bolt sticking out of the right side of the processor-resident chassis ... it's supposed to be for a ground connection ... what (if anything) is connected there? ... if there's a wire there, where is the other end of the wire connected? ...

Dravik · Jul 3, 2011

Hmm, There is a ground wire attached there, I'll have to grab the print and figure out where that got earthed.

Ron Beaufort · Jul 3, 2011

I'll have to grab the print and figure out where that got earthed.

suggestion: follow it hand-over-hand instead ...

Dravik · Jul 3, 2011

Agreed, Before I left plant today I had already found 2 ground wires that were loose in panels out by the machine. I suspect more of the same in the MCC. Oh and someone had removed the termination resistor from the RIO at the PLC end. Bah!

Ron Beaufort · Jul 3, 2011

and of course the classic questions:

(1) has this been an ongoing problem - or something that just suddenly popped up? ...

(2) has anything recently been changed in (or around) the system? ...

the more information you post, the more detailed the answers will be ...

Ken will be along shortly ...

Dravik · Jul 3, 2011

Um, the system is about 9 years old now. This particular problem has been cropping up occasionally over the last few months. I found out that normally the operator will shut the PLC off and go take a break, and then upon returning they can normally run for another few hours. On friday everything went to heck and it hasn't run for more than 5 mins since.
There have been no electrical changes recently, but the weekend it first started we did have a nasty storm. Hence why we changed out the processor.

Mickey · Jul 3, 2011

Power supply?

Dravik · Jul 4, 2011

Well, Replaced that, but in the process found out that the electricians didn't check the line voltage like I asked. Guess I should have watched to make sure, lesson learned!
Soo, The isolation transformer is only putting out 103v. Sigh. Thanks for the help all!

Dravik · Jul 4, 2011

Hmm, and the problem isn't completely solved yet, but it is better now.
Do you think a VFD->Motor w/ a ground fault somewhere could cause a PLC to reset?

Dravik · Jul 4, 2011

We isolated it to only happen when this particular VFD starts up, now just trying to figure out what about it is borked.

Christoff84 · Jul 5, 2011

Dravik said:
We isolated it to only happen when this particular VFD starts up, now just trying to figure out what about it is borked.

If this is a larger VFD could it be dropping the line voltage on startup to a point where that 103V form the isolation transformer is dropping below the level needed to keep the PLC alive?

I'd put a meter on both the line voltage for the VFD (assuming 480V) and the line voltage to the PLC and see what the minimum values you read are.

Lancie1 · Jul 5, 2011

PLC5 80c w/ 2 remote racks via RIO.

If one of the remote PLCs is lost, shut off, has temporary low supply voltage, or the comm cable between Remote and Main is loose, disconnected, or affected by VFD drive noise, then you can see all sorts of weird problems in the main PLC. Loss of a RIO input is loss of a critical piece of data for main PLC to make decisions.

PLC5 drops comms and seems to reset some outputs? No errors logged..

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