Micrologix 1400 Faulting

jasonh

Member
Join Date
Sep 2010
Location
Minnesota
Posts
7
I have a Micrologix 1400 series A PLC with 3 1762 IA8 Input cards, 1 1762 OW16 relay output card, and 1 1762 IF4 Analog input card. The problem I am having is that every week to 1 1/2 weeks, the PLC will fault out saying that it cannot communicate with one of the expansion cards. This is a repetitive issue and I was just wondering if anyone else has had an issue like this and maybe what they did to resolve it? I have looked at noise and grounding to maybe be at fault but am not sure. I have attached a screenshot of the fault. Thanks for the input in advance.

untitled.jpg
 
i had this problem with device net.
we had to reseat the cards. that solved the problem with most cards. we had to replace one card, the pin had broken at the solder joint. we eventually moved the control panel 4 inches onto an independent stand. the machine vibration was causing the problems.

hasn't stopped in two years now.

regards,
james
 
If I re-seat the cables on all of the cards then it will run for a couple weeks and then fault out all over again. My next move was to move the control panel like you suggested. Glad to hear it took care of your problems hopefully will help with mine. Thanks for the advice.
 
I'll bet you a case of donuts that one or more of the OW16's relay outputs is connected to an inductive load without a snubber.

That's a classic fault for any controller; the collapse of the magnetic field in a large inductor (pneumatic or hydraulic valves, motor starters, etc) causes a large back-EMF of up to many hundreds of volts, which interferes with the backplane or expansion bus.

Every manufacturer's installation manuals strongly recommend spike suppression circuitry for inductive outputs, both for solid-state (transistor and triac) and relays.

I would build a little data trap to see if there's a consistent output that turns on or off just prior to the fault each time; that would help you narrow it down.
 
Yeah i am looking at getting snubbers put on the outputs coming off this relay card. So is it a voltage spike then that is causing the card to fault out somewhere on the "backplane"?
 
Ken Roach said:
I would build a little data trap to see if there's a consistent output that turns on or off just prior to the fault each time; that would help you narrow it down.

I had to do this once on a PLC-5 system with remote 1746 I/O. The relay card that ended up being the culprit was driving a solenoid valve buried deep under the machine, so I switched to a triac output to solve my problem (after looking at the prints and the loads, there was no reason I couldn't in this particular situation).

Setting the logic trap was not too hard, but waiting up to a week between faults was a PITA. I finally nailed down which card and which point was to blame. Point 7 on a 1746-OW16 is right next to the backplane.

jasonh said:
Yeah i am looking at getting snubbers put on the outputs coming off this relay card. So is it a voltage spike then that is causing the card to fault out somewhere on the "backplane"?

Yes.
When the PLC can't figure out which module is fouled up, that is donut-bet worthy evidence it's significant backplane noise.

Another quick 'n dirty fix might be to use an interposing relay. You can get them as small as terminal blocks.
 
I just thought of something else you might try.
we had several units to fault out on us at power up and after discussing the problem with AB, we went into the scan setting screen and toggled the fault over ride at power up bit.

some ab modules do not initialize within the alloted time and cause a fault exactly like you stated. by toggling the bit, you keep this from happening.

regards,
james
 
OK. So I installed snubbers on the solenoid valves coming off the relay output card and also installed some ferrite core noise filters on the cables coming out of same card. Both machines have been running for over a week now without a problem. Thank you all!
 

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