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Question from a PLC newbie....  We had a "blip" during a thunderstorm where we saw multiple signals on a 1734-IB8S point I/O transition from true to false for ~ 4 seconds (based on our DCS history from a serial connection).  There are several other modules on the backplane, and we only saw state change on this one card.  The 24VDC power is sourced from T-0 for points 0-3, and T-2 for points 4-7, so we don't suspect a short.  It's been suggested that we either had an I/O card "blip", or temporary loss of power to our field contacts (limit switches) caused by dust/vibration of the removeable terminal block.  Our controller is a 1756-L73S GuardLogix 5570.  My question is should I expect to see an I/O fault in the Minor Fault history (we did not)?  Would an I/O fault necessarily show up in this history, or does it need to be configured somewhere?

Our points on this card are set up as single points with test pulse from T-0 and T-2.  We are reading status of limit switches in the field.

Thank you in advance for any response.



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Electricity is extremely predictable except in three scenarios: ground faults, arc faults, and lightning. In those cases, typically all bets are off. It is interesting that you had false indications on only one of several I/O cards on a backplane. When something like this happens, I usually ask myself, "what's different". Some random thoughts, in no particular order:

  • With no minor faults in the fault log, I'd be inclined to think that no faults occurred and this is truly an electrical field device/wiring anomaly, rather than a card problem.
  • Are the limit switches physically located where they might be more susceptible to stray currents during bad weather than other related I/O?
  • Is the shielding or grounding different on this card or its I/O points/field devices compared to others on the backplane? 
  • Did you get this error on inputs 0-3 and 4-7?
  • Is the 24V field power supply shared with any other I/O?
  • Have you checked the RTBs for dust or loose connections? Find anything that looks suspicious?

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