Posted 13 May 2021 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. Kathy Share this post Link to post Share on other sites
Posted 17 May 2021 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? Share this post Link to post Share on other sites
