1756-IF8I Card Power Loss

[Danster11]

We had a problem that I have never seen before. 1756-IF8I card with channels 1-4 powering field devices. This card has been working for several years without issue. The maintenance guys noticed that the devices being powered by the card were blank / had no power. They opened the PLC cabinet and the IF8I module in slot 4 which had no lights on it at all. HOWEVER (this is the weird part), somehow at least 1 channel, channel 4 maintained a milliamp value of 13.76mA (4-20MA signal) for the 12 hours that the card was in this state. The channel data is trended by an external software application and the trend flat lined at 86% (13.6mA) the entire time the card was in this state. The technicians cycled power on the PLC enclosure and the card returned to normal operation as did the flow meter and other input devices. Unfortunately engineering was not able to observe or capture the PLC program in the faulted state. Has anyone seen this issue before? Summary: Analog input module appears to be "dead" in the rack but continues to act like a channel is receiving a 13.6mA signal. Thanks!

 

[rupej]

Yep, that's normal for all input cards, digital or analog. If the card fails, the last reading is what is stored. Same thing with Ethernet comms too.

Best practice IMO is to use a GSV instruction to monitor that the card or Enet device is connected and alive, and clear all values (or write whatever numbers are a safe fail-to), as well as trigger an alarm for operators.

 

[Danster11]

Thanks rupej!