CompactLogix Analog Input Lost Connection

The story you are about to read is true. Only the names have been changed to protect the innocent…

CompactLogix Analog Input Lost Connection

Case File: 2016_05_26_001

Problem: Lost Connection to CompactLogix Analog Input Module 1769-IF8

In this week’s Tech Files case review, we look at a perplexing case that involved a 1769-IF8 analog input card losing its connection to the CompactLogix CPU.  As with most cases in the Tech Files, all is not as it seems in the beginning.

Our Tech Files case started with a simple installation of a CompactLogix L30ER CPU and just two I/O modules attached to the CPU rack.  One of those modules was an analog input card, 1769-IF8, the other a discrete combo module.  There were 5 identical system installed at the same time.  Two of the system were exhibiting strange behavior.  After some amount of time in operation, the 1769-IF8 analog input module would lose its OK light, and freeze the values at the last known state.  The initial course of action for this user was to replace the analog input module with a new one.  But the issue occurred again with the newly replaced analog input module.  Oddly enough, a second identical system installed in another area of the facility did the same exact thing.  And they also replaced that 1769-IF8 module with a brand new one, only to find it too would lose its OK light after some time in operation.

This led the customer to contact us and discuss the issue in more detail.  Upon initial review we could not find any configuration issues, and what made the problem even more puzzling was that three system were operating with no issues, and two were not.  All five systems were running the same configuration and program.

Initial thoughts turned to the RPI setting, which was set to 80 msec for the 1769-IF8.  But the system would run for several weeks before choking, and again three identical systems were operating with no issues at all.

Next discovery was a knowledge base article that at first appeared to provide some explanation.  AID# 558834 described a known issue that affected some 1769 I/O modules. The problem existed in versions 20-23 of the L3Y family of CompactLogix CPUs.  This application was running version 20.12.  The resolution was to move to Version 24 of Studio 5000.

CompactLogix controller does not properly handle some 1769 I/O module faults

The user decided to convert their applications to version 24, and did so after an planned outage.  But it did not take long for the same issue to return and on the same two systems that had the issues previously.  Same symptoms, the 1769-IF8 module would lose its OK light during operation.

We now had to go back to the knowledge base well to look for more information.  Luckily for us, we got a hot tip from a tech support engineer.  There was a knowledge base article that was written for a temperature input module, 1769-IT6, that gave an important clue.

AID# 65227 described a problem with the thermocouple input card that was induced because of its proximity to the AC power supply.

Values from the 1769-IT6 module are fluctuating

Going back to the basics, we realized that the 1769-IF8 module in these racks was placed immediately to the right of the AC power supply, 1769-PA4.  Our next troubleshooting suggestion was to either move the 1769-IF8 to the second slot to the right of the power supply, or change the AC power to a DC power supply.

The user decided to relocate the 1769-IF8 module, and keep the AC power supply.  To this date, the system has remained in operation.

This was one of the more challenging cases from the Tech Files, but demonstrates the importance of gathering all of the facts, and working to eliminate all of the possibilities.

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