Best Practices for Troubleshooting Intermittent PLC I/O Issues

[lostintheether]

Hi all,

I’m looking for some advice on troubleshooting intermittent I/O issues on PLC-controlled equipment. We have a production line where a few input signals randomly drop out for a split second, causing nuisance shutdowns that are hard to reproduce.

So far, we’ve checked wiring and terminal connections, replaced a few suspect sensors, and even swapped input cards, but the issue still pops up occasionally.

I’d like to know:
Any tips for logging or isolating signal drops in real time?

I’d really appreciate any insights or examples from similar experiences. These random faults can be such time-eaters!

Thanks for your help in advance.

 

[ptonsparky]

I've had an 'alarm' set to log the time and which input failed. It would indicate what step in the process failed. PITA for even the small projects I had and Mine were all small and relatively low speed.

 

[ModbusMan]

Are these inputs dry contact, voltage, current loop, something else...?

 

[jim dungar]

All of the I/O cards in a rack all all multiplexed into some interface device within the communicates with the CPU, often remotely.
While it is extremely rare, these interface modules and rack assemblies have been known to 'not multiplex correctly'. When the multiplexing fails the CPU reads data from the wrong I/O even though the I/O modules lights are operating correctly.

I found one of these issues when the process was shutting down on a stop command even though the stop button was physically jumpered out. The screen update was to slow to see a contact blip so to confirm the I/O was correct we started checking each input as if this was a startup. We replaced the rack and every thing went back to normal. I had never seen a rack fail like this before.

 

[4-20mA]

Trends... Check your trends, look for correlations, something might line up... I don't hate the alarm idea but that can all be found via trends as well.

Intermittent issues are the worse. Good luck, let us know what is found.

 

[ModbusMan]

Trends... Check your trends, look for correlations, something might line up...

And look for things that aren't immediately obvious... perfect example:
Years ago (not long after being laid off from my first automation-type job), I was interviewing with a company that worked in machine vision based quality control. Think "snap a picture of the widget as it moves down the conveyor, compare against a template, kick out any with a deviation >some percentage." A technician of theirs had been sent out to diagnose a line that was rejecting samples at a far greater rate than anybody could explain. After several days and a couple of camera replacements, it was discovered that the strobe light on a neighboring piece of gear was blinding the camera, causing their algo to reject a perfectly compliant part. Solution was a piece of cheap plastic corrugated board from the office supply store to block the light pollution.