The top five problems with PLCs

By performing sequences of instructions such as timing, counting, storing memory, relaying logic and arithmetic computation, the PLC's duties are crucial and imperative for the complex processes needed. So what can possibly go wrong?

There is a propensity amongst those unfamiliar with the PLC to fear the mysterious 'black box' when troubleshooting. However, PLCs are in fact easier to troubleshoot than hard-wired control systems, with more open and easy diagnosis due to the black box.

Putting aside the universal signs of something going wrong, such as programme bugs and wiring errors, here we will explore the top five things that can go wrong with the PLC.

1) I/O modules
Everyone generally assumes that when something goes wrong with a PLC, it is due to internal processor problems. Wrong!

A big percentage of problems are the result of I/O modules or field equipment. No need to panic either as it is not difficult to diagnose whether the problem is emanating from the I/O system or in the processor. Both types of problems have unique signatures allowing an even easier examination, and therefore conclusion.

2) In or out?
If the problem is traced back to a specific I/O module, this means that it is usually an external one, like the aforementioned wiring errors. If it is an internal problem, this could actually result in erratic behaviour, large groups of failures or even total failure of the PLC system!

However, one of the first checks to do now is the 'integrity' of the PLC. This doesn't mean questioning whether the PLC is a good, honest citizen with morals and a clean tax record, rather the integrity of the ground has to be electronically checked.

The power and ground wiring also needs to be examined further to ascertain whether it is loose, corroded or has damaged connections. The power supply should be checked using a digital meter and both AC/DC voltages should be zero.

3) Interference
Another problem for the PLC can be the effects of EMI or RFI. These can be related to lightning strikes, welding in the area or handheld radio transmitters. The handheld radios used by maintenance staff, emit powerful radio frequency radiation, disrupting and interfering with any unprotected electronic equipment. Improvements in shielding, grounding and power conditioning can combat any EMI or RFI problems.

4) Corrupted memory
Frequency interference, power and grounding are all problems that can disrupt and corrupt the PLC's memory, so it is crucial to verify the programme is still correct and comparable with a backup copy on tape, disk or in the cloud. As with all data backups, ensure they are up to date and kept away from EMI, RFI, high temperatures and humidity.

5) Confusion
When PLC troubleshooting, a major aim is to find out why the internal status of the PLC is in conflict and not in agreement with the external situation.

Our job is to determine the status of the relationship between physical I/O modules and I/O instructions in the PLC programme. Different manufacturers have different solutions and schemes, usually a terminal, handheld unit or PC. Either can be used to check the internal status of the input/output in question.

To conclude, the PLC can be both advantageous and disadvantageous. Yes, there is and always will be a lot of work required in connecting wires, a difficulty with changes and replacements and potentially long hold up times when something does go wrong.