Getting started with PLC programming could make you instantly more valuable as an engineer. Without PLCs, we would not be able to run our industrial factories that contribute to every economy all over the world. We need PLCs to use real-time software to monitor what happens inside an industrial system and control what the results will be. The running joke is that a person could come into contact with a PLC without even realizing it due to PLC uses in elevators, amusement parks, etc.

EIT Stock ImageThe PLCs are programmed to understand input conditions of whatever machinery they are attached to so that the PLC can decide what the outcome of that input is going to be. We need PLCs to keep the proverbial show on the road and ensure that industrial manufacturing works efficiently.

The PLC does a program scan that uses the entire system to scan for any incoming inputs and then does what it has been programmed to do. The scan takes one to two milliseconds. The scan happens as follow:

  • The state of inputs (sensors, switches, pushed buttons) are scanned ; what the PLC needs to continue its scan
  • The program is executed the pre-programmed logic of what the engineer wants the PLC to do with the input values
  • Housekeeping of any devices connected or related to the PLC currently scanning
  • 'Scan' the user's ladder logic left to right, sequencing through the 'rungs'
  • Compute the results and write updates to the outputs
  • Do diagnostics and if all is well, repeat the scan

Sometimes things can go wrong. Once a PLC malfunctions, the reasons might not be glaringly obvious. The PLC might need troubleshooting. Whether you are an engineer-in-training or a veteran, learning how to troubleshoot a PLC could save an industrial complex some money.

The Dean of Engineering at the Engineering Institute of Technology, Steve Mackay, has spoken about troubleshooting PLCs in the fortieth episode of his online series named the Engineering News Network. He says there are two areas that experience two types of faults in PLCs; internal and external. He says that eighty percent of the faults are usually the external components being faulty. 


The internal issues within the PLC could relate to the decision-making CPU. The CPU is the main decision-making module within the PLC so it is a good idea to keep that as safe as possible. The communication ports are also important to keep safe consider that they can be connected to other PLCs and so they can both share data. Mackay says the earthing and grounding of the chassis that houses the PLC's elements could not have been done properly. He says: "That has to be done firmly and effectively. I've had many problems with that occasionally. Obviously, the power supplies to your PLC and the racks need to be continuously operative." The internal workings of the PLC could also be experiencing electromagnetic interference problems. "Those [electromagnetic interference problems] are intermittent, very difficult to trace but can be the cause for a lot of angst. Corrupted programs need to be looked for," Mackay added.

The batteries also need to be monitored to ensure that they are working all of the time so the PLC can function.



"Check power supplies, the power, the fuses - you've got voltage continuously to your inputs," Mackay warned. Checking the wiring is also a good idea, noticing any corrosion could lead to thwarting a problem before it gets worse. He points out that there could be problems with a leakage current: "You think the input is off when its actually on. There is enough leakage current coming in that makes it appear off when its actually off."

This would be done by checking the digital outputs which would tell an engineer whether or not there are irregularities in the PLC's operations. This is done by using a meter to ensure that the proper voltage is coming out of a PLC.

Outside noise could also be a cause of a PLC malfunctioning. Ensuring that external sources are not producing noise is sometimes inevitable and so a filtering process then needs to be followed, by installing a noise filter.

Due to eighty percent of PLC faults being external, the PLCs are equipped with diagnostic LED indicators that will show whether or not a module is performing as normal or malfunctioning. Tracing the source of a fault is, therefore, easy to locate.

Finding the faults in PLC controllers are paramount to the continual operation of industrial automation to ensure that manufacturing plants work around the clock and work efficiently. Learning how to program is all good and well, however, learning how to troubleshoot is as important so that you can keep an industrial factory working optimally.




The Engineering Institute of Technology (EIT) is dedicated to ensuring our students receive a world-class education and gain skills they can immediately implement in the workplace upon graduation. Our staff members uphold our ethos of honesty and integrity, and we stand by our word because it is our bond. Our students are also expected to carry this attitude throughout their time at our institute, and into their careers.