This article is only to cover the basics of PLC troubleshooting. The first step in PLC troubleshooting is to decide if the problem is internal to the processor or in the I/O system. It seems to be natural to assume that most malfunctions of PLC systems are due to processor problems. But from the experience mostly PLC malfunctions can be traced to problems with I/O modules or field equipment.
It's relatively easy to determine whether a problem is located in the processor or in the I/O system because each type of problem has a unique signature. Problems that can be localized to a specific I/O module or even a specific input or output device are usually external, while internal problems normally result in large groups of failures. Let's look first at the possible causes for internal problems.
The first thing to check is the integrity of the PLC's power and ground. Visually inspect the power and ground wiring, looking for loose, corroded, or otherwise questionable connections. The integrity of the ground can be electrically checked by measuring the voltage between the PLC ground terminal and a known ground.
Second is to check the DC supplies for AC ripple. This can be done using a digital meter set on a low AC range, and the value measured should be well below the manufacturer's specifications.
The final power check is to measure the voltage of any batteries in the system. Battery power is often used to prevent a PLC from losing its program during power outages, and battery voltages should be within recommended values.