what is the first thing you should do when troubleshooting a device?
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, globally erratic behavior, or even total failure of the PLC system. 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.