What is a PLC? 

First I’d like to explain in the most non-technical terms possible, What a PLC is. As this article is not just for the maintenance technician, but for maintenance managers, plant managers and corporate managers. A PLC (Programmable Logic Controller) is the type of computer that controls most machines today. The PLC is used to control AND to troubleshoot the machine. The PLC is the brain of the machine. Without it, the machine is dead. The maintenance technicians we train, are the brain surgeons. That is how I explain it to my doctor any way. (His mouth drops open, "... you train brain surgeons?")

Important Note

Just as a doctor asks the patient questions to figure out what is wrong, a maintenance technician asks the PLC questions to troubleshoot the machine. The maintenance technician uses a laptop computer to see what conditions have to be met  in order for the PLC to cause an action to occur (like turn a motor on). In a reliable maintenance management environment, the maintenance technician will be using the PLC as a troubleshooting tool to reduce downtime.

A little more detailed definition of a PLC

A programmable controller is a small industrial strength computer used to control real world actions, based on its program and real world sensors. The PLC replaces thousands of relays that were in older electrical panels, and allows the maintenance technician to change the way a machine works without having to do any wiring. The program is typically in ladder logic, which is similar to the wiring schematics maintenance electricians are already accustomed to working with. Inputs to a PLC can be switches, sensors, bar codes, machine operator data, etc. Outputs from the PLC can be motors, air solenoids, indicator lights, etc.