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Programmable Logic Controller A programmable logic controller (PLC) is a specialized computer used to control machines and process. Advantages of a PLC Control System 1 Eliminates much of the hard wiring that was associated with conventional relay control circuits. The program takes the place of much of the external wiring that would be required for control of a process. 2 Increased Reliability: Once a program has been written and tested it can be downloaded to other PLCs. Since all the logic is contained in the PLC’s memory, there is no chance of making a logic wiring error. 3 More Flexibility: Original equipment manufacturers (OEMs) can provide system updates for a process by simply sending out a new program. It is easier to create and change a program in a PLC than to wire and rewire a circuit. End-users can modify the program in the field. 4 Lower Costs: Originally PLCs were designed to replace relay control logic. The cost savings using PLCs have been so significant that relay control is becoming obsolete, except for power applications. Generally, if an application requires more than about 6 control relays, it will usually be less expensive to install a PLC. 5 Communications Capability: A PLC can communicate with other controllers or computer equipment. They can be networked to perform such functions as: supervisory control, data gathering, monitoring devices and process parameters, and downloading and uploading of programs. 6 Faster Response Time: PLCs operate in real-time which means that an event taking place in the field will result in an operation or output taking place. Machines that process thousands of items per second and objects that spend only a fraction of a second in front of a sensor require the PLC’s quick response capability. 7 Easier To Troubleshoot: PLCs have resident diagnostic and override functions that allows users to easily trace and correct software and hardware problems. The control program can be watched in real-time as it