Programmable Logic Controller, (PLC) is a digital computer used for automation of electromechanical processes, such as liquid control levels, pressure relief or flow control. The abbreviation “PLC” and the term “Programmable Logic Controller” are registered trademarks of the Allen-Bradley Company. Unlike general purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Consequently, it can be understood why PLC’s are an excellent tool for the extreme conditions often seen in the oil and gas extraction, refining and distribution world.
Programs to control machine operations are typically stored in a battery-backed-up system or non-volatile memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a limited time, otherwise an adverse condition could result.
Modern PLCs can be programmed in a variety of ways, from the relay-derived ladder logic to programming languages such as specifically adapted dialects of Basic and C. Another method is State logic, a very high-level programming language designed to program PLCs based on state transition diagrams.
The main difference from other computers is that PLCs are armored for severe conditions (such as dust, moisture, heat, cold) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read the limit switches, analog process variables (such as temperature and pressure) and the positions of complex positioning systems. On the actuator side, PLCs operate electric motors. pneumatic or hydraulic cylinders, magnetic relays, solenoids or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.
PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday control. A human-machine interface (HMI) is employed for this purpose. PLCs have built in communication ports, usually 9-pin RS-232, but optionally EIA-485 or Ethernet, Modbus, BACnet or DFI. Most modern PLCs can communicate over a network to some other system, such as a computer running a SCADA system or web browser.
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