Programmable Logic Controller (PLC)

The Programmable Logic Controller (PLC) (engl. Programmable Logic Controller, PLC) or the programmable controller — an electronic component of the industrial controller, the specialized (computerized) device used for automation of technology processes. As the main mode of long work of a PLC, often in adverse environmental conditions, his autonomous use, without serious service and practically without intervention of the person acts.^[1]

Sometimes on a PLC the systems of numerical control by the machine are under construction (the CNC, engl. Computer numerical control, CNC). PLCs are devices of real time.

Unlike

• the microcontroller (the single-crystal computer), the chip intended for control of electronic devices a scope of a PLC usually are the automated processes of industrial production, in the context of manufacturing enterprise;
• computers, PLCs have the developed input/output devices of signals of sensors and actuators as opposed to weak opportunities of input output of management (the keyboard, a mouse, the monitor, etc.);
• the built-in systems - the PLC is installed separately from the equipment managed with his help.

The first logic controllers appeared in the form of cabinets with a set of the relays connected among themselves and contacts. This scheme was set strictly at a design stage and could not be changed further.

The first-ever PLC — MOdular DIgital CONtroller (Modicon) 084 having memory of 4 kb is made in 1968.

In the first PLCs who came to replacement to normal logic controllers, the logic of connections was programmed by the connection diagram LD (Ladder logic Diagram). The device had the same principle of work, but the relay and contacts (except input and output) were virtual, i.e. existed in the form of the program executed by the PLC microcontroller. Modern PLCs are "svobodnoprogrammiruyemy".

In management systems for technology objects logical commands prevail over numerical transactions that allows at comparative simplicity of the microcontroller (buses 8 or 16 bits wide), to receive the powerful systems operating in real time. In modern PLCs numerical transactions are implemented on an equal basis with logical. At the same time, unlike the majority of processors of computers, in a PLC access to separate bits of memory is provided.

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