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20 Programmable logic controller

A programmable logic controller (PLC) is a digital device adapted for control of processes which require high reliability, ease of programming and real­time responses. Such functionality has long since replaced hard­wired relays, timers and sequencers which would be required to complete various tasks.

Programmable logic controllers usually had to conform to IEC 61131­3 standard which defines four programming languages: function block diagram (FBD), ladder diagram (LD), structured text (ST) and sequential function chart (SFC). This standard does not support distributed control systems therefore IEC 61499 standard was published in 2005. The standard is considered an extension of IEC 61131­3 standard.

WCC Lite supports PLC functionality while conforming to specifications of IEC 61499 standard.