A discrete control system mainly has discrete sensors and actuators, that is, sensors and actuators that have one of two values (e.g., on/off or open/closed). Though Ladder Diagram (LD) is the primary language of discrete control, the industry trend is toward using the IEC 61131-3 (formerly 1131-3) standard. Besides Ladder Diagram, IEC 61131-3 defines four additional languages: Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL), and Sequential Function Chart (SFC). Even though Ladder Diagram was originally developed for the programmable logic controller (PLC) and Function Block Diagram (FBD) was originally developed for the distributed control system (DCS), a PLC is not limited to ladder logic and a DCS is not limited to function block. The five IEC languages apply to all platforms for implementation of discrete control.
K. T. Erickson, "Discrete Control," A Guide to the Automation Body of Knowledge, 3rd Ed., pp. 57 - 78, International Society of Automation (ISA), Sep 2018.
Electrical and Computer Engineering
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24 Sep 2018