plcLib (Arduino): Instruction List Programming

Instruction list programming is a text based language, used to describe PLC programs, and is one of five methods specified by international standard IEC 61131-3, with the others being ladder diagram, function block diagram, sequential function chart and structured text.

Examples of instruction list commands include reading inputs, performing logical operations and controlling outputs, although details of command syntax tend to vary between manufacturers. A close relationship exists between instruction list and graphical design methods, including ladder diagrams and function block diagrams, as shown by the following illustration.

Instruction list is sometimes claimed to be a 'low level language', reminiscent of assembly language, but 'slightly lower level' (than a graphical method) might be more accurate. In some cases, several instruction list commands may be needed to perform the same task as a single graphical block.

Instruction list is not directly related to the microprocessor running the program, unlike assembly language or its close relative machine code, and is itself most likely to have been coded using a high level language such as C or C++.

Instruction list may be used as a program entry method in its own right, although this is less common in modern, commercially available PLCs.

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