Ladder logic

Ladder logic was originally a written method to document the design and construction of relay racks as used in manufacturing and process control.^[1] Each device in the relay rack would be represented by a symbol on the ladder diagram with connections between those devices shown. In addition, other items external to the relay rack such as pumps, heaters, and so forth would also be shown on the ladder diagram.

Ladder logic has evolved into a programming language that represents a program by a graphical diagram based on the circuit diagrams of relay logic hardware. Ladder logic is used to develop software for programmable logic controllers (PLCs) used in industrial control applications. The name is based on the observation that programs in this language resemble ladders, with two vertical rails and a series of horizontal rungs between them. While ladder diagrams were once the only available notation for recording programmable controller programs, today other forms are standardized in IEC 61131-3 (For example, as an alternative to the graphical ladder logic form, there is also a language more like C called Structured text within the IEC 61131-3 standard).

^ "The Basics of Ladder Logic". Ladder logic uses switch or relay contacts to implement Boolean expressions. In years past, ladder logic was made possible with discrete relays and was sometimes termed relay logic.

[1] "The Basics of Ladder Logic". Ladder logic uses switch or relay contacts to implement Boolean expressions. In years past, ladder logic was made possible with discrete relays and was sometimes termed relay logic.

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