PLC & Programming
Ladder logic
Ladder Diagram (LD)
Definition
A graphical PLC programming language drawn to resemble relay ladder diagrams, where a rung energizes its output when a continuous path of true conditions exists across it.
Contacts in series form a logical AND and parallel branches form an OR, so the visual layout of a rung is a direct picture of the boolean expression behind it.
It remains the dominant language in water, wastewater, and most discrete manufacturing, largely because maintenance staff can read it online while the process runs.
Also known as: ladder diagram, relay ladder logic.
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