Ladder rung, series and parallel branch symbols
A ladder diagram is drawn between two power rails, with rungs across them. Contacts in series on a rung make AND logic; contacts on parallel branches make OR logic.
SymbolIEC 61131-3What it is
Rung and power railsThe two vertical power rails and a rung across them, with its conditions on the left and the coil on the right.
Series contacts (AND)Two contacts one after the other on a rung: power passes only when both are true, which is AND logic.
Parallel branch (OR)Two paths side by side on a rung: power passes when either one is true, which is OR logic.
Rungs and branches
The PLC solves the rungs from top to bottom and each rung from left to right, then updates the outputs, and repeats: the scan. A rung is true when there is a path of true contacts from the left rail to the coil.
Frequently asked questions
Which rungs and branches appear on a ladder diagram?
This legend covers rung and power rails, series contacts (AND), parallel branch (OR). Project legends may add more.
Can I download these symbols?
Yes. Each symbol page has free SVG files, and the hub has printable PDF charts.
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