Ladder logic

Ladder logic is a programming language that represents a program by a graphical diagram based on the circuit diagrams of relay-based logic hardware. It is primarily used to develop software for programmable logic controllers used in industrial control applications. The name is based on the observation that programs in this languages resemble ladders, with two vertical rails and a series of horizontal rungs between them.

Each rung of ladder language typically has one coil at the far right. Some manufacturers may allow more than one output coil on a rung.

• ----( )----- A regular coil, energizer whenever its rung is closed.
• ----(/)----- A "not" coil, energized whenever its rung is open.
• ----[ ]----- A regular contact, closed whenever its corresponding coil or an input which controls it is energized.
• ----[/]----- A "not" contact, closed whenever its corresponding coil or an input which controls it is not energized.

The "coil" (output of a rung) may represent a physical output which operates some device connected to the programmable controller, or may represent an internal storage bit for use elsewhere in the program.

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