Example of A Simple Ladder Logic Program
The language itself can be seen as a set of connections between logical checkers (contacts) and actuators (coils). If a path can be traced between the left side of the rung and the output, through asserted (true or "closed") contacts, the rung is true and the output coil storage bit is asserted (1) or true. If no path can be traced, then the output is false (0) and the "coil" by analogy to electro-mechanical relays is considered "de-energized". The analogy between logical propositions and relay contact status is due to Claude Shannon.
Ladder logic has contacts that make or break circuits to control coils. Each coil or contact corresponds to the status of a single bit in the programmable controller's memory. Unlike electromechanical relays, a ladder program can refer any number of times to the status of a single bit, equivalent to a relay with an indefinitely large number of contacts.
So-called "contacts" may refer to physical ("hard") inputs to the programmable controller from physical devices such as pushbuttons and limit switches via an integrated or external input module, or may represent the status of internal storage bits which may be generated elsewhere in the program.
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, energized 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, open whenever its corresponding coil or an input which controls it is 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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