Ladder logic's elements

•	Power rails: Ladder Logic networks are delimited on the left and right by vertical lines known as left and right power rails, respectively. The right power rail may be explicit or implied.

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Link elements and states: Links indicate power flow in the rungs of the Ladder Logic diagram. A link element may be horizontal or vertical. A horizontal link transmits the state of the element to its immediate left to the element to its immediate right. The state of an element can be either ON or OFF. A vertical link intersects with one or more horizontal links on each side and its state is the inclusive OR of the states of the horizontal links on its left. This state is transmitted to all horizontal links attached to the vertical link on its right.

•	Contacts: A contact is an element which imparts a state to the horizontal link on its right side equal to the AND of the state of the horizontal link on its left side with an appropriate function. A contact does not modify the value of the associated Boolean variable.

•	Coils: A coil copies the state of the link on its left to the link on its right without modification, and stores an appropriate function of the state or transition of the left link into the associated Boolean variable.

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Functions and function blocks: A function is a program unit which, when executed, yields exactly one result. A function block may yield more than one result. Internal variables of a function or function block are not accessible to users of the function. In Ladder Logic, at least one Boolean input and one Boolean output is shown for each function block to allow power to flow through the block.


Ladder logic's elements