Program Ladder Diagram

On a program ladder diagram are two basic types of instructions:

Input Instructions: An instruction that checks, compares, or examines specific conditions in your machine or process.

Output Instructions: An instruction that takes some action, such as turn on a device, turn off a device, copy data, or calculate a value.

Branch
A branch is two or more instructions in parallel.

There is no limit to the number of parallel branch levels that you can enter. This example shows a parallel branch with five levels. The main rung is the first branch level, followed by four additional branches.

You can nest branches to as many as 6 levels. This example shows a nested branch. The bottom output instruction is on a nested branch that is three levels deep.

Rung Condition
The controller evaluates ladder instructions based on the rung condition preceding the instruction (rung-condition-in).

Only input instructions affect the rung-condition-in of subsequent instructions on the rung.

If the rung-condition-in to an input instruction is true, the controller evaluates the instruction and sets the rung-condition-out to match the results of the evaluation.

• If the instruction evaluates to true, the rung-condition-out is true.

• If the instruction evaluates to false, the rung-condition-out is false.

An output instruction does not change the rung-condition-out.

• If the rung-condition-in to an output instruction is true, the rung-condition-out is set to true.

• If the rung-condition-in to an output instruction is false, the rung-condition-out is set to false.