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.

The ControlLogix controller supports these programming languages, both online and offline.

ControlLogix Controller Programming Languages

Language	Is best-used in programs with
Relay ladder	Continuous or parallel execution of multiple operations (not sequenced)
	Boolean or bit-based operations
	Complex logical operations
	Message and communication processing
	Machine interlocking
	Operations that service or maintenance personnel may have to interpret in order to troubleshoot the machine or process
Function block diagram	Continuous process and drive control
	Loop control
	Calculations in circuit flow
Sequential function chart (SFC)	High-level management of multiple operations
	Repetitive sequence of operations
	Batch process
	Motion control using structured text
	State machine operations
Structured text	Complex mathematical operations
	Specialized array or table loop processing
	ASCII string handling or protocol processing

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