Rockwell Automation Logix5000 Controllers Ladder Diagram Programming Manual - Manual (Page 11)

Program ladder diagram Chapter 1
Rung condition
The controller evaluates ladder instructions based on the rung condition
preceding the instruction (rung-condition-in).
Only create instructions that 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.
Writing ladder logic requires that you choose the input and output instructions,
and choose the tag names for operands.
Choose the required instructions
1. Identify the conditions to check and separate them from the action to take
for the rung.
2. Choose the appropriate input instruction for each condition and the
appropriate output instruction for each action.
For more information on specific instructions, see:
Write ladder logic
Rockwell Automation Publication 1756-PM008E-EN-P - October 2014 11