Ladder Logic Programming Examples

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Let’s start with the Set (Latch) and Reset (Unlatch) logic. Remember Allen Bradley PLC’s use Latch and Unlatch symbols.

The rungs below show the basic code.

When both Input A and Input B are FALSE then the state of Output Y does not change. If Output Y is FALSE then it stays FALSE….

If Input A goes TRUE momentarily then the SET symbol changes the state of Output Y to TRUE.

Then, after subsequent scans, if Input A changes state to FALSE it does not affect the state of Output Y. In other words, Output Y is latched TRUE…

 Only when Input B changes state to TRUE will the RESET symbol change the state of Output Y back to FALSE. In other words, Output Y is now unlatched…. 

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