How to Use Latch and Unlatch in Ladder Logic for PLCs

In ladder logic, use the LATCH instruction to turn on an output and keep it on even if the input turns off. Use the UNLATCH instruction to turn off that output when needed. This lets you control devices that stay on until explicitly turned off.

Syntax

The LATCH and UNLATCH instructions control outputs that maintain their state independently of the input signal after activation.

• LATCH (Set): Turns on the output and keeps it on until an UNLATCH command is given.
• UNLATCH (Reset): Turns off the output latched previously.

Typical syntax in ladder logic:

ladder_logic

----[Input Condition]----(LATCH)----(Output Coil)
----[Input Condition]----(UNLATCH)----(Output Coil)

Example

This example shows a start button that latches a motor output on, and a stop button that unlatches it off.

ladder_logic

----[Start Button]----(LATCH)----(Motor Output)
----[Stop Button]----(UNLATCH)----(Motor Output)

Output

When the Start Button is pressed, Motor Output turns ON and stays ON even if Start Button is released.
When the Stop Button is pressed, Motor Output turns OFF.

Common Pitfalls

Common mistakes include:

• Using LATCH without a matching UNLATCH, causing outputs to stay on forever.
• Confusing LATCH with a normal coil that only stays on while input is true.
• Not ensuring UNLATCH conditions are reachable, so the output never turns off.

Always pair LATCH and UNLATCH properly to avoid stuck outputs.

ladder_logic

Wrong:
----[Start Button]----(LATCH)----(Motor Output)
(no UNLATCH instruction)

Right:
----[Start Button]----(LATCH)----(Motor Output)
----[Stop Button]----(UNLATCH)----(Motor Output)

Quick Reference

Instruction	Function	Effect
LATCH	Set output	Turns output ON and keeps it ON until UNLATCHed
UNLATCH	Reset output	Turns output OFF that was previously latched
Normal Coil	Momentary output	Output ON only while input condition is true

Key Takeaways

Use LATCH to turn on an output that stays on after input turns off.

Use UNLATCH to turn off a latched output when needed.

Always pair LATCH and UNLATCH to avoid outputs stuck ON.

LATCH differs from normal coils which only stay ON while input is true.

Check that UNLATCH conditions are reachable to reset outputs properly.