PLC latching, also known as latch logic or memory bit logic, is a method used in Programmable Logic Controllers (PLCs) to maintain the state of an output even after the input condition that caused it to change has been removed. In other words, it allows the output to "latch" or stay in its current state until a reset condition is met, regardless of the status of the input signal.

Here's how PLC latching typically works:

1. Latching Circuit: In a PLC program, a latching circuit is created using memory or auxiliary bits. These memory bits store the state of the output.

2. Set Condition: When the input condition or event occurs that triggers the output to turn on, the corresponding memory bit is set or activated.

3. Output Activation: The output associated with the memory bit is turned on or activated in response to the set memory bit.

4. Removal of Input Condition: Even if the input condition that triggered the output to turn on is removed or goes false, the memory bit remains set, keeping the output on.

5. Reset Condition: To reset or turn off the output, a separate condition or event is defined in the PLC program. When this reset condition is met, the memory bit associated with the output is reset or deactivated, turning off the output.

6. Output Reset: Once the memory bit is reset, the output associated with it is turned off, and the latching circuit returns to its initial state.

PLC latching is commonly used in applications where it's necessary for an output to maintain its state until explicitly reset, such as maintaining motor operation after a momentary start button press or holding a valve open until a specific condition is met. It helps improve control flexibility and efficiency in various industrial automation processes.