Allen Bradley PLC Latching and Unlatching Explained

Latching and Unlatching in Allen Bradley PLCs

Latching and unlatching are essential concepts in PLC programming, particularly when dealing with outputs that require persistent states. This blog post will delve into the fundamentals of latching and unlatching in Allen Bradley PLCs, explaining how they work and providing practical examples to illustrate their application.

What are Latching and Unlatching?

In essence, latching and unlatching refer to the ability to maintain an output ON or OFF state even after the triggering condition is removed. Imagine a scenario where you want a motor to run continuously until a specific button is pressed. This is where latching comes into play.


Latching allows an output to remain active, even after the input condition that initially triggered it is no longer present. For example, if you press a button to start a motor, the motor will continue running even if you release the button. This is achieved by using a latching instruction within the PLC program.


Unlatching, on the other hand, is the process of releasing the latch and returning the output to its default state. In our motor example, pressing a separate “stop” button would unlatch the output, causing the motor to stop.

How Latching and Unlatching Work in Allen Bradley PLCs

Allen Bradley PLCs typically use a combination of instructions and data structures to implement latching and unlatching. One common approach involves using the following:

  • Latching Instruction: This instruction is used to set the output state to ON and maintain it until unlatched.
  • Unlatching Instruction: This instruction is used to reset the output state to OFF, releasing the latch.
  • Latch Bit: A dedicated bit within the PLC memory that stores the latch state. When the latch bit is set to 1, the output is latched, and when it’s set to 0, the output is unlatched.

Example: Controlling a Motor with Latching and Unlatching

Let’s consider a simple example where we want to control a motor using a start button and a stop button. Here’s how we can achieve this using latching and unlatching:

  1. Start Button: When the start button is pressed, the PLC program sets the latch bit to 1, turning the motor ON. The motor will continue running even if the button is released.
  2. Stop Button: When the stop button is pressed, the PLC program sets the latch bit to 0, unlatching the output and turning the motor OFF.

In this example, the latch bit acts as a memory to keep track of the motor’s state, allowing it to run continuously until explicitly stopped.

Practical Applications of Latching and Unlatching

Latching and unlatching are widely used in various industrial applications, including:

  • Automatic Door Control: A door can be latched open or closed based on sensor input, ensuring it remains in that state until an unlatching condition occurs.
  • Emergency Stop Systems: Latching mechanisms can be used to maintain an emergency stop state, preventing dangerous operations until a specific procedure is followed.
  • Process Control: Latching can be used to maintain a process variable at a specific setpoint, even if there are temporary disturbances.
  • Sequence Control: Latching can be used to ensure that specific steps in a sequence are completed before proceeding to the next step.


Latching and unlatching are fundamental concepts in PLC programming that enable you to create outputs with persistent states. Understanding how these functions work and their applications is crucial for developing effective and reliable PLC programs. By using latching and unlatching appropriately, you can control various industrial processes and devices with greater flexibility and precision.