What is Latch?
The latch is used to keep an output active even when the input is turned off. In some industries, such a situation is required.
A motor, for example, can be started by pressing a push button switch. Despite the fact that the switch contacts are not closed, the motor must continue to run until a stop pushbutton switch is pressed.
PLC Latching
When the output must be activated even after the entry has ended, latching is used.
A motor that is started by pressing a button switch is a simple example of such a situation. The motor must continue to run until the stop button switch is pressed, despite the fact that the switch connections are not closed. The latching allowed the motor to continue running until the push button was pressed again.
In logic control, circuits that are characteristic of the previous conditions are frequently required. The output is latched in this series by using the output contact itself, so even if the input changes, the output condition remains constant:
When an exit occurs, another set of exit-related contacts is closed. These contacts form an OR logic gate system using the input contacts. Therefore, even if input A is open, the circuit will output the output. Activating the normally closed Contact B is the only way to release the output.
Consider a motor that controls the stop and start button switches, for which a signal light must be turned on when power is applied to the motor and another must be turned off when power is not applied. A ladder diagram with Mitsubishi notation for directions is shown below.
Concept of Latching
When the start pushbutton is pressed, the contact becomes I0.0 (N-O) N-C. Since the N-O contact I0.1 has already become N-C (due to the normally closed push button on the physical connection), the color corrects and the coil (bulb) Q0.0 becomes energized.
Now, the bulb will also be on when the start push button is released (here comes the idea of latching).
As the Q0.0 coil becomes energized, the latched contact of this coil, represented as Q0.0 (NO), becomes NC, the logic passing through this room and the bulb being on.
If we want to turn off the bulb, we have to press the stop push button. By pressing the stop push button, the contact becomes I0.1 NO (normally opens), and the color logic changes incorrectly, giving D-energy to the Q0.0 coil and turning off the bulb.