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Tuesday, August 19, 2025

Programmable Logic Controller - Latch Application in PLC

The latch application, meaning latching of push button outputs are one of the most common use cases of the PLC Application. The following experiments demonstrates how that is implemented. These were the experiments and records i did as a student. Posting it here as a blog content.

Variation - 1

Most of the Industries use push buttons to start and stop some devices. In a logical case, when the push button is released, the device will be turned off. But using a latch we will keep the input to the device on.

Input – Output Allocation:


Inputs:
	Start Push button – 0000, Stop Push button – 0001.
Output:
	Motor – 0500.

Ladder Logic Program - Using Non retentive instruction

Ladder Program - Using Non Retentive Instruction

Ladder Logic Program - Using retentive instruction

Ladder Program - Using retentive Instruction

Variation - 2

Develop a relay based controller that will allow three switches in a room to control a single electrical device. Note: control represents both on and off

Input – Output Allocation:


<Relay>
<No.> <Label> <Comment1> <Comment2> <Comment3>
0000 :S1 Switch 1
0001 :S2 Switch 2
0002 :S3 Switch 3
0500 :DEVICE Control Device

Ladder Logic Program

Ladder Program - 3 Switch Control

Variation - 3

Three start – stop switch stations (each station has 2 switches) are used to control a single motor. Develop ladder logic for this.

Input – Output Allocation:


<Relay>
<No.> <Label> <Comment1> <Comment2> <Comment3>
0000 :START1 Start Switch of Station1
0001 :STOP1 Stop Switch of Station1
0002 :START2 Start Switch of Station2
0003 :STOP2 Stop Switch of Station2
0004 :START3 Start Switch of Station3
0005 :STOP4 Stop Switch of Station3
0500 :MOTOR Motor Output

Ladder Logic Program

Ladder Program - 3 start stop switch

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