Ladder Diagram Example using Memory Bits and Set Coils

In this article, you will learn the PLC ladder diagram example using memory bits and set coils to control the motors.

Note: The ladder diagram example is for students and fresh engineering graduates.

Memory Bits and Set Coils

Problem Statement

Design a PLC ladder logic for the following application.

We are using one toggle switch to control four motors.

If Switch 1 is ON, then Motor I, Motor II, Motor III, and Motor IV will be ON.

If Switch 1 is OFF, then Motor I and Motor II, Motor III, and Motor IV remain ON.

PLC Training Video

Watch this video to understand the given example program with a step-by-step approach.

Inputs in PLC

The inputs of this PLC program are given below.

Switch 1: I0.0

Memory bit: M1

Outputs in PLC

The outputs of this PLC program are given below.

Motor 1: Q0.0

Motor 2: Q0.1

Motor 3: Q0.2

Motor 4: Q0.3

Ladder Diagram for Motors

Program Explained

In this program, we used the Schneider Electric PLC software.

In the above program, we have used Normally Open Contact for Switch 1 (I0.0).

We have also used a set coil for the Memory bit (M0).

In Rung1, switch 1 is connected with a set coil for the Memory Bit.

Motor 1, Motor 2, Motor 3, and Motor 4 are connected to the input Memory Bit present in Rung1.

For Motor 1, Motor 2, Motor 3, and MOTOR 4 to turn ON, input Switch 1 should be ON.

All Motors will remain ON if switch 1 is turned OFF as switch 1 is connected with a set coil representing the memory bit.

PLC Result

When Switch 1 is ON

In Rung 0, there is use of Set Coil for Memory Bit and switch 1 is used as Normally Open Contact. When switch 1 is turned ON, it will allow signal to flow through it and it will make the memory bit true

Then in Rung 1, Motor 1, Motor 2, Motor 3 and Motor 4 connected to the Memory Bit will turn ON.

When Switch 1 is OFF

In Rung 0, Memory Bit remains true even when the input switch 1 is turned OFF.

It is so because the set coil used sets itself true when the input gets ON and retains its true state even when its input is turned OFF. Then in Rung 1, Memory Bit which is still true will keep Motor 1, Motor 2, Motor 3, and Motor 4 ON.

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