Siemens PLC Comparator Logic

This article shows the different types of Siemens PLC Comparator Logics and their ladder diagrams.

Problem Description

Implement equal, not equal, greater or equal, less or equal, greater than, check validity and check invalidity comparators in Siemens S7-1200 PLC using ladder diagram language.

Problem Diagram

Problem solution

Comparators are used for comparing two values. Say, If we want to compare two physical quantities (temperature, pressure etc.).

We can use the equal instruction to determine, if a first comparison value is equal to a second comparison value or not.

We can use the not equal instruction to determine, if a first comparison value is not equal to a second comparison value or not.

We can use the greater or equal instruction to determine, if a first comparison value is greater than or equal to a second comparison value or not. Both values to be compared must be of the same data type.

We can use the less or equal instruction to determine, if a first comparison value is less than or equal to a second comparison value or not. Both values to be compared must be of the same data type.

We can use the greater than instruction to determine, if a first comparison value is greater than a second comparison value or not. Both values to be compared must be of the same data type.

We can use the less than instruction to determine, if a first comparison value is less than a second comparison value or not. Both values to be compared must be of the same data type.

List of Inputs/Outputs

Output List

Result 1 = Q0.0
Result 2 = Q0.1
Result 3 = Q0.2
Result 4 = Q0.3
Result 5 = Q0.4
Result 6 = Q0.5

Memory

MW0 :- Comparator 1
MW2 :- Comparator 2
MW4 :- Comparator 3
MW6 :- Comparator 4
MW8 :- Comparator 5
MW10 :- Comparator 6

Ladder diagram for comparators used in S7-1200 PLC

Program Description

In this program we have used Siemens S7-1200 PLC and TIA Portal Software for programming.

Network 1 :

If value in MW0 is equal to 2, result 1 (Q0.0) will be ON.

Network 2 :

If value in MW2 is not equal to 2, result 1 (Q0.1) will be ON.

Network 3 :

If value in MW4 is greater or equal than 2, result 3 will be ON.

Network 4 :

If value in MW6 is less or equal than 2, result 4 will be ON

Network 5 :

If value in MW8 is greater than 2, result 5 will be ON.

Network 6 :

If value in MW10 is less than 2, result 6 will be ON

Test Cases

Author : Bhavesh

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