How to Use Compare Instruction in PLC Programming

In PLC programming, the compare instruction checks the relationship between two values, such as equal, greater than, or less than. You use it by specifying the two values and the comparison type, which then outputs a true or false result to control logic flow.

Syntax

The compare instruction syntax typically involves two operands and a comparison operator. It evaluates the condition and returns a boolean result.

• Operand1: First value to compare.
• Operand2: Second value to compare.
• Operator: Type of comparison (e.g., =, >, <, >=, <=, <>).
• Result: Boolean output (true if condition met, false otherwise).

plaintext

COMPARE Operand1, Operand2, Operator
// Example: COMPARE 10, 20, <

Example

This example shows how to use the compare instruction to check if a sensor value is greater than a threshold and turn on an output if true.

structured_text

VAR
  SensorValue : INT := 75;
  Threshold : INT := 50;
  OutputSignal : BOOL := FALSE;
END_VAR

// Compare if SensorValue is greater than Threshold
IF SensorValue > Threshold THEN
  OutputSignal := TRUE;
ELSE
  OutputSignal := FALSE;
END_IF;

Output

OutputSignal = TRUE

Common Pitfalls

Common mistakes when using compare instructions include:

• Mixing data types (e.g., comparing integer with string) which causes errors.
• Using incorrect operators that do not match the intended logic.
• Not handling the false case, leading to unexpected outputs.

Always ensure operands are compatible and operators are correctly chosen.

structured_text

VAR
  Value1 : INT := 10;
  Value2 : STRING := '10';
  Result : BOOL;
END_VAR

// Wrong: comparing INT with STRING
// Result := (Value1 = Value2); // This will cause a type error

// Correct:
VAR
  Value3 : INT := 10;
  Value4 : INT := 10;
END_VAR
Result := (Value3 = Value4);

Quick Reference

Operator	Meaning	Example
=	Equal to	COMPARE 5, 5, = (true)
>	Greater than	COMPARE 10, 5, > (true)
<	Less than	COMPARE 3, 7, < (true)
>=	Greater than or equal	COMPARE 5, 5, >= (true)
<=	Less than or equal	COMPARE 4, 6, <= (true)
<>	Not equal	COMPARE 5, 3, <> (true)

Key Takeaways

Use the compare instruction to evaluate conditions between two values in PLC logic.

Ensure operands are of compatible data types to avoid errors.

Choose the correct comparison operator to match your control logic needs.

Always handle both true and false outcomes for reliable automation.

Refer to the quick reference table for common comparison operators and their meanings.