Comparison OperationsBeginnerIEC 61131-3

GT (Greater Than)

Returns TRUE when first input is strictly greater than second input.

The GT (Greater Than) comparison function evaluates whether the first input is strictly greater than the second input and returns TRUE if so. It works with all numeric and comparable data types including INT, REAL, DINT, and TIME. GT is fundamental to conditional logic in PLC programs, used for high-limit alarms, threshold detection, and process control decisions.

Parameters

Inputs

Name	Type	Description
IN1	ANY_NUM	First value (left operand)
IN2	ANY_NUM	Second value (right operand)

Outputs

Name	Type	Description
OUT	BOOL	TRUE when IN1 > IN2

Ladder Logic Example

// Ladder Logic - GT Comparison (High Temperature Alarm)
//
// |----[GT]---( High_Temp_Alarm )----|
// | IN1: Temperature_PV              |
// | IN2: 85.0 (High Limit)           |

Structured Text Example

IF Temperature_PV > 85.0 THEN
    High_Temp_Alarm := TRUE;
ELSE
    High_Temp_Alarm := FALSE;
END_IF;

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Platform-Specific Implementation

Siemens (TIA Portal)

Use CMP > instruction or GT in TIA Portal. Works in LAD, FBD, and SCL.

Allen-Bradley (Studio 5000)

Use GRT (Greater Than) instruction in Studio 5000. Works with all numeric types.

CODESYS

Use > operator in ST or GT function block in FBD/LD.

Common Applications

High-limit alarm detection
Threshold-based control
Level monitoring (tank overflow)
Speed overshoot detection
Pressure relief activation

Common Mistakes to Avoid

Comparing REAL values for exact equality instead of using GT/LT with tolerance
Mixing data types without proper conversion

Frequently Asked Questions

What is GT (Greater Than) in PLC programming?

Returns TRUE when first input is strictly greater than second input. The GT (Greater Than) comparison function evaluates whether the first input is strictly greater than the second input and returns TRUE if so. It works with all numeric and comparable data types including INT, REAL, DINT, and TIME.

What are common applications of GT (Greater Than)?

GT (Greater Than) is commonly used for: High-limit alarm detection, Threshold-based control, Level monitoring (tank overflow), Speed overshoot detection, Pressure relief activation.

How do I use GT (Greater Than) in different PLC platforms?

Siemens: Use CMP > instruction or GT in TIA Portal. Works in LAD, FBD, and SCL. Allen-Bradley: Use GRT (Greater Than) instruction in Studio 5000. Works with all numeric types. CODESYS: Use > operator in ST or GT function block in FBD/LD.