LT (Less Than)
Returns TRUE when first input is strictly less than second input.
The LT (Less Than) comparison function returns TRUE when the first input is strictly less than the second input. It is used for low-limit alarms, underflow detection, and minimum threshold monitoring in PLC programs.
Parameters
Inputs
| Name | Type | Description |
|---|---|---|
| IN1 | ANY_NUM | First value |
| IN2 | ANY_NUM | Second value |
Outputs
| Name | Type | Description |
|---|---|---|
| OUT | BOOL | TRUE when IN1 < IN2 |
Ladder Logic Example
// Ladder Logic - LT Comparison (Low Level Alarm) // // |----[LT]---( Low_Level_Alarm )----| // | IN1: Tank_Level | // | IN2: Low_Setpoint (10.0) |
Structured Text Example
Low_Level_Alarm := (Tank_Level < Low_Setpoint);
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Platform-Specific Implementation
Siemens (TIA Portal)
Use CMP < instruction in TIA Portal.
Allen-Bradley (Studio 5000)
Use LES (Less Than) instruction.
CODESYS
Use < operator in ST or LT function block.
Common Applications
- Low-limit alarms
- Underflow detection
- Minimum pressure monitoring
- Low temperature alerts
- Empty tank detection
Frequently Asked Questions
What is LT (Less Than) in PLC programming?
Returns TRUE when first input is strictly less than second input. The LT (Less Than) comparison function returns TRUE when the first input is strictly less than the second input. It is used for low-limit alarms, underflow detection, and minimum threshold monitoring in PLC programs.
What are common applications of LT (Less Than)?
LT (Less Than) is commonly used for: Low-limit alarms, Underflow detection, Minimum pressure monitoring, Low temperature alerts, Empty tank detection.
How do I use LT (Less Than) in different PLC platforms?
Siemens: Use CMP < instruction in TIA Portal. Allen-Bradley: Use LES (Less Than) instruction. CODESYS: Use < operator in ST or LT function block.
Related Function Blocks
LE (Less Than or Equal)
Returns TRUE when first input is less than or equal to second input.
View Reference →GT (Greater Than)
Returns TRUE when first input is strictly greater than second input.
View Reference →GE (Greater Than or Equal)
Returns TRUE when first input is greater than or equal to second input.
View Reference →EQ (Equal)
Returns TRUE when both inputs are exactly equal.
View Reference →NE (Not Equal)
Returns TRUE when two compatible operands compare unequal at execution.
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