Selection & LimitingBeginnerIEC 61131-3

LIMIT (Value Limiter)

Q: What are common applications of LIMIT (Value Limiter)?

LIMIT (Value Limiter) is commonly used for: Speed command limiting, Analog output range clamping, Setpoint validation, Position command limiting, Safe operating range enforcement.

Q: How do I use LIMIT (Value Limiter) in different PLC platforms?

Siemens: Use LIMIT instruction in TIA Portal. Allen-Bradley: Implement with GRT/LES comparisons and conditional MOV instructions. CODESYS: Use LIMIT() function in ST. Standard IEC function.

Constrains input value to specified minimum and maximum range.

The LIMIT function in the selection category operates as a value clamp, ensuring an input stays within a defined range. It is functionally identical to the process control LIMIT block but is classified under IEC 61131-3 as a selection function. OUT = MAX(MN, MIN(IN, MX)). This function is used wherever values need to be constrained to valid operating ranges.

Parameters

Inputs

Name	Type	Description
MN	ANY_NUM	Minimum boundary
IN	ANY_NUM	Value to limit
MX	ANY_NUM	Maximum boundary

Outputs

Name	Type	Description
OUT	ANY_NUM	Clamped value: MN <= OUT <= MX

Structured Text Example

// Limit speed command to safe range
Safe_Speed := LIMIT(MN := 0, IN := Speed_Cmd, MX := 3000);

// Limit analog output to valid range
AO_Value := LIMIT(MN := 0, IN := Calculated_AO, MX := 27648);

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Platform-Specific Implementation

Siemens (TIA Portal)

Use LIMIT instruction in TIA Portal.

Allen-Bradley (Studio 5000)

Implement with GRT/LES comparisons and conditional MOV instructions.

CODESYS

Use LIMIT() function in ST. Standard IEC function.

Common Applications

Speed command limiting
Analog output range clamping
Setpoint validation
Position command limiting
Safe operating range enforcement

Frequently Asked Questions

What is LIMIT (Value Limiter) in PLC programming?

Constrains input value to specified minimum and maximum range. The LIMIT function in the selection category operates as a value clamp, ensuring an input stays within a defined range. It is functionally identical to the process control LIMIT block but is classified under IEC 61131-3 as a selection function.

What are common applications of LIMIT (Value Limiter)?