Selection & LimitingBeginnerIEC 61131-3

MAX (Maximum Selection)

Returns the larger of two or more input values.

The MAX function returns the larger of two (or more) input values. It is used for selecting the highest reading from redundant sensors, implementing high-select control strategies, finding peak values, and ensuring minimum output levels.

Parameters

Inputs

Name	Type	Description
IN1	ANY_NUM	First value
IN2	ANY_NUM	Second value

Outputs

Name	Type	Description
OUT	ANY_NUM	Maximum value (larger of inputs)

Ladder Logic Example

// Ladder Logic - MAX (High-Select)
//
// |----[MAX]----|
// | IN1: Sensor_1        |
// | IN2: Sensor_2        |
// | OUT: Highest_Reading  |

Structured Text Example

// High-select from redundant sensors
Highest_Temp := MAX(Sensor_1, Sensor_2);

// Ensure minimum output level
Actual_Output := MAX(Calculated_Output, Min_Output);

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Platform-Specific Implementation

Siemens (TIA Portal)

Use MAX instruction in TIA Portal SCL. In LAD/FBD, use comparison with MOVE.

Allen-Bradley (Studio 5000)

No direct MAX; implement with GRT comparison and MOV.

CODESYS

Use MAX() function in ST. Supports multiple inputs.

Common Applications

High-select from redundant sensors
Peak value tracking
Minimum output enforcement
Worst-case selection
Maximum demand calculation

Frequently Asked Questions

What is MAX (Maximum Selection) in PLC programming?

Returns the larger of two or more input values. The MAX function returns the larger of two (or more) input values. It is used for selecting the highest reading from redundant sensors, implementing high-select control strategies, finding peak values, and ensuring minimum output levels.

What are common applications of MAX (Maximum Selection)?

MAX (Maximum Selection) is commonly used for: High-select from redundant sensors, Peak value tracking, Minimum output enforcement, Worst-case selection, Maximum demand calculation.

How do I use MAX (Maximum Selection) in different PLC platforms?

Siemens: Use MAX instruction in TIA Portal SCL. In LAD/FBD, use comparison with MOVE. Allen-Bradley: No direct MAX; implement with GRT comparison and MOV. CODESYS: Use MAX() function in ST. Supports multiple inputs.