User Defined Function

Introduction

A User Defined Function is a sequence of statements which may return an optional value.

A function that returns a value can be called from an Expression.

A function that does not return a value can be called through a Structured Text Function Call.

A Function uses the following syntax:

Function name [ : return-type ]
    variable-declarations

    statement;
    statement;
    ...
    statement;
End_Function

Where:

name is any suitable Identifier.
return-type is an optional Elementary Data Type, or Derived Data Type defining the return type of the function. If not specified, the function does not return a value.
variable-declarations is zero or more Variable Declarations.
statement is any Structured Text Statement.

Example

A function to calculate the standard deviation of four values:

Function CalcStdDev : LREAL
	var_input v1,v2,v3,v4 : LREAL; end_var
	
	var 
		SumOfSquares : LREAL;
		Mean : LREAL;
	end_var

	(* Calculate the mean *)
	Mean := (v1 + v2 + v3 + v4) / 4;

	(* Calculate the standard deviation *)
	SumOfSquares := (v1 - Mean) ** 2 +
				(v2 - Mean) ** 2 +
				(v3 - Mean) ** 2 +
				(v4 - Mean) ** 2;

	CalcStdDev := sqrt( SumOfSquares / 4 );
End_Function

Standards Compliance

Table 20a - IEC 61131-3 Second Edition.

Table 19 - IEC 61131-3 Third Edition.

Further Information

Program Units

To learn about other program units.

Variable Declarations

To learn about variable declarations.

Structured Text (ST) Statements

To learn about Structured Text (ST) Statements.

Glossary

For the meaning of terms used in Fernhill SCADA.