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.

Syntax

A Function uses this syntax:

function ::=
FUNCTION function-name [ : return-type ]
{ variable-declaration }
[ function-code ]
END_FUNCTION

return-type ::= [ Elementary Data Type | Derived Data Type ]

function-code ::= structured-text

Where:

function-name is the name of the function. This must be a valid Identifier.
return-type is the optional data type returned by the function.
variable-declaration defines the variables available to the Program. You can declare multiple blocks of variables.
function-code is the code to run.

Remarks

Use any of the following for the return-type:

If the return-type is not used, the function does not return a value.

Use any of the following for the program-code:

A function can call other functions.

Examples

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

IEC 61131-3 Second Edition: Table 20a.

IEC 61131-3 Third Edition: Table 19.

Further Information

Program Units

To learn about other program units.

Variable Declarations

To learn about variable declarations.

ST Statements

To learn about ST Statements.

Glossary

For the meaning of terms used in Fernhill SCADA.