Assignment Statement

Introduction

The IEC 61131-3 ST assignment statement assigns the value of an expression to a variable. The assignment statement uses this syntax:

variable-reference := expression;

Where

variable-reference is the target of the assignment. This can be any writable Variable Reference.
expression is the source of the assignment. This can be any Expression that is compatible with the assignment target.

String Assignment

If a string value is too long to fit a Variable Reference, a runtime error is reported.

If you need to truncate a string to fit a Variable Reference, use the LEFT() Function to limit the string length. For example:

StringVariable := LEFT( LongString, 254 );

Note: Before Fernhill SCADA Version 3.81, no runtime error was reported if a string was truncated during variable assignment.

Examples

Assignment of a constant to a variable:
```
A := 12.5;
```
Assign a value to an element of an Array:
```
C[0] := -12;
```

Assign members of a Struct:

Position.Angle := ATAN2( Y, X );
Position.Distance = SQRT( X * X + Y * Y );

Assign one Struct to another:
```
PreviousPosition := Position
```

The above examples use these Variable Declarations:

TYPE
    PolarCoordinates : STRUCT
        Angle : LREAL;
        Distance : LREAL;
    End_STRUCT
End_TYPE
VAR
    A : LREAL;
    X, Y : LREAL;
    C : ARRAY [0..5] OF INT;
    NewPosition, PreviousPosition : PolarCoordinates;
End_VAR

Standards Compliance

IEC 61131-3 Second Edition: Table 56.1.

IEC 61131-3 Third Edition: Table 72.1.

Further Information

Structured Text

For other Structured Text (ST) statement types.

Variable Reference

To learn about variable references that can be used on the left of the := assignment operator.

Expressions

To learn about expressions that can be used on the right of the := assignment operator.

Glossary

For the meaning of terms used in Fernhill SCADA.