Variable Declaration

Introduction

A variable is a named unit of data that is assigned a value.

You can declare variables:

Globally
In a Program Unit
In a Class Declaration

Syntax

All variable declarations begin with a keyword, which defines the storage of the variables in the block:

Keyword	Usage
VAR	Declare local variables in: A Program A Function A Function Block A Class Declaration or a Method in a Class Declaration
VAR_INPUT	Declare input parameters to: A Function A Function Block A Method in a Class Declaration
VAR_OUTPUT	Declare output parameters from: A Function A Function Block A Method in a Class Declaration
VAR_IN_OUT	Declare parameters passed by reference to: A Function A Function Block A Method in a Class Declaration
VAR_TEMP	Declare a temporary variable in: A Function A Function Block A Method in a Class Declaration
VAR_GLOBAL	Declare a global variable.
VAR_EXTERNAL	Declare a reference to a global variable declared in another file.

You use the END_VAR keyword to indicate the end of a variable block.

Storage Modifiers

You can use storage modifiers after the variable declaration keyword:

Storage Modifier	Used With	Description
CONSTANT	VAR	Declares a constant variable locally in a Program Unit.
CONSTANT	VAR_GLOBAL	Declares a constant variable globally in a file.
CONSTANT	VAR_EXTERNAL	Declares a constant reference to a global variable. The global variable may or may not be constant.

Access Modifiers

You can use access modifiers after the variable declaration keyword:

Access Modifier	Description
PUBLIC	Code outside of the Class Declaration can access the variable.
PRIVATE	Only code in the Class Declaration can access the variable.
PROTECTED	Reserved for future use.
INTERNAL	Reserved for future use.

Variable Declaration Syntax

Inside a variable declaration block, you declare variables using this syntax:

variable-name [, variable-name ] : variable-type [ := initial-value ];

variable-name ::= name [ AT location ]

Where:

variable-type is an Elementary Data Type, or Derived Data Type.
initial-value is an Expression providing the initial value of the variable.
name is any suitable Identifier.
location is a Tag Reference.

Directly Located Variables

A directly located variable uses the AT location syntax to associate a variable name with a Tag Reference. For best results use a variable-type that is compatible with the type of the Tag Reference.

Examples

Declaration of global variables:

Var_Global
    X, Y, Z : LReal; (* Coordinates *)
End_Var

Declaration of global constant value:

Var_Global Constant
    MaxTags : Int := 12;
End_Var

Input variable to a program unit:
```
Var_Input
    V1, V2 : Vector;
End_Var
```
This example assumes Vector is a previously declared derived data type, for example:
```
Type
    Vector : Struct X, Y : REAL; End_Struct;
End_Type
```

Temporary array:

Var_Temp
    Buffer : Array[0..5] Of Int;
End_Var

Directly located variable:

Var
    Pressure AT %".TankPressure" : LREAL
End_Var

Further Information

Elementary Data Type

For the elementary types you can use for variable-type.

Derived Data Type

For the derived types you can use for variable-type.

Glossary

For the meaning of terms used in Fernhill SCADA.