Variables

Declaration

Every program component (POU) declaration, i.e. declaration of function, functional block or program begins with variable sections description. Variables are declared inside variable section.

Example of variable section declaration:

VAR

   isEnabled : BOOL;

   bits : ARRAY[0..7] OF BOOL;

   data : INT := 100;

END_VAR

Supported Variable Sections

Type

Example

VAR ... END_VAR

Available only inside the component, there is no access from outside. Can have initial values. Local variables of functions are located in dynamic memory. At the end of function operation, memory is cleared. VAR variables save their values between a program and instance calls inside functional block programs and instances. Local variables are not displayed in graphical component representation.

VAR_INPUT ... END_VAR

Transmitted by value via copying.  Upon block call through such variable a value of other variable (compatible type) or expression can be assigned. Any changes of this variable inside component are not displayed in calling component data. Applied inside any component. Default values are available. Shown on the left In the graphical representation.

VAR_IN_OUT ... END_VAR

This parameter is input and output simultaneously. Variable transmission to block instance is performed by reference. It means that outer variable operates as inner variable inside the block. Only its location address in the data memory is transmitted to the component. For VAR_IN_OUT variable, it is forbidden to:

• assign initial value;

• assign constant as a current parameter.

An outer variable can be assigned for VAR_IN_OUT only on block call. The Most important VAR_IN_OUT feature is external data copying absence. VAR INPUT and VAR_OUTPUT parameters can process arrays and structures, but every time upon component call full data copying will take place. It can take up a lot of time. Assigning one array to another one means the component switch from one array to another one will be performed for VAR_IN_OUT. Local data copy is not created in this case. VAR_IN_OUT parameters break the rule of components independence. The right component shouldn't affect other component's memory. That's why they should be processed attentively and only in cases when it's really necessary.

VAR_OUTPUT ... END_VAR

Reflect component operation results. Transmitted by value via copying. It is reasonable to read output value after block execution. VAR_OUTPUT parameters are available only for reading outside the component. Can have initial values. Displayed on the right in the graphical representation.

 

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