Variable: PERSISTENT

Persistent variables are declared in the declaration section VAR_GLOBAL RETAIN PERSISTENT in the persistent global variable list. For variables that are marked with the PERSISTENT keyword outside of the persistence editor, instance paths are added there.

Tip

As of CODESYS V3.3.0.1, a variable declaration with PERSISTENT RETAIN has the same effect as with RETAIN PERSISTENT or PERSISTENT.

Syntax of the declaration in the global persistent variable list PersistentVars

VAR_GLOBAL PERSISTENT RETAIN
    <identifier> : <data type> (:= <initialization>)?;
 <instance path to POU variable>
END_VAR

Syntax of the declaration in POUs

<scope> PERSISTENT RETAIN
    <identifier> : <data type> ( := <initialization> )?; // ( ... )? : Optional
END_VAR
<scope> : VAR | VAR_INPUT | VAR_OUTPUT | VAR_IN_OUT | VAR_STAT | VAR_GLOBAL

An assignment of inputs, outputs, or memory addresses with the AT keyword is not permitted.

Tip

Never use the POINTER TO data type in persistent variable lists. If the application is downloaded again, their addresses could change. The corresponding compiler warnings are shown in the message window.

Tip

If you frequently change the names or data types of remanent variables, then it is better to declare them as retain variables with the RETAIN keyword only.

Important

Avoid inserting instance paths because in this case twice as much memory is used and a higher cycle time can occur. Instead, declare variables in the list of persistent variables.

Example 96. Example

Declaration in the PersistentVars persistent variable list

{attribute 'qualified_only'}
VAR_GLOBAL PERSISTENT RETAIN
        g_iCounter : INT;
        // Generated instance path of persistent variable
        PLC_PRG.fb_A.iPersistentCounter_A: INT;
END_VAR

Declaration in the FB_A function block

FUNCTION_BLOCK FB_A
VAR_INPUT
END_VAR
VAR_OUTPUT
END_VAR
VAR PERSISTENT
        iPersistentCounter_A : INT;
END_VAR

Declaration in the PLC_PRG program

VAR
        fb_A1 : FB_A;
END_VAR

Possible declaration locations	Description
Directly in the persistent global variable list	The variable is persistent and located in the protected memory area.
Locally in a program with an instance path in the persistent variable list Locally in a function block with an instance path in the persistent variable list	The variable is persistent and located in the protected memory area and in the memory (double allocation).
Only locally in a program Only locally in a function block	The variable is not persistent. A warning is shown in the message window. Tip Click Declarations → Add All Instance Paths to import the variables into the persistent variable list.
Locally in a function	This kind of declaration does not have any effect. The variable is not persistent.

Tip

In the persistence editor, click Declarations → Add All Instance Paths if local variables are marked as PERSISTENT.

Tip

Whenever possible, avoid marking variables as PERSISTENT which are declared in a function block. This is because the function block instance is stored entirely in remanent memory and not just the marked variable.