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.

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

In a function block, there must not exist any variable declaration exclusively with ‎PERSISTENT‎, but only with ‎PERSISTENT RETAIN‎ or ‎RETAIN 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.

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.

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.

NOTICE

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

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 RETAIN
        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.

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

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.