Persistent 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.

Note

As of CODESYS version 3.3.0.1, 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.

Note

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.

Note

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.

Hint

Declare variables directly in the list of persistent variables and avoid inserting instance paths. As a result, this takes up twice as much memory and also increases the cycle time.

Example

Declaration in the persistent variable list PersistentVars:

{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 function block FB_A:

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

Declaration in the program PLC_PRG:

VAR
    fb_A1 : FB_A;
END_VAR

Possible declaration locations

Directly in the persistent global variable list The variable is persistent and lies 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 lies in the protected memory area and in the memory (double allocation).

Only locally in a program

Only locally in a function block

This variable is not persistent. A warning is shown in the message window.

Hint: Click Declarations ‣ Add all instance paths to import the variables into the persistent variable list.
Locally in a function This declaration does not have any effect. This variable is not persistent.

Note

In the persistence editor, click Add all instance paths if local variables are marked with PERSISTENT.

Note

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

See also