Retain Variable

Retain variables are declared by the keyword ‎RETAIN‎ is added in programming objects in the scope ‎VAR‎, ‎VAR_INPUT‎, ‎VAR_OUTPUT‎, ‎VAR_IN_OUT‎, ‎VAR_STAT‎, or ‎VAR_GLOBAL‎.

Syntax for the declaration

<scope> 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.

Example

In a POU:

VAR RETAIN
	iVarRetain: INT;
END_VAR

In a GVL:

VAR_GLOBAL RETAIN
	g_iVarRetain: INT;
END_VAR

Possible declaration locations

Locally in a program	Only the variable is located in the retain memory area. Info: When using redundancy, the entire program with all of its data is located in the retain memory area.
Globally in a global variable list	Only the variable is located in the retain memory area. Info: When using redundancy, the entire global variable list with all of its data is located in the retain memory area.
Locally in a function block	The entire instance of the function block with all of its data is located in the retain memory area. Only the declared retain variable is protected.
Locally in a function	The variable is not located in the retain memory area. This declaration does not have any effect.
Locally and persistently in a function	The variable is not located in the retain memory area. This declaration does not have any effect.

Whenever possible, avoid using ‎RETAIN‎ to mark the variables of a function block.