The ‎VAR_INST‎ instance variables are allowed only in methods. This kind of variable can be accessed only within the method. You can monitor the variable values of instance variables in the declaration part of the method.

CODESYS does not save a ‎VAR_INST‎ method variable in a method stack, but in the stack of the function block instance. This means that the ‎VAR_INST‎ variable functions like other Variables of the function block instance, and it is not reinitialized each time the method is called.

The variable can be extended with an attribute keyword (‎CONSTANT‎, ‎RETAIN‎, or ‎PERSISTENT‎).

METHOD meth_last : INT
VAR_INPUT
  iVar : INT;
END_VAR
VAR_INST
  iLast : INT := 0;
END_VAR
meth_last := iLast;
iLast := iVar;