Remanent variables¶
Remanent variables can retain their value throughout the usual program run period. These include Retain variables and Persistent variables.
Example:
VAR RETAIN
iRem1:INT; (* 1. Retain variable*)
END_VAR
Retain variables are identified by the keyword RETAIN. These variables maintain their value even after an uncontrolled shutdown of the controller as well as after a normal switch off and on of the controller (corresponding to command Online Reset). When the program is run again, the stored values will be processed further. A concrete example would be an piece-counter in a production line, that recommences counting after a power failure. All other variables are newly initialized, either with their initialized values or with the standard initializations.
Retains are reinitialized at Reset (cold) and Reset (original) and - contrary to Persistent variables - at a new download of the program.
Persistent variables are identified by the keyword PERSISTENT. Unlike Retain variables, these variables retain their value only after a re-download, but not after an Online Reset, Online Reset (original) or Online Reset (cold), because they are not saved in the “retain area”. If also persistent variables should maintain their values after a uncontrolled shutdown of the controller, then they have to be declared additionally as VAR RETAIN variables. A concrete example of “persistent Retain-Variables” would be a operations timer that recommences timing after a power failure.
x = value will be retained
- = value gets reinitialized
| after Online command | VAR | VAR RETAIN | VAR PERSISTENT | VAR RETAIN PERSISTENT VAR PERSISTENT RETAIN |
|---|---|---|---|---|
| Reset | - | x | - | x |
| Reset cold | - | - | - | - |
| Reset origin | - | - | - | - |
| Download | - | - | x | x |
| Online Change | x | x | x | x |
Note
- If a local variable in a program is declared as VAR RETAIN, then exactly that variable will be saved in the retain area (like a global retain variable).
- If a local variable in a function block is declared as VAR RETAIN, then the complete instance of the function block will be saved in the retain area (all data of the POU), whereby only the declared retain variable will be handled as a retain.
- If a local variable in a function is declared as VAR RETAIN, then this will be without any effect. The variable will not be saved in the retain area ! If a local variable is declared as PERSISTENT in a function, then this will be without any effect also!