1

The application must maintain device settings.

Example: After a power failure, the building control has to have information available about how long a window blind needs to be raised.

Suitable

Preferred use case

In this case, you can also use retain variables instead of persistent variables. This is advantageous for variables whose declaration is often changed.

Suitable

Preferred use case

Retain variables are an advantage when their declarations are changed often.

Suitable

Advantageous for controllers which do not have any hardware support

Special functionalities make this possible, such as double file buffering.

Possible, but very complicated and therefore not recommended

2

The application must maintain values also after program changes or extensions.

2a: Rare extensions

Example: An application programmer extends the program with a new switch and installs a new light. The building control must still have saved values available until then.

Suitable

Preferred use case

Suitable

Suitable

Possible, but complicated

2b: Unrestricted changes, including deleting or changing the data type of variables

The building control is running and is persistent. When an application programmer adds a new functionality to the controller and therefore adds another persistent variable to a function block, the values saved up to that point must be retained. For example, the program in an FB is extended with a variable that controls the automatic switching off of a previously uncontrolled lamp after a certain time. The building control must have the times of all controlled lamps available after the extension.

Not suitable

Suitable

Data from retain variables are preserved as far as possible after an online change.

Suitable as far as possible

Preferred use case

Possible if textual, but complicated

2c: The application must maintain values after a download.

Suitable

Not suitable

Suitable

Suitable

3

The application must be able to use different value sets.

Example: The operating settings for summer, winter, and holidays must be saved and imported when needed.

Not suitable

Not suitable

Not suitable

Suitable

Preferred use case

4

The application must be able to use settings from another system.

It must be possible to transfer settings to another plant using similar variables.

Not suitable

Not suitable

Suitable

Suitable

5

The application must provide human readable data.

The user must be able to read, compare, and edit the data.

Not suitable

Not suitable

Suitable

Suitable