Feedback variables in FBD/LD networks

In FBD/LD function block and program POUs, implicit feedbacks can be programmed by connecting an output back to an input.

In FBD/LD, feedbacks can be realized via connection lines or using variables. A variable which is inserted into a feedback connection line is considered as feedback variable. It provides two connection points (in/out). The menu item 'Toggle Feedback Variable' in the context menu of such a variable is marked as selected. Feedback variables must be connected on both sides.

A feedback variable can be created in several ways:

By right-clicking the feedback connection line and selecting the context menu item 'Insert > Insert Feedback Variable'.
By inserting an existing variable into the feedback connection line. This can be done by drag & drop from the variables grid or using Intellisense. This way, the variable automatically becomes a feedback variable.
By right-clicking any connected or unconnected variable and selecting 'Toggle Feedback Variable' from the context menu. This adds the second connection point which must not remain unconnected.

Feedback lines are displayed red in the code, feedback variables have a red border.

Note
No special declaration is necessary for a standard feedback variable in FBD/LD. For safety-related feedback variables in SNOLD the 'Feedback' property must be set in the declaration.

When disconnecting a feedback variable it remains a feedback variable (maintains both connection points) and can be inserted into another feedback loop. To make a feedback variable a variable, deselect its context menu item 'Toggle Feedback Variable'.