Combination of Elements

Elements of the FBD language must be interconnected by signal flow lines.Outputs of blocks shall not be connected together. In particular, the “wired-OR” construct of the LD language is not allowed, as an explicit Boolean “OR” block is required.FeedbackA feedback path is said to exist in a network when the output of a function or function block is used as the input to a function or function block which precedes it in the network; the associated variable is called a feedback variable.Feedback paths can be utilized subject to the following rules:

• Feedback variables must be initialized, and the initial value is used during the first evaluation of the network. See the Global variables editor, the Local variables editor, or the Parameters editor to know how to initialize the respective item.
• Once the element with a feedback variable as output has been evaluated, the new value of the feedback variable is used until the next evaluation of the element.

For instance, the Boolean variable RUN is the feedback variable in the example shown below.Explicit Loop Implicit Loop