Connecting Instructions in FBD

FBD offers horizontal and vertical lines to help connect logic in series and parallel arrangements. By connecting the logic flow output of one instruction to the logic flow input of another, you create the logic in your program. FBD provides some horizontal connections automatically, and can even expand boxes vertically to align logic flows.

Connecting Boxes in Series

Boxes can be connected in a logical series. Horizontals are sometimes needed to connect the logic flows of a series of instructions. Horizontals are used here to connect the Add Real and Multiply Real boxes.

Multiple Box Branch Connection

When a specific output is needed as input to multiple instructions, verticals can be used to make the needed connection. A vertical can extend up and/or down from the cell of origin. Here, verticals enable the result of Compare Integer to be used as logic flow input by two boxes.

Combining Branch Connections

Note that verticals should not be used to connect multiple outputs to a specific input. Such a configuration will not be compiled.

Instead, an AND or OR box should be used to tie together multiple logic flows.

Automatic Logic Flow Connections

When an element is placed in a network, the FBD editor automatically connects the logic flow input of the element to any logic flow indicator to the left. In the network to the right, FBD automatically extended logic flow to the Read Real-Time Clock.

Automatic extension of logic flow is not unconditional. The first example above demonstrates that horizontals are not extended to a logic flow output with the Immediate setting.

The second example shows that an assigned logic flow parameter is not replaced with logic flow when the box is placed to its right.

The final example results when the box is placed directly on a logic flow output. The logic flow output parameter is replaced with logic flow.

When a box is placed in a network, and the box contains two or more logic flow inputs, the editor will automatically align the logic flow inputs with the logic flow indicators and outputs of the network.

The Table Find box occupies many rows of the network. In order for the second logic flow input of the AND box to be connected to logic flow output of the second box, the box was automatically expanded.