Rockwell Automation Logix5000 Controllers Function Block Diagram Programming Manual - Manual (Page 14)

Chapter 1 Program a Function Block Diagram
The order of execution is only relative to the blocks that are wired together. The
two groups of blocks in this example are not wired together. The blocks within a
specific group execute in the appropriate order in relation to the blocks in that
group.
Resolve a loop
To create a feedback loop around a block, wire an output pin of the block to an
input pin of the same block. In this example, the loop contains only a single block,
so execution order does not matter.
If a group of blocks are in a loop, the controller cannot determine which block to
execute first, and it cannot resolve the loop, as illustrated in this example.
14 Rockwell Automation Publication 1756-PM009E-EN-P - October 2014