Logix5000 Controllers General Instructions - SLAC Confluence

Function Block Attributes 645Resolve a LoopTo create a feedback loop around a block, wire an output pin of theblock to an input pin of the same block. The following example isOK. The loop contains only a single block, so execution order doesnot matter.This input pin uses an output thatthe block produced on theprevious scan.If a group of blocks are in a loop, the controller cannot determinewhich block to execute first. In other words, it cannot resolve theloop.? ??To identify which block to execute first, mark the input wire thatcreates the loop (the feedback wire) with the Assume Data Availableindicator. In the following example, block 1 uses the output fromblock 3 that was produced in the previous execution of the routine.1 2 3This input pin uses the outputthat block 3 produced on theprevious scan.Assume Data Available indicatorThe Assume Data Available indicator defines the data flow within theloop. The arrow indicates that the data serves as input to the firstblock in the loop.Publication 1756-RM003I-EN-P - January 2007