gary,
The next thing you need to learn about LabVIEW is dataflow. This is the fundamental paradigm of LV and will explain your slow down issues.
The basic dataflow concept is that any node (node = function, subVI, structure) can begin to execute when data is present on all its inputs and that no data will be present at its outputs until the node comopletes execution.
How does this affect your VI? The true case has a 1000 ms Wait (actually two, one in each case of the inner case structure = redundant code). The case structure will not complete execution until the wait completes. And, the while loop does not complete its iteration until the case structure completes. The result: When the PID.vi output = 0, the loop will run at 1 second per iteration.
What is the fix? Continue running the loop at 100 ms intervals. Keep a count in the case structure (true case) and another shift register of how many 100 ms intervals have passed. When the count reaches ten, toggle the output line. Do not use the 1000 ms Wait.
All the code in your inner case structure is unnecessary. Wire the Remainder output directly to the Arduino Digital Write VI.
Be careful with the comparison to zero with floating point data. If the calculation which produces the data has round off errors you might get a value like 0.573E-15, but that is not equal to zero. It is better to compare to a small range of values above and below zero.
Right to left wiring is much easier to follow when trying to understand what a program does. Using the Clean Up tool occasionally will help (although it will eventually frustrate you also).
Lynn
