I recommend the use of Custom Command for the purpose.
In the firmware, you edit the code to put your PID logic inside the custom command function, then you can transfer bytes from and to arduino using custom command function on labview.
Sure, let me try.
First, i'm assuming that you may be using community edition. If that is true, i'd recommend the use of control toolkit, since it is available in community edition and have useful PID functions for your block diagram, and the connection with linx device can be used only for reading and writing analog values.
However you may be somewhat like me, who likes to try different things.
two different paths to follow: (i dont know which is better, you might need to test)
1) put your logic in main loop and transfer data via custom command function
2) put your logic inside custom command function.
Since official website is deprecated, i'll post a link for internet archive:
Hope it helps.