I just ran into an issue where a VFD with option modules would kick back "Illegal Data Address" when attempting to read/write Holding Registers. Upon discussion with the manufacturer it was found that the unit ID for the various option modules is different than the VFD itself (255).
I changed the Unit ID constant in the "Create TCP Session" vi and it fixed the issue, as well as using a property node to change the Unit ID after creating the TCP Session. Would you consider changing the constant to a control with the default being 255?
Keep up the great work, I use this library daily and it has been a life saver.
Thanks!