There is a great KnowledgeBase that discusses communicating with modbus devices in LabVIEW. This article can be found:
http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/2cd034f223c650e9862569570041995c?OpenDocument
I found this entry by going www.ni.com, clicking on the KnowledgeBase link, and searching for "modbus labview."
