National Instruments has a product called "Industrial Automation OPC Servers" that provides an OPC Server for a very large number of PLC communication protocols, including Modbus RTU. Industrial Automation OPC Servers installs a program called "Lookout Protocol Drivers", or LPD for short. In LPD, you would configure communication settings to your PLC, in this case the Modbus RTU. LPD then runs as the OPC Server communicating with the PLC, and exposes all of the registers of the PLC as OPC items. You would then write your CVI application as an OPC client, using either OPC functions or datasocket to read/write to the PLC. This is actually the very same method that is used with LabVIEW.
If you have CVI installed, then you should already have some examples on your machine
that demonstrate the OPC Server / Client architecture. I believe they are under the "datasocket" example directory (I'm not at a machine with CVI installed right now, so I'm going on memory). By default, these examples will connect you to the OPCDemo server, which is a software OPC Server that also installs with CVI.
There is much more info about Industrial Automation OPC Servers at the link below:
http://sine.ni.com/apps/we/nioc.vp?lang=US&pc=mn&cid=4584
Regards,
Greg
