Hello!
There's a few different options for Modbus communication with LabVIEW. The options supported by National Instruments are covered here:
Introduction to Modbus using LabVIEW:: https://www.ni.com/en/shop/labview/introduction-to-modbus-using-labview.html
These are your best option for good documentation and examples on how to use the examples. However, if you don't have the LabVIEW add-ons to use one of the above options, there's an API here, but it is not supported by National Instruments, and you will need to rely on the Industrial Communications forums for assistance.
LabVIEW Modbus API:: https://forums.ni.com/t5/Reference-Design-Content/LabVIEW-Modbus-API/ta-p/3524019
Overall, you'll probably get better assistance deciding what's best for you application and how to accomplish this by posting your questions to the Industrial Communications forum instead of the LabVIEW forums.
Industrial Communications:: https://forums.ni.com/t5/Industrial-Communications/bd-p/260
Hope that gives you an idea of where to start!
R Dahlman
