Getting modbus devices to communicate to continuum may require some trial and error.
Modbus RTU, ModbusRTU
Windows Server 2003
Windows Server 2008
Manufacturers of modbus devices may have an offset for their register values and addressing these values requires trial an error.
This article was created as a starting point. There are a lot of tools and methods to get values from modbus devices communicating. The document that can be downloaded with the link below discusses some important concepts as well methods for trying to bring values in.