How can one use the Modbus protocol to communicate with an external device?

[darksidePrinceton]

I'm using a 9871 module to connect an external serial device to a cRIO controller. The communication protocol required is Modbus. However, there don't seem to be resources online that indicate how to construct a Modbus string command. I downloaded the Modbus library available for LabView, but that does not show how they construct the string that they send through the serial port. Are there any resources recommended? The device I have plugged in is a Compod controller on a Sierra Smar-Trak 2 flow meter.

[jareds]

Hi darksidePrinceton, 

This is a pretty good resource about Modbus commands.  As you'll see in that tutorial, Modbus communication generally involves reading from and writing to registers on the device.  Your device should have details about which ones to read and write to in order to perform different operations or get data back from the device.