My best guess is you need to set the address, change from input registers to holding registers, and set the correct number of words you want to read. The Modbus spec (see David Wisti's link) is pretty readable and does contain the error codes you'll get back from the device.
One caveat about Modbus addresses is the address you need to request may not be the one documented by the manufacturer, but it will be near that address (+/- a couple); I've found this to be the case on Festo and Cuttler Hammer equipment (to boot the PLC and PC request different addresses for the same register... arg!).
Tim
I too am looking for a RTU MODBUS example. I need to read and write to a modbus slave. I tried the sample program that uses the MODBUS vi master with no sucsess. I have written seueral programs in VB and am new to LAbView. An example of a vi that can read and write to a 40000 register would be helpfull.
Example VI to communicate with Modbus equipment
in Remote Control, Monitoring and the Internet
Posted
I too am looking for a RTU MODBUS example. I need to read and write to a modbus slave. I tried the sample program that uses the MODBUS vi master with no sucsess. I have written seueral programs in VB and am new to LAbView. An example of a vi that can read and write to a 40000 register would be helpfull.
Thanks