For convenience, I'd recommend trying the MB_Serial_Master Comm Tester (attached). You can try with different serial port settings and register addresses. Once you find the winning combination you can put it in your code.
To use:
- Run the VI
- Set the Communication Settngs: COM1 or COM3 or COM4 (whichever is connected to the instrument), Baud Rate 9600 or 19200, Address 1, Mode RTU, Stop Bits 1.0, Flow Control None, Parity None
- Click Open Port (The LED should turn green)
- Set starting address of Holding Registers to 0 (also try 1), quantity 1
- Click Read Registers
If this produces an error, the serial port will be closed automatically.
MB_Serial_Master Comm Tester.vi
Porter's post in Modbus communication problem was marked as the answer