Modbus Read Input Registers (FC04)

Function Code 04

Request

This command is requesting the content of analog input register # 30009
from the slave device with address 17.

11 04 0008 0001 B298

11: The SlaveID (11 hex = address 17 )
04: The Function Code 4 (read Analog Input Registers)
0008: The Data Address of the first register requested.
             ( 0008 hex = 8 , + 30001 offset = input register #30009 )
0001: The total number of registers requested. (read 1 register) 
B298: The CRC (cyclic redundancy check) for error checking.

Response

11 04 02 000A F8F4

11: The SlaveID (11 hex = address 17 )
04: The Function Code 4 (read Analog Input Registers)
02: The number of data bytes to follow (1 registers x 2 bytes each = 2 bytes)
000A: The contents of register 30009
F8F4: The CRC (cyclic redundancy check).