Modbus communication example - GMW83 - GMW83A - GMW83D - GMW83DRP - GMW83DRPC - GMW83RPC - GMW84 - GMW84S - GMW86P - GMW87M

GMW80 Series User Guide
Document code
M211748EN
Revision
H
Language
English
Product
GMW83
GMW83A
GMW83D
GMW83DRP
GMW83DRPC
GMW83RPC
GMW84
GMW84S
GMW86P
GMW87M
Document type
User guide

Reading CO2 concentration value

The device address used in the following example is 240 (F0hex). The values returned by the device differ depending on the ambient conditions. Your device might not return exactly the same values.
Request Response
Bytes on the line Description Bytes on the line Description
(silence for 3.5 bytes) Start of Modbus RTU frame (silence for 3.5 bytes) Start of Modbus RTU frame
F0hex Transmitter address F0hex Transmitter address
03hex Function (Read Holding Registers) 03hex Function (Read Holding Registers)
00hex Register address 04hex Number of data bytes
00hex E6hex Value of first register (least significant word)
00hex Number of 16-bit registers to read (2) 3Ahex
02hex 44hex Value of second register (most significant word)
D1hex Modbus RTU checksum 17hex
2Ahex 7Ehex Modbus RTU checksum
(silence for 3.5 bytes) End of Modbus RTU frame B7hex
(silence for 3.5 bytes) End of Modbus RTU frame
Communication description
Register number 1 (1-based Modbus register number) = address 0000hex (0-based address used in actual communication).
Data format Two 16-bit Modbus registers interpreted as IEEE 754 binary32 floating point value, least significant word first.
Returned value 4417E63Ahex, which is binary32 representation of 607.6 (ppm CO2).