Vaisala GMP252 - Modbus Communication Examples

A.3 Modbus Communication Examples
Reading CO
2
Value
Device address used in the following examples is 240 (F0
hex
).
The values returned by the device dier depending on the ambient conditions and/or
device settings. Your device might not return exactly same values.
Request Response
Bytes on the Line
(Hexadecimal)
Description Bytes on the Line
(Hexadecimal)
Description
(silence for 3.5 bytes) Start of Modbus RTU
frame
(silence for 3.5 bytes) Start of Modbus RTU
frame
F0
hex
GMP25x address F0
hex
GMP25x address
03
hex
Function (Read Holding
Registers)
03
hex
Function (Read Holding
Registers)
00
hex
Register address 04
hex
Number of data bytes
00
hex
D4
hex
Value of first register
(least significant word)
00
hex
Number of 16-bit
registers to read (2)
7A
hex
02
hex
43
hex
Value of second register
(most significant word)
D1
hex
Modbus RTU checksum E8
hex
2A
hex
33
hex
Modbus RTU checksum
(silence for 3.5 bytes) End of Modbus RTU
frame
AB
hex
(silence for 3.5 bytes) End of Modbus RTU
frame
Communication Description
Register address 1 (1-based Modbus documentation format) = 0000
hex
(0-based format 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 43E8D47Ah, which is binary32 representation of
465.65997 (ppm).
GMP252 User Guide M211897EN-D
118