Vaisala HMP3 - Modbus Communication Examples

A.4.6 Modbus communication examples
Reading relative humidity 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
Probe address F0
hex
Probe 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
7A
hex
Value of first register
(least significant
word)
00
hex
Number of 16-bit
registers to read (2)
E1
hex
02
hex
41
hex
Value of second
register (most
significant word)
D1
hex
Modbus RTU
checksum
F4
hex
2A
hex
05
hex
Modbus RTU
checksum
(silence for 3.5 bytes) End of Modbus RTU
frame
06
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 41F47AE1
hex
, which is binary32 representation of
30.56 (%RH).
HMP3, HMP4, HMP5, HMP7, HMP8, HMP9, MMP8, TMP1 User Guide M212022EN-D
88
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