Despite the specification, some Modbus masters may expect "big-endian" word order (most
significant word first). In such case, you must select "word-swapped" floating point format in
your Modbus master for the Modbus registers of the device.
Reading the measurement data registers with incorrect floating point format setting may
occasionally result in correct-looking values. It is highly recommended to first try reading the
register number 7938 (floating point test) to see that you have configured the floating point
format correctly on your Modbus host system.
A complete 32-bit floating point value should be read and written in a single
Modbus transaction.
A.4 Modbus Registers
Registers are numbered in decimal, starting from one. Register
addresses in actual Modbus messages (Modbus Protocol Data Unit (PDU)) start
from zero.
Please check the reference documentation of your Modbus host (PLC) for the
notation of Modbus register addresses.
CAUTION!
A.4.1 Measurement Data Registers
Table 51 Modbus Measurement Data Registers (Read-Only)
Register
Number
(Decimal)
Address
(Hexadecimal)
Register Description Data Format Unit
1 0000
hex
Relative humidity 32-bit float %RH
3 0002
hex
Temperature 32-bit float °C
9 0008
hex
Dew/frost point
temperature
32-bit float °C
19 0012
hex
Wet bulb temperature 32-bit float °C
27 001A
hex
Enthalpy 32-bit float kJ/kg
Accessing unavailable (temporarily missing) measurement data does not generate an
exception. “Unavailable” value (a quiet NaN) is returned instead.
Appendix A – Modbus Reference
79
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