Vaisala HMDW110 Kurzanleitung Seite 17

Modbus Communication
Transmitters ordered with the Modbus configuration option are accessed using the Modbus
serial communication protocol. The supported Modbus variant is Modbus RTU (Serial Modbus)
over RS-485 interface.
Table 2 Default Modbus Serial Communication Settings
Description
Serial bit rate
Parity
Number of data bits
Number of stop bits
Modbus device address
Serial delay
Communication mode
Use Vaisala Insight software to change the Modbus serial communication settings if needed.
Download Insight software at www.vaisala.com/insight.
Data Encoding
Registers using "32-bit float" data format are encoded using the "binary32" encoding defined
in IEEE 754 (also known as "single-precision floating point format").
The least significant 16 bits of a floating point number are placed at the Modbus register listed
in the table, while the most significant 16 bits are placed in the register with number/
address + 1, as specified in Open Modbus TCP Specification, Release 1.0. This is also known as
"little-endian" or "Modicon" word order.
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.
Test Value Registers
Read the known test values from the test registers to verify the functionality of your Modbus
implementation.
Table 3 Modbus Test Registers (Read-Only)
Register Number Address
(Decimal) (Hexadecimal)
7937 1F00
7938 1F01
Register Description
Signed integer test
hex
Floating point test
hex
Default Value
19200
N
8
2
240
0
Modbus RTU
Data Format
16-bit integer
32-bit float
Test Value
-12345
-123.45
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