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 Default Value
Serial bit rate 19200
Parity N
Number of data bits 8
Number of stop bits 2
Modbus device address 240
Serial delay 0
Communication mode Modbus RTU
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
(Decimal)
Address
(Hexadecimal)
Register Description Data Format Test Value
7937 1F00
hex
Signed integer test 16-bit integer -12345
7938 1F01
hex
Floating point test 32-bit float -123.45
17
ENGLISH
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