Modbus RTU protocol - WM80

WM80 User Guide

Document code
M212995EN
Revision
A
ft:locale
en-US
Product
WM80
Document type
User guide

The WM80 interface supports Modbus RTU.

Data encoding

In the data registers, the numeric values are available in one or two formats with separate register addresses: 32-bit IEEE floating point format and/or 16-bit signed integer format.

32-bit floating point or 32-bit integer format

Registers using 32-bit float data format are encoded using the binary32 encoding defined in IEEE 754. The format is 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.

Despite the specification, some Modbus clients may expect a big-endian word order (most significant word first). In such case, you must select "word-swapped" floating point format in your Modbus client for the Modbus registers of the device.

A complete 32-bit floating point or 32-bit integer value should be read and written in a single Modbus transaction.

CAUTION Reading the measurement data registers with incorrect floating point format setting may occasionally result in correct-looking, but nevertheless incorrect values.
Verify that you have configured the floating point format correctly on your Modbus host system by reading a floating point value from a test value register.