Rosemount 3095FB - Examples of Modbus Commands; Reading and Writing Floating Point Data

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Model 3095FB Multivariable Transmitter with Modbus Protocol

Modbus Protocol Guide

Page 16 of 79

2.6 Examples of Modbus Commands

2.6.1 Reading and Writing Floating Point Data

Floating point values must be read or written in a single command to a series of two

consecutive registers. If half of a floating point register is written the 3095FB will return

the Modbus exception Illegal Data Value(03). There are two ways to read a floating point

always read at least 2 registers to get a valid floating point number, unless they are reading a

32-bit floating point register. Function 69 reads a register pair (two consecutive 16 bit regis-

ters). Function 03 and function 04 are interchangeable when working with the 3095FB.

There are two ways to write a 16-bit register pair, using function 16 and function 70. When

writing to a 32-bit register either function 6 or function 16 can be used.

The first three examples show the Modbus message for reading a Differential Pressure (DP)

of 100.00. The last four examples show the Modbus message for writing a DP upper operat-

ing limit of 230.00.

See Section 3.4 for information on changing the byte transmission order of the floating point

registers.

Function 04: Reading Floating Point Data from a 16-bit Register Pair

Address Function Starting reg-

ister

# of registers Error check

Query 01 04 01 90 00 02 XXXX

Address Function Byte count Register data

bytes

Error check

Response 01 04 04 42 C8 00 00 XXXX

Function 04: Reading Floating Point Data from a 32-bit Register