Rosemount 3095FB, Справочное руководство

Страница: 47 / 170

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
register pair, using function 03/04 or function 69. When using function 03/04 the user must
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
Address Function Starting reg-
ister
# of registers Error check
Query 01 04 1C E8 00 01 XXXX
Address Function Byte count Register data
bytes
Error check
Response 01 04 04 42 C8 00 00 XXXX