17
14063434.02 10/2019 EN/DE
WIKA operating instructions transmitter, models GDT-20, GDHT-20
EN
6. Commissioning, operation
6.4.2.4 Modbus
®
register and functional description
The following documents (available under www.Modbus.org) are recommended for under-
standing the Modbus
®
architecture which the following chapters will refer to.
■
Modbus APPLICATION PROTOCOL SPECIFICATION
■
Modbus over serial line specification and implementation guide
The register structure is described in the following.
Communication via messages
General form of the messages
Instrument address
Function
Data
CRC check
8 Bit
8 Bit
n x 8 Bit
16 Bit
In accordance to Modbus
®
specification, separate messages must be divided by a break of at
least 3.5 characters.
The characters within one message may not have spacing of more than 1.5 characters.
Examples of a typical transmission:
Intermission Message 1
▶
Intermission Message 2
▶
Intermission Message 3
...
Valid function calls
Function
Designation
Description
03
Read holding registers
Reading of one or more register values or the instrument
configuration
04
Read input register
Reading a register value or the instrument configuration
06
Write single register
Writing a register value or the instrument configuration
16
Write multiple registers
Writing of one or more register values or the instrument
configuration
08
Diagnostic - Sub code 00
Diagnostic function
23
Read/write multiple registers
configuration
Writing or reading of one/several register values or the
instrument configuration
