8 Communication
8.2 Modbus
139
SICAM I/O Unit, 7XV5673, Device Manual
E50417-H1040-C484-A3, Edition 05.2012
8.2
Modbus
For the communication via the Ethernet interface, the Modbus TCP protocol or Modbus UDP protocol is used.
For serial FO and RS485 interface, the Modbus RTU protocol is used. The Modbus specification with a detailed
explanation of the Modbus protocol is provided in:
•
Modbus over Serial Line
Specification & Implementation Guide
http://www.modbus.org
•
Modbus Application Protocol Specification
http://www.modbus.org
•
Modbus Messaging on TCP/IP Implementation Guide
http://www.modbus.org
The following features are implemented in the SICAM I/O Unit:
•
Modbus TCP server
•
Modbus UDP server
•
Modbus UDP client (only I/O mirror function)
•
Modbus RTU master (only I/O mirror function)
•
Modbus RTU slave
•
Support of the Modbus function codes 3, 6, 16, and 23
•
Support of the data types: ASCII strings, date/time, indications, controllable indications, and command
8.2.1
Modbus Functions
The Modbus server of SICAM I/O Unit supports the following Modbus functions:
Table 8-1
Supported Modbus Functions
NOTE
The Modbus functions are the same for Modbus TCP and Modbus UDP (Ethernet) and Modbus RTU (serial).
Function
Number
Function
Name
Description
03
(03H)
Read Holding
Registers
Reading one or more holding registers from the Modbus server
Up to 125 registers can be read with one message.
06
(06H)
Preset Single
Register
Writing of value in a single holding register
Function 16 is used for writing several holding registers via a Modbus
telegram.
16
(10H)
Preset
Multiple
Registers
Writing of values in several holding register (sequential)
A maximum of 123 registers can be written with a telegram.
23
(17H)
Read/Write
4x Registers
Combination of writing (request indication) and reading (answer indication)
in one or more holding registers; writing and reading do not have to take
place in the same register.
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