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Functions
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Modicon Premium automation
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Ethernet Modbus/TCP network
Ethernet Modbus/TCP communication services
Functions
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Modbus standard communication protocol
Modbus, the industry communication standard since 1979, has been combined with
Ethernet Modbus/TCP, the medium for the Internet revolution, to form Modbus/TCP,
a completely open Ethernet protocol. The development of a connection to Modbus/
TCP does not require any proprietary component, or the purchase of a licence.
This protocol can easily be combined with any product supporting a standard TCP/IP
communication stack. The specifications can be obtained free of charge from the
following website:
www.modbus-ida.org.
Modbus/TCP, simple and open
The Modbus application layer is very simple and universally familiar with its 9 million
installed connections. Thousands of manufacturers have already implemented this
protocol. Many have already developed a Modbus/TCP connection and numerous
products are currently available.
The simplicity of Modbus/TCP enables any field device, such as an I/O module, to
communicate on Ethernet without the need for a powerful microprocessor or a lot of
internal memory.
Modbus/TCP, high-performance
Due to the simplicity of its protocol and the fast speed of 100 Mbps Ethernet, the
performance of Modbus/TCP is excellent. This allows this type of network to be used
in real-time applications such as I/O scanning.
Modbus/TCP, a standard
The application protocol is identical on Modbus serial link, Modbus Plus or
Modbus/TCP. This means that messages can be routed from one network to the
other without converting the protocol.
Since Modbus is implemented on top of the TCP/IP layer, users can also benefit from
IP routing enabling devices located anywhere in the world to communicate without
worrying about the distance between them.
Schneider Electric offers a complete range of gateways for interconnecting a
Modbus/TCP network to existing Modbus Plus or Modbus serial link networks.
The IANA organization (Internet Assigned Numbers Authority) has allocated the fixed
port TCP 502 (Well known port) to the Modbus protocol. Thus Modbus has become
an Internet standard.
A study by the ARCAdvisory Group, a leading analyst in the automation and
software sectors, shows that Modbus/TCP is the world's leading Ethernet industrial
protocol in terms of units sold in 2004.
Modbus and Modbus/TCP are recognized by the IEC/EN 61158 international
standard as a fieldbus. They are also compliant with the Chinese National Standard
managed by ITEI.
Interfacing CANopen with Modbus/TCP
CiADSP 309-2 provides standardized mapping of CANopen data for transport on
Ethernet Modbus/TCP networks. The specification reserves the Modbus 43/13
function code for this purpose. This function code is reserved exclusively for
CANopen.
Modbus TCP/IP characteristics
Maximum size of data:
b
Read: 125 words or registers
b
Write: 100 words or registers
Modbus/TCP function codes
dec hex
Bit
access
Read n input bits
02
02
Read n output bits
01
01
Read exception status
07
07
Write 1 output bit
05
05
Write n output bits
15
0
F
Read 1 input word
04
04
Read n input words
03
03
Write 1 output word
06
06
Write n output words
16
10
Read device ID
43/14 2
B/0E
Examples of Modbus/TCP function codes for accessing data
and diagnostics
2
1
3
4
5
6
7
8
9
10