Ethernet
TCP/IP Protocol
Modbus was originally developed
by Modicon and today, it is managed by the Modbus-IDA
User Organization. Modbus is an open Master/Slave
application protocol, that can be used on several
different physical Layers. Modbus is an application-layer
messaging protocol, positioned at level 7 of
the OSI model. It provides client/server communication
between devices connected on different types
of buses or networks. Modbus-TCP means that the
Modbus protocol is used on top of Ethernet-TCP/IP.
Modbus-TCP is an open Industrial Ethernet network
which has been specified by the Modbus-IDA User
Organization in co-operation with the Internet
Engeneering Task Force (IETF) as an RFC Internet
standard. Modbus devices are certified by the
Modbus-IDA User Organization for interoperability
and conformance to the Modbus specification.
Disparaged by critics but highly valued by those who have used it,
Modbus-TCP isn't really anything new. Rather more, it was solely
necessary to approve Ethernet-TCP/IP as an additional data transmission
technology for the Modbus Protocol, which has been available since
1979. The well-proven Modbus services and the object model which has
been available since the original Modbus protocol version are
unchanged, and have simply been adapted to TCP/IP as the data
transmission protocol. This extends the Modbus family with an
additional product range, which now consists of the classical
Modbus-RTU (asynchronous data transmission via RS-232 or RS-485),
Modbus-Plus (high speed communication via a Token Passing Network) and
Modbus-TCP (Ethernet-TCP/IP-based client/server communication). All of
these versions share the same application protocol, which specifies a
universal object module for user data and communication services.
Modbus is a request/reply protocol and offers services
specified by function codes. Modbus function codes
are elements of Modbus request/reply PDU's. Modbus
provides a set of functions to read and write data
in the field devices. Modbus supports bit or word
data transfers.
The performance of a Modbus-TCP network is highly
dependent on the type and design of the Ethernet
network which is used and on the performance of
the processors in the communication interfaces
of the respective devices.
Modbus-TCP is a pragmatic approach to use Ethernet as
a data transmission medium for automation applications.
The additional costs of the network infrastructure
(star topology with intelligent switches) can be
justified by the advantages of Ethernet such as
the large number of stations in a network and by
substantial benefits due to additional IT functions
embedded Internet, email and file transfer) which
can use the same medium.
HMS is an active member of the Modbus-IDA User Group
and provides many certified Modbus-TCP products. 
See
Anybus products supporting the Modbus TCP network
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