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For a complete description of the Modbus protocol, please look at the Modicon
Modbus Protocol Reference Guide (PI–MBUS–300 Rev. J).
Modbus TCP Framing
Modbus TCP/IP (also Modbus-TCP) is simply the Modbus RTU protocol with a TCP
interface that runs on Ethernet.
The Modbus messaging structure is the application protocol that defines the rules for
organizing and interpreting the data independent of the data transmission medium.
TCP/IP refers to the Transmission Control Protocol and Internet Protocol, which
provides the transmission medium for Modbus TCP/IP messaging.
Simply stated, TCP/IP allows blocks of binary data to be exchanged between computers.
It is also a world-wide standard that serves as the foundation for the World Wide Web.
The primary function of TCP is to ensure that all packets of data are received correctly,
while IP makes sure that messages are correctly addressed and routed. Note that the
TCP/IP combination is merely a transport protocol, and does not define what the data
means or how the data is to be interpreted (this is the job of the application protocol,
Modbus in this case).
So in summary, Modbus TCP/IP uses TCP/IP and Ethernet to carry the data of the
Modbus message structure between compatible devices. That is, Modbus TCP/IP
combines a physical network (Ethernet), with a networking standard (TCP/IP), and a
standard method of representing data (Modbus as the application protocol). Essentially,
the Modbus TCP/IP message is simply a Modbus communication encapsulated in an
Ethernet TCP/IP wrapper.
In practice, Modbus TCP embeds a standard Modbus data frame into a TCP frame,
without the Modbus checksum, as shown in the following diagram.
RVT communication ç Modbus protocol overview 11
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