Hitachi ABB RVT Installation Und Betriebsanleitung Seite 551

The first field then transmitted is the device address.
Networked devices monitor the network bus continuously, including during the 'silent' intervals. When the first field
(the address field) is received, each device decodes it to find out if it is the addressed device.
Following the last transmitted character, a similar interval of at least 3.5 character times marks the end of the mes-
sage. A new message can begin after this interval.
The entire message frame must be transmitted as a continuous stream. If a silent interval of more than 1.5 character
times occurs before completion of the frame, the receiving device flushes the incomplete message and assumes that
the next byte will be the address field of a new message.
Similarly, if a new message begins earlier than 3.5 character times following a previous message, the receiving device
will consider it is a continuation of the previous message. This will set an error, as the value in the final CRC field will
not be valid for the combined messages. A typical message frame is shown below.
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 stand-
ard 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 be-
tween compatible devices. That is, Modbus TCP/IP combines a physical network (Ethernet), with a networking stand-
ard (TCP/IP), and a standard method of representing data (Modbus as the application protocol). Essentially, the Mod-
bus 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.
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