Modbus-Rtu; Introduction In The Modbus-Rtu Protocol; Transmission Modes; Message Structure In Rtu Mode - WEG CFW08 Vector inverter User Manual

CFW-08 OPTIONS AND ACCESSORIES

8.20 MODBUS-RTU

8.20.1 Introduction to
Modbus-RTU Protocol
8.20.1.1 Transmission
Modes
8.20.1.2 Message Structure
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Modbus protocol has been already developed 1979 firstly. Currently it is a
wide diffused open protocol, used by several manufacturers in different
equipment. The Modbus-RTU communication of the do CFW-08 has been
developed by considering two documents:
1. MODBUS Protocol Reference Guide Rev. J, MODICON, June 1996.
2. MODBUS Application Protocol Specification, MODBUS.ORG,
may 8th 2002.
In these documents are defined the format of the messages used by
these elements that are part of the Modbus network, the services (or
functions) that can be made available via network, and also how these
elements exchange the data on the network.
Two transmission modes are defined in the protocol definition: ASCII and
RTU. The transmission modes define the form how the message bytes
are transmitted. It is not permitted to use the two transmission modes on
the same network.
In the RTU mode each transmitted word has one start bit, eight data bits,
1 parity bit (optional) and 1 stop bit (2 stop bits, if no parity bit is used).
Thus the bit sequence for the transmission of 1 byte is as follows:
Start B0 B1 B2
In the RTU mode each transmitted word has 1 start bit, eight data bits, 1
parity bit (optional) and 1 stop bit (2 stop bits, if parity bit is not used).
Thus the bit sequence for the transmission is as follows:
In RTU ModeThe Modbus RTU network operates in Master-Slave system
and it can consist of up to 247 slaves but only one Master. The master
always initiates the communication with a question to a slave and the
slave answers the question. Both messages (question and answer) have
the same structure: Address, Function Code, and CRC. Depending on
what is being requested, only the data field has variable length.
Master Query Message
Address (1 byte)
Function Code (1 byte)
Data (n bytes)
CRC (2 bytes)
B3 B4 B5 B6
Figure 8.26 - Message Structure
B7 Parity or Stop
Address (1 byte)
Function Code (1 byte)
Data (n bytes)
CRC (2 bytes)
Slave Answer Message
Stop