Chapter 9 Appendix
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Chapter 9 Appendix Modbus communication
Appendix A Modbus communications
VFD300 series of inverter provides RS485 communication interface, and adopts MODBUS
communication protocol. User can carry out centralized monitoring through PC/PLC to get
operating requirements. And user can set the running command, modify or read the function
codes, the working state or fault information of frequency inverter by Modbus communication
protocol.
A.1 About Protocol
This serial communication protocol defines the transmission information and use format in the
series communication and it includes master-polling (or broadcasting) format, master coding
method and the content includes function code of action, transferring data and error checking.
The response of slave is the same structure, and it includes action confirmation, returning the
data and error checking etc. If slave takes place the error while it is receiving the information or
cannot finish the action demanded by master, it will send one fault signal to master as a
response.
A.2 Application Methods
The frequency inverter will be connected into a “Single-master Multi-slave” PC/PLC control net
with RS485 bus as the communication slave.
A.3 Bus structure
(1) Hardware interface.
The “485+” and “485-“terminals on frequency inverter are the communication interfaces of
Modbus
(2) Topological mode
It is a “Single-master Multi-slave” system. In this network, every communication machine has a
unique slave address. One of them is as “master” (usually PC host machine, PLC and HMI, etc.),
actively sends out the communication, to read or write the parameters of slave. Other machines
will be used as slave and response to the inquiry/command from master. At one time only one
machine can send the data and other machines are in the receiving status. The setup range of
slave address is 0 to 247. Zero refers to broadcast communication address. The address of
slave must is exclusive in the network.
(3) Transmission mode
There provide asynchronous series and half-duplex transmission mode. In the series
asynchronous communication, the data is sent out frame by frame in the form of message.
According to the Modbus-RTU protocol, when the free time of no transmission in communication
data lines is more than the transmission time of 3.5byte, it indicates that a new start of
communication frame.