EM12-SP User Manual
Appendix I Modbus communication protocol
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Appendix I. Modbus communication protocol
EM12series 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.
I. 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.
II. 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.
III. 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.
EM12 series inverter has built-in the Modbus-RTU communication protocol, and is applicable to response the slave
“Inquiry/command” or doing the action according to the master’s “Inquiry / Command” and response to the data.
Here, master is personnel computer (PC), industrial machine or programmable logical controller (PLC), and the
slave is inverter. Master not only visits some slave, but also sends the broadcast information to all the slaves. For
the single master “Inquiry/Command”, all of slaves will return a signal that is a response; for the broadcast
