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.
Modbus protocol communication data format of the series inverter is
shown as following. The inverter only support the reading and writing of Word
type parameters, the corresponding reading operation command is “0x03”,
the writing operation command is “0x06”. The writing and reading operation
of byte or bit is not supported.
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 information provided by master,
slave needs not feedback a response to master machine.
Communication data structure
The 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.
In theory, the host computer can continuously read several function codes
once (that is, the maximum value of “n” is 12), but note that not to jump across
the last function code in this function group to avoid the wrong reply.
User Manual
170
User Manual
171
APPENDIX
Appendix A Modbus communications
The 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
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.
(1) Hardware interface.
First sending of
master
First replying
of slave
Second sending
of master
Second relaying
of slave
>3.5byte
Transmission time
Data frame
A
B
Data frame
>3.5byte
Transmission time
Master reads
command
frame
>3.5byte
1byte
2byte
Free
(Start frame)
Target
station
address
Read the
command
(0x03)
Function
code
address
(H... ...L)
Number
of
function
code (n)
CRC
correction
and H... ...L
Free
Calculate CRC
correction
1byte
1byte
2byte
Chapter 9
Chapter 9