AC70 VC CONTROL FREQUENCY INVERTER MANUAL
APPENDIX
135
Appendix 2: RS485 Communication Protocol
1 Communication Protocol
AC70 series frequency inverter can select the RS485 communication interface. The international standard ModBus
communication protocol is adopted for master-slave communication. The consumer can carry out centralized control by
PC/PLC, upper machine, main station frequency inverter etc (Setting of the frequency inverter control command,
running frequency, relative function parameters modification, frequency inverter working state and malfunction
information monitoring etc.. to adapt to the special application requirements.
2 Protocol Content
This ModBus protocol defines the information content and format of asynchronous transmission in series
communication. It includes: host machine polling, broadcast and the format of slave machine response. Host machine
data frame includes: slave machine address (or broadcast address), ask action code, data and fault check. Slave
machine response is same structure: action check, back data and fault check. If slave machine meet fault while accept
frame, or can’t compete the action asked, fault information will be feed back to host machine.
3 Application Mode
AC70 series frequency inverter controls network for “single host machine and many slave machines” with
RS232/RS485.
Remote RS485 communication needs shield cable and shield grounding.
For long distance communication, we suggest to open J6 and add
120Ω
resistance to prevent signal reflection.
4 Main Line Structure
1, Port mode: RS485 port
2, Transmission mode: Asynchronous series, half duplex transmission mode. At same time, one of the host machine
and slave machine sends data, anther receives data. Data is sent one by one frame as report form in asynchronous
series communication.
3, Topological structure: One host machine with several slave machines. Range of slave machine address is 1 to 247.
O is broadcast address. Every slave machine address is only one. It is the base of ModBus series communication.
5 Protocol Explain
AC70 series frequency inverter communication protocol is main-slave ModBus communication protocol of
asynchronous series. In the net, only the host machine can set up protocol “inquire/ order”. Slave machines can only
respond to host machine. Host machine means PC, main frequency inverter, industrial control equipment or PLC…
Slave machines are AC70 frequency inverters and other control equipments with same communication protocol. Host
machine can communicate with only one slave machine or broadcast to all slave machines. Slave machine need
feedback every “inquire/order” of host machine, but no need feedback broadcast.
5.1 Communication frame structure
The ModBus protocol communication data format of AC70 series frequency inverter is RTU (remote terminal unit) mode.
Communication data format is as follows:
The byte composition: Include initiation bit, 8 data bit, check bit and stop bit.
Initiation
bit
Bit1
Bit2
Bit3
Bit4
Bit5
Bit6
Bit7
Bit8
No check bit
Even check bit
Odd check
stop bit
In RTU mode, it always starts after at least 3.5 byte transmission time which is easy realized under Baud rate. And next
data is: slave machine address, operation order code, data and CRC check. Every domain transmission is hexadecimal
0...9
，
A...F. Network equipment detects the network bus unceasingly, including the interval time. While receiving the first
domain (address information, each network equipments carry out decoding to judge whether the byte is for itself. While
the final byte transmission is completed, there will be at least 3.5 bytes transmission time interval to indicate that this
frame is over. Then a new information’ transmission can begin
One frame message must be transmitted as a continued data flow. If there is a pause over 1.5 byte before the end, the
receiving equipment will clear the half-baked information. And the next byte will be considered as the address domain of
