Appendix 2: Rs485 Communication Protocol - Veichi AC90 Manual

AC90 TENSION CONTROL FREQUENCY INVERTER MANUAL

10.2 Appendix 2: RS485 communication protocol

Communication protocol
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The AC90 series frequency inverter can select the RS485 communication interface. The ModBus communication
protocol of international standard 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.
Application styles
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1. AC90 series frequency inverter is connected to "single host and many slave machines" control network with
RS232/RS485 master line. Slave machine do not response while main machine order with broadcast (while
slave machine add is 0).
2. AC90 afford RS485 port only, asynchronous half-duplex. While the communication port of the external
equipment is RS232, RS232/RS485 converter should be added.
3. This ModBus serial communication protocol defines asynchronous transfer information content and formats
used in the serial communication, which can be divided into RUT and ASCII mode. AC90 is RTU（remote
terminal unit）mode.
Communication frame structure
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Communication data format is as follows:
The byte composition: Include initiation bit, 8 data bit, check bit and stop bit.
initiation
Bit1 Bit2
bit
One frame message must be transmitted as a continued data flow, if there is a pause over1.5 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
a new frame. Similarly, if the interval between a new frame start-up and the former frame is smaller than 3.5 byte time,
the receiving equipment will think that it is the former one frame continuation. Because of the jumbled frame, finally
CRC checking value is incorrect, what leads to the communication mistake.
RTU frame's standard structure:
Frame START
Slave machine address
Function code
Data domain
CRC CHK low-order
CRC CHK high-order
Frame ending
In RTU mode, it always begins with transmission time pause gap of at least 3.5 bytes. The successive order of the
following transmission data domains: slave machine address, operation command code, data and CRC checking bit.
Transmission byte of each domain is sexadecimal 0...9; A...F. Network equipment detects the network bus unceasingly,
including the pause gap time. While receiving the first domain (address information), each network equipment carries
out decoding to judge whether the byte is sent to it. While the final byte transmission is completed, there will be an at
least 3.5 bytes transmission time pause gap to indicate that this frame is over. Then a new information' transmission
Bit3 Bit4 Bit5
Transmission time of 3.5 bytes
Communication address:
0～247（decimal）（0 is broadcast address）
03H: read slave machine parameter
06H: write slave machine parameter
08H: circuit auto-detection
parameter address, parameter number, parameter values
Detecting value: 16 bit CRC Detecting value
transmission time of 3.5 bytes
179
BASIC OPERATION AND TRIAL RUN
Bit6 Bit7 Bit8
stop
check bit
bit