Selec VFD Series

63
User Manual
Standard Structure of RTU Frame :
Frame header
T1-T2-T3-T4 (3.5-byte transmission time)
Slave address
Communication address: 0~147(decimal system) /
(Zero stands for broadcast communication address)
Function domain
0x03: reading parameters of slave controller
0x06: programming parameters of slave controller
Data field
2*N-byte data is not only the main content of communication
but also the core of data exchange during the communication.
Checksum
Estimated value: CRC value (16BIT)
Frame termination
T1-T2-T3-T4 (3.5-byte transmission time)
9.3. Frame Structure
of new frame will always be restarted with a interval of 3.5 bytes transmission time. When it's
connected with the Internet of which transmission rate is calculated by Baud rate, 3.5-byte
transmission time is easy to be controlled. The data fields of transmission will be sent in sequence as
below: slave address - operating command code - data &
In the RTU mode, the format of each byte is as follow: encoding system -8-bit binary system, there
are two hexadecimal characters included in every 8-bit frame (hexadecimal characters include 0~9 &
A~F). The transmission
CRC check word; and transmission byte of each field are hexadecimal (0~9, A~F). The networking
devices will always monitor the operation of communication bus; when the first domain (address
information) is received, it will be confirmed again by each networking device; when the transmission
of last byte is done, there will be a interval of 3.5 bytes transmission time to identify the end of this
frame. After that, transmission of new frame will be started again.
Frame format of RTU data
MODBUD Message
Initiate; an
interval of at
least 3.5 bytes
Slave
address
Function
Code
Data
Verification
Terminate; an
interval of at
least 3.5 bytes
A frame message must be transmitted in a consecutive data stream; if the interval is more than 1.5-
byte transmission time before transmission of the whole frame is finished, those incomplete
messages will be cleared by the receiver which will take the next byte as part of address field of new
frame by mistake; likewise, if the interval is less than 3.5- byte transmission time, the receiver will
identify it as the extension of last frame. Because of disorder of frames, CRC value will be incorrect,
which may result in malfunction.