Goodrive350A series high-performance multifunction VFD
Communication
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RTU data frame format
Modbus packet
Start with a time gap
(with a min. length of 3.5
bytes)
Slave
address
Function
code
Data
Check
End with a time gap
(with a min. length of
3.5 bytes)
The information of a frame must be sent in a continuous data flow. If there is an interval greater than
the transmission time of 1.5 bytes before the transmission of the entire frame is complete, the
receiving device deletes the incomplete information, and mistakes the subsequent byte for the
address domain of a new frame. Similarly, if the transmission interval between two frames is shorter
than the transmission time of 3.5 bytes, the receiving device mistakes it for the data of the last frame.
The CRC check value is incorrect due to the disorder of the frames, and thus a communication fault
occurs.
The following table describes the standard structure of an RTU frame.
START (frame header)
T1-T2-T3-T4(transmission time of 3.5 bytes)
ADDR
(slave
address
domain)
Communication address: 0
–247 (in decimal system) (0 indicates the
broadcast address)
CMD (function domain)
03H: read slave parameters
06H: write slave parameters
(Data domain)
DATA(N-1)
…
DATA(0)
Data of 2*N bytes, main content of the communication as well as the
core of data exchanging
CRC CHK low-order bits
Detection value: CRC (16 bits)
CRC CHK high-order bits
END (frame tail)
T1-T2-T3-T4 (transmission time of 3.5 bytes)
9.3.2.2 RTU communication frame error check modes
During the transmission of data, errors may occur due to various factors. Without check, the data
receiving device cannot identify data errors and may make a wrong response. The wrong response
may cause severe problems. Therefore, the data must be checked.
The check is implemented as follows: The transmitter calculates the to-be-transmitted data based on
a specific algorithm to obtain a result, adds the result to the rear of the message, and transmits them
together. After receiving the message, the receiver calculates the data based on the same algorithm
to obtain a result, and compares the result with that transmitted by the transmitter. If the results are
the same, the message is correct. Otherwise, the message is considered wrong.
The error check of a frame includes two parts, namely, bit check on individual bytes (that is, odd/even
check using the check bit in the character frame), and whole data check (CRC check).
Bit check on individual bytes (odd/even check)
