Goodrive350 IP55 High-ingress Protection Series VFD
Communication protocol
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RTU data frame format
Modbus packet
Start with an idle time (at
least the transmission
time of 3.5 bytes)
Slave
address
Function
code
Data
Check
End with an idle time (at
least the transmission
time of 3.5 bytes)
The information of a frame must be transmitted 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 (decimal system) (0 is the
broadcast address)
CMD (function domain)
03H: read slave parameters
06H: write slave parameters
DATA (N-1)
...
DATA (0)
(data domain)
Data of 2×N bytes, main content of the communication as well
as the core of data exchanging
CRC CHK (LSBs)
Detection value: CRC (16 bits)
CRC CHK high bit (MSBs)
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)
You can select the bit check mode as required, or you can choose not to perform the check,
which will affect the check bit setting of each byte.
Definition of even check: Before the data is transmitted, an even check bit is added to indicate
whether the number of "1" in the to-be-transmitted data is odd or even. If it is even, the check
