INVT IPE300 Series, Руководство по эксплуатации

Страница: 287 / 383

IPE300 series engineering VFD Communication protocol
-276-
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 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 (time gap with a min. length 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 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 LSB
Detection value: CRC (16 bits)
CRC CHK MSB
END (frame tail) T1-T2-T3-T4 (time gap with a min. length 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 an incorrect response. The incorrect
response may cause severe problems. Therefore, the data must be checked.
The check is implemented as follows: The sender calculates the to-be-sent data based on a specific
algorithm to obtain a result, appends the result at the end of the message, and sends 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 sent by the sender. 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 bit using the check bit in the character frame), and whole data check (CRC check).