Kruger Ecowatt K320 Series Operation Manual page 289

Ecowatt Drive K320 series Communication
®
RTU data frame format
Modbus packet
Start with a time gap End with a time gap
Slave Function
(with a min. length of 3.5 Data Check (with a min. length of
address code
bytes) 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)
Communication address: 0–247 (in decimal system) (0
ADDR (slave address domain)
indicates the broadcast address)
03H: read slave parameters
CMD (function domain)
06H: write slave parameters
(Data domain)
DATA(N-1) Data of 2*N bytes, main content of the communication
... as well as the core of data exchanging
DATA(0)
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).
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