INVT Goodrive270 Series Operation Manual page 307

Goodrive270 series VFD for fan and pump
transmission time of 3.5 bytes can be easily obtained. After the idle time ends, the data domains are
transmitted in the following sequence: slave address, operation command code, data, and CRC
check character. Each byte transmitted in each domain includes 2 hexadecimal characters (0–9, A–F).
The network devices always monitor the communication bus. After receiving the first domain (address
information), each network device identifies the byte. After the last byte is transmitted, a similar
transmission interval (the transmission time of 3.5 bytes) is used to indicate that the transmission of
the frame ends. Then, the transmission of a new frame starts.
Start 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)
ADDR (slave address
domain)
CMD (function domain)
Data domain
DATA (N-1)
...
DATA (0)
CRC CHK LSB
CRC CHK MSB
END (frame tail)
9.3.2.2 RTU communication frame error check methods
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 transmitter calculates the to-be-transmitted data based on
RTU data frame format
Modbus packet
Slave Function
address domain
T1-T2-T3-T4 (transmission time of 3.5 bytes)
Communication address: 0–247 (decimal system) (0 is the broadcast
address)
03H: read slave parameters
06H: write slave parameters
Data of 2×N bytes, main content of the communication as well as the
core of data exchanging
Detection value: CRC (16 bits)
T1-T2-T3-T4 (transmission time of 3.5 bytes)
-297-
End with an idle time (at
Data Check least the transmission
Communication protocol
time of 3.5 bytes)