Goodrive350-19 series VFD for crane
Communication protocol
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In a character frame, only the data bits carry information. The start bit, check bit, and end bit are used
to facilitate the transmission of the data bits to the destination device. In practical applications, you
must set the data bits, parity check bits, and end bits consistently.
In RTU mode, a new frame must be always preceded by a time gap with a minimum length of 3.5
bytes. On a network where the transmission rate is calculated based on the baud rate, the
transmission time of 3.5 bytes can be easily obtained. After the idle time ends, the data domains are
sent in the following sequence: slave address, operation command code, data, and CRC check
character. Each byte sent 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 sent, a similar transmission
interval (with a minimum length of 3.5 bytes) is used to indicate that the frame transmission ends.
Then, the transmission of a new frame starts.
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 lists 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 (N-1)
...
DATA (0)
(data domain)
Data of 2xN 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)
