GD350A series high-performance multifunction VFD
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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 stop bits consistently.
In RTU mode, a new frame always must be 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 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 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 (in decimal system) (0 indicates 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 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
