Communication Protocol
130
Start bit
BIT1
BIT2
BIT3 BIT4 BIT5 BIT6
BIT7
Check bit
End bit
In one character frame, the digital bit takes effect. The start bit, check bit and end bit is used to
send the digital bit right to the other device. The digital bit, even/odd checkout and end bit should
be set as the same in real application.
The MODBUS minimum idle time between frames should be no less than 3.5 bytes. The network
device is detecting, even during the interval time, the network bus. When the first field (the address
field) is received, the corresponding device decodes next transmitting character. When the interval
time is at least 3.5 byte, the message ends.
The whole message frame in RTU mode is a continuous transmitting flow. If there is an interval
time (more than 1.5 bytes) before the completion of the frame, the receiving device will renew the
uncompleted message and suppose the next byte as the address field of the new message. As
such, if the new message follows the previous one within the interval time of 3.5 bytes, the
receiving device will deal with it as the same with the previous message. If these two phenomena
all happen during the transmission, the CRC will generate a fault message to respond to the
sending devices.
The standard structure of RTU frame:
START
T1-T2-T3-T4(transmission time of 3.5 bytes)
ADDR
Communication address: 0
–
247(decimal system)(0 is the broadcast
address)
CMD
03H: read slave parameters
06H: write slave parameters
DATA (N-1)
...
DATA (0)
The data of 2*N bytes are the main content of the communication as
well as the core of data exchanging
CRC CHK low bit
Detection value: CRC (16BIT
）
CRC CHK high bit
END
T1-T2-T3-T4(transmission time of 3.5 bytes)
9.3.2.2 RTU communication frame error checkout
Various factors (such as electromagnetic interference) may cause error in the data transmission.
For example, if the sending message is a logic "1",A-B potential difference on RS485 should be 6V,
but in reality, it may be -6V because of electromagnetic interference, and then the other devices
take the sent message as logic"0". If there is no error checkout, the receiving devices will not find
the message is wrong and they may give incorrect response which cause serious result. So the
checkout is essential to the message.
The theme of checkout is that: the sender calculate the sending data according to a fixed formula,
and then send the result with the message. When the receiver gets this message, they will
calculate anther result according to the same method and compare it with the sending one. If two
results are the same, the message is correct. If not, the message is incorrect.
The error checkout of the frame can be divided into two parts: the bit checkout of the byte and the
whole data checkout of the frame (CRC check).
