99
10-bit character frame (BIT1~BIT7 are the digital bits)
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)
7.2.2.2 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).
Bit checkout of the byte
The user can select different bit checkouts or non-checkout, which impacts the check bit setting of each byte.
To Next Page
Loading...
Need help?
General