Salicru CV10-002-S2 User Manual page 77

The standard structure of RTU frame:
START
ADDR
CMD
DATA (N-1)
...
DATA (0)
CRC CHK low bit
CRC CHK high bit
END
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 another 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.
The definition of even checkout: add an even check bit before the data transmission to illustrate the number of
"1" in the data transmission is odd number or even number. When it is even, the check byte is "0", otherwise,
the check byte is"1". This method is used to stabilize the parity of the data.
The definition of odd checkout: add an odd check bit before the data transmission to illustrate the number of "1"
in the data transmission is odd number or even number. When it is odd, the check byte is "0", otherwise, the
check byte is"1". This method is used to stabilize the parity of the data.
For example, when transmitting "11001110", there are five "1" in the data. If the even checkout is applied, the even
check bit is "1"; if the odd checkout is applied; the odd check bit is "0". The even and odd check bit is calculated on
the check bit position of the frame. And the receiving devices also carry out even and odd checkout. If the parity of
the receiving data is different from the setting value, there is an error in the communication.
CRC check
The checkout uses RTU frame format. The frame includes the frame error detection field which is based on the CRC
calculation method. The CRC field is two bytes, including 16 figure binary values. It is added into the frame after
calculated by transmitting device. The receiving device recalculates the CRC of the received frame and compares them
with the value in the received CRC field. If the two CRC values are different, there is an error in the communication.
During CRC, 0*FFFF will be stored. And then, deal with the continuous 6-above bytes in the frame and the value in the register. Only
the 8Bit data in every character is valid to CRC, while the start bit, the end and the odd and even check bit is invalid.
Communication protocol
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
The data of 2*N bytes are the main content of the communication as well as
the core of data exchanging
Detection value:CRC (16BIT）
T1-T2-T3-T4 (transmission time of 3.5 bytes)
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