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)
10.3.2.2 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).
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
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