INVT Goodrive18 Series Operation Manual page 120

Goodrive18 series two-in-one VFD Communication
CRC CHK MSBs
END (frame tail) T1-T2-T3-T4 (time gap with a min. length of 3.5 bytes)
7.2.2.2 RTU communication frame error check modes
During data transmission, errors may occur due to various reasons. For example, if a sending
message contains a logic "1", A-B potential difference on RS485 should be 6V, but the
difference changes to -6V in reality because of electromagnetic interference, and then the
other devices take the logic in the message as "0". Without check, the data receiving device
cannot identify data errors and may make an incorrect response. The incorrect response may
cause severe problems. Therefore, the data must be checked.
The check is implemented as follows: The sender calculates the to-be-sent data based on a
specific algorithm to obtain a result, appends the result at the end of the message, and sends
them together. After receiving the message, the receiver calculates the data based on the
same algorithm to obtain a result, and compares the result with that sent by the sender. If the
results are the same, the message is correct. Otherwise, the message is considered wrong.
The error check of a frame includes two parts, namely bit check on individual bytes (that is,
odd/even check bit using the check bit in the character frame), and whole data check (CRC
check).
Bit check on individual bytes (odd/even check)
You can select the bit check mode as required, or you can choose not to perform the check,
which will affect the check bit setting of each byte. Goodrive18 series two-in-one VFD uses the
even parity check by default.
Definition of even check: Before the data is sent, an even check bit is added to indicate
whether the number of "1" in the to-be-sent data is odd or even. If it is even, the check bit is set
to "0"; and if it is odd, the check bit is set to "1".
Definition of odd check: Before the data is sent, an odd check bit is added to indicate whether
the number of "1" in the to-be-sent data is odd or even. If it is odd, the check bit is set to "0";
and if it is even, the check bit is set to "1".
For example, the data bits to be sent are "11001110", including five "1". If the even check is
applied, the even check bit is set to "1"; and if the odd check is applied, the odd check bit is set
to "0". During the transmission of the data, the odd/even check bit is calculated and placed in
the check bit of the frame. The receiving device performs the odd/even check after receiving
the data. If it finds that the odd/even parity of the data is inconsistent with the preset
information, it determines that a communication error occurs.
CRC check mode
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