Frequency Inverter ST600 Series High-Performance Multifunction VFD Communication protocol
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The error check of a frame includes two parts, namely, bit check on individual bytes (that is, odd/even
check 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.
Definition of even check: Before the data is transmitted, an even check bit is added to indicate whether
the number of "1" in the to-be-transmitted 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 transmitted, an odd check bit is added to indicate whether
the number of "1" in the to-be-transmitted 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 transmitted 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
A frame in the RTU format includes an error detection domain based on the CRC calculation. The
CRC domain checks all the content of the frame. The CRC domain consists of two bytes, including 16
binary bits. It is calculated by the transmitter and added to the frame. The receiver calculates the CRC
of the received frame, and compares the result with the value in the received CRC domain. If the two
CRC values are not equal to each other, errors occur in the transmission.
During CRC, 0xFFFF is stored first, and then a process is invoked to process a minimum of 6
contiguous bytes in the frame based on the content in the current register. CRC is valid only for the 8-
bit data in each character. It is invalid for the start, end, and check bits.
During the generation of the CRC values, the "exclusive or" (XOR) operation is performed on the each
8-bit character and the content in the register. The result is placed in the bits from the least significant
bit (LSB) to the most significant bit (MSB), and 0 is placed in the MSB. Then, LSB is detected. If LSB
is 1, the XOR operation is performed on the current value in the register and the preset value. If LSB
is 0, no operation is performed. This process is repeated 8 times. After the last bit (8
th
bit) is detected
and processed, the XOR operation is performed on the next 8-bit byte and the current content in the
register. The final values in the register are the CRC values obtained after operations are performed
on all the bytes in the frame.
The calculation adopts the international standard CRC check rule. You can refer to the related
standard CRC algorithm to compile the CRC calculation program as required.
The following is a simple CRC calculation function for your reference (using the C programming
language):
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