Goodrive600 series high-performance multifunction VFD
Communication networking
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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.
Cyclical Redundancy Check (CRC) method
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 low-order bit to the high-order bit, and 0 is placed
in the high-order bit. Then, the low-order bit is detected. If the low-order bit is 1, the XOR operation is performed on the
current value in the register and the preset value. If low-order bit is 0, no operation is performed. This process is repeated
8 times. After the last bit (8th 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 example is a simple CRC calculation function for your reference (using the C programming language):
unsigned int crc_cal_value (unsigned char*data_value,unsigned char data_length)
{
int i;
unsigned int crc_value=0xffff;
while (data_length--)
{
crc_value^=*dat+;
for (i=0;i<8;i++)
{
if (crc_value&0x0001)
crc_value= (crc_value>>1)^0xa001;
else
crc_value=crc_value>>1;
}
}
return (crc_value);
}
In the ladder logic, CKSM uses the table look-up method to calculate the CRC value according to the content in the
frame. The program of this method is simple, and the calculation is fast, but the ROM space occupied is large. Use this
program with caution in scenarios where there are space occupation requirements on programs.
