Goodrive3000 series medium voltage VFD
Moudbus protocol
-196-
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 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
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 for 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.
