Inovance MD500T75G - Page 231

MD500 User Manual 8. Interfaces and Communication
- 231 -
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CRC check
In the Modbus-RTU mode, a message includes a CRC-based error-check field. The CRC field checks the content of the entire
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and then added to the message. The receiving device recalculates a CRC value after receiving the message, and compares the
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in the register. Only the eight bits in each character are used for the CRC. The start bit, stop bit and the parity bit do not apply to
the CRC.
During generation of the CRC, each eight-bit character is in exclusive-OR (XOR) with the content in the register. Then the result
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is extracted and examined. If the LSB was a 1, the register then performs XOR with a preset value. If the LSB was a 0, no XOR is
performed. This process is repeated until eight shifts have been performed. After the last (eighth) shift, the next eight-bit byte is
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register, after all the bytes of the message have been applied, is the CRC value.
The CRC is added to the message from the low-order byte followed by the high-order byte. The CRC simple function is as follows:
unsigned int crc_chk_value
（
unsigned char *data_value,unsigned char length
）
{
unsigned int crc_value=0xFFFF;
int i;
while
（
length--
）
{
crc_value^=*data_value++;
for
（
L LL
）
{
if
（
crc_value&0x0001
）
{
crc_value=
（
crc_value>>1
）
^0xa001;
}
else
{
crc_value=crc_value>>1;
}
}
}
return
（
crc_value
）
;
}
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