Mitsubishi Electric MELSEC iQ-R Series Reference Manual page 9

4.
If the least significant bit of the step 2 is '1', calculate the exclusive OR (XOR) from the result of step 3 and the generator
polynomial (A001H).
If the least significant bit is '0', do not calculate the exclusive OR (XOR), but shift it by 1 bit to the right.
5.
Repeat the step 3 and 4 until the bit is shifted up to 8 times.
6.
Calculate the exclusive OR (XOR) from the result of step 5 and the next 8 bits of the frame.
7.
Repeat the step 3 to 6.
8.
Repeat the above operations until the end of the data unit is reached. The final value is a calculated CRC value.
9.
The CRC value is stored in the frame in the order of lower 8 bits to upper 8 bits.
Ex.
The following is a calculation example in the case where function code 07H is sent to station No.2.
Error check range is as follows:
• Address field (1 byte): 02H
• Function code (1 byte): 07H
• Data (0 bytes)
CRC error checking procedure
1 Load the 16-bit register whose bits are all '1'.
2 Calculate the exclusive OR (XOR) from the first 8 bits of the
frame and the bits in the above.
3 Shift by 1 bit to the right.
4 Since the flag is '1', calculate the exclusive OR (XOR) from the
result in 'Shift 1' and generator polynomial (A001H).
5 Shift by 1 bit to the right.
Since the flag is '1', calculate the exclusive OR (XOR) from the
result in 'Shift 2' and generator polynomial (A001H).
Shift by 1 bit to the right.
Shift by 1 bit to the right.
Since the flag is '1', calculate the exclusive OR (XOR) from the
result in 'Shift 4' and generator polynomial (A001H).
Shift by 1 bit to the right.
Shift by 1 bit to the right.
Since the flag is '1', calculate the exclusive OR (XOR) from the
result in 'Shift 6' and generator polynomial (A001H).
Shift by 1 bit to the right.
Shift by 1 bit to the right.
Since the flag is '1', calculate the exclusive OR (XOR) from the
result in 'Shift 8' and generator polynomial (A001H).
6 Calculate the exclusive OR (XOR) from the next 8 bits of the
frame and the bits in the above.
16-bit register (MSB)
16-bit register whose bits are all '1'.
Address field (02H)
Exclusive OR (XOR)
Shift 1
Generator polynomial (A001H)
Exclusive OR (XOR)
Shift 2
Generator polynomial (A001H)
Exclusive OR (XOR)
Shift 3
Shift 4
Generator polynomial (A001H)
Exclusive OR (XOR)
Shift 5
Shift 6
Generator polynomial (A001H)
Exclusive OR (XOR)
Shift 7
Shift 8
Generator polynomial (A001H)
Exclusive OR (XOR)
Function code (07H)
Exclusive OR (XOR)
1.1 MODBUS Protocol Frame Specifications
Flag
1111 1111 1111 1111 

0000 0010
1111 1111 1111 1101
0111 1111 1111 1110 1
1010 0000 0000 0001
1101 1111 1111 1111
0110 1111 1111 1111 1
1010 0000 0000 0001
1100 1111 1111 1110
0110 0111 1111 1111 0
0011 0011 1111 1111 1
1010 0000 0000 0001
1001 0011 1111 1110
0100 1001 1111 1111 0
0010 0100 1111 1111 1
1010 0000 0000 0001
1000 0100 1111 1110
0100 0010 0111 1111 0
0010 0001 0011 1111 1
1010 0000 0000 0001
1000 0001 0011 1110
 
0000 0111
1000 0001 0011 1001
1 FRAME SPECIFICATIONS
1
*1
7