CHAPTER 13 RS-485 COMMUNICATIONS

(3) CRC-16 calculation example

The [Table 13-7] is the result obtained from CRC-16 calculated according to its algorithm using

the query to read parameters PA1_41 to 47 (7 pcs). The last data No.52: C651h will be added to

the end of the frame in order of digits from lower to upper.

Station No.

FC

01h

03h

Address

40h

28h

No. of registers

00h

0Eh

CRC check

51h

C6h

[Table 13-7] calculation examples

No.

Calculations

15 14 13 12 11 10 9

bit

87

6

5

4

3

2

1

0 Shift carry

1

CRC (initial value)

11 1 11 1 1 11 1 1 1 111 1

2

POLY (initial value)

10 1 00 0 0 00 0 0 0 000 1

3

DT[0] (station no.)

00 0 00 0 0 00 0 0 0 000 1

4

CRC = No.1 XOR No.3

11 1 11 1 1 11 1 1 1 111 0

5

Shift CRC by 2 bits to the right

(until shift-carry occurs.)

0

0

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

6

CRC = No.5 XOR No.2

10 0 11 1 1 11 1 1 1 111 0

7 Shift CRC by 2 bits to the right 0 0 1 0 0 1 1 1 1 1 1 1 1 1 1 1

1

8

CRC = No.7 XOR No.2

10 0 00 1 1 11 1 1 1 111 0

9 Shift CRC by 2 bits to the right 0 0 1 0 0 0 0 1 1 1 1 1 1 1 1 1

1

10 CRC = No.9 XOR No.2 1 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0

11

Shift CRC by 2 bits to the right

(Finished with sht =8.)

0

0

1

0

0

0

0

0

0

1

1

1

1

1

1

1

1

12 CRC = No.11 XOR No.2 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0

13

DT[1] (FC)

00 0 00 0 0 00 0 0 0 001 1

14 CRC = No.12 XOR No.13 1 0 0 0 0 0 0 0 0 1 1 1 1 1 0 1

15 Shift CRC by 1 bit to the right 0 1 0 0 0 0 0 0 0 0 1 1 1 1 1 0

1

16 CRC = No.15 XOR No.2 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 1

17 Shift CRC by 1 bit to the right 0 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1

1

18 CRC = No.17 XOR No.2 1 1 0 1 0 0 0 0 0 0 0 1 1 1 1 0

19 Shift CRC by 2 bits to the right 0 0 1 1 0 1 0 0 0 0 0 0 0 1 1 1

1

20 CRC = No.19 XOR No.2 1 0 0 1 0 1 0 0 0 0 0 0 0 1 1 0

21 Shift CRC by 2 bits to the right 0 0 1 0 0 1 0 1 0 0 0 0 0 0 0 1

1

22 CRC = No.21 XOR No.2 1 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0

23 Shift CRC by 2 bits to the right 0 0 1 0 0 0 0 1 0 1 0 0 0 0 0 0

24

DT[2] (Address (H))

00 0 00 0 0 00 1 0 0 000 0

25 CRC = No.23 XOR No.24 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0

26 Shift CRC by 8 bits to the right 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1

27

DT[3] (Address (L))

00 0 00 0 0 00 0 1 0 100 0

28 CRC = No.26 XOR No.27 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1

29 Shift CRC by 1 bit to the right 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0

1

30 CRC = No.29 XOR No.2 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0 1

31 Shift CRC by 1 bit to the right 0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0

1

32 CRC = No.31 XOR No.2 1 1 1 1 0 0 0 0 0 0 0 0 0 0 1 1

33 Shift CRC by 1 bit to the right 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 1

1

34 CRC = No.33 XOR No.2 1 1 0 1 1 0 0 0 0 0 0 0 0 0 0 0

35 Shift CRC by 5 bits to the right 0 0 0 0 0 1 1 0 1 1 0 0 0 0 0 0

36 DT[4] (No. of registers (H)) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

37 CRC = No.35 XOR No.36 0 0 0 0 0 1 1 0 1 1 0 0 0 0 0 0

38 Shift CRC by 7 bits to the right 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 1

1

39 CRC = No.38 XOR No.2 1 0 1 0 0 0 0 0 0 0 0 0 1 1 0 0

40 Shift CRC by 1 bit to the right 0 1 0 1 0 0 0 0 0 0 0 0 0 1 1 0

Modbus RTU Communications 13-29

13