CHAPTER 13 RS-485 COMMUNICATIONS

13

6. CRC-16

(1) Outline of CRC

CRC (Cyclic Redundancy Check) is a system to check if communications data are correct.

In the CRC calculation, data expressed as a polynomial are divided by a generating polynomial,

and the residue is used as CRC data.

Modbus RTU uses the CRC-16 which performs calculation using X16 + X15 + X2 + 1 as the

generating polynomial.

(2) CRC-16 calculation algorithm

The algorism for calculating CRC-16 on the data (N bytes) from the station number field to the

information field is as follows:

START

Default settings

CRC = FFFFh

POLY = A001h

dtn = 0

sft = 0

CRC

POLY

dtn

sft

DT[dtn]

N

XOR

･･･ Calculated value of CRC-16

･･･ Generating polynomial

･･･ Data counter

･･･ Shift counter

･･･ Nth data (one byte)

･･･ Number of data bytes

･･･ exclusive OR

CRC = CRC XOR DT[dtn]

Shift CRC to the right by one bit

DT[0] is station number, DT[1] is FC, and

DT[2]-DT[N-1] are data in the information

field.

Shift carry available?

No

Yes

CRC = CRC XOR POLY

sft = sft + 1

Yes

sft ＜ 8

No

dtn = dtn + 1

Yes

dtn ＜ N ?

No

END

13-28 Modbus RTU Communications