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LRI2K

CRC (Informative)

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Appendix B

CRC (Informative)

B.1

CRC error detection method

The cyclic redundancy check (CRC) is calculated on all data contained in a message, from

the start of the Flags through to the end of Data. The CRC is used from VCD to LRI2K and

from LRI2K to VCD.

To add extra protection against shifting errors, a further transformation on the calculated

CRC is made. The one’s complement of the calculated CRC is the value attached to the

message for transmission.

To check received messages the 2 CRC bytes are often also included in the re-calculation,

for ease of use. In this case, the expected value for the generated CRC is the residue

F0B8h.

B.2

CRC calculation example

This example in C language illustrates one method of calculating the CRC on a given set of

bytes comprising a message.

C-Example to calculate or check the CRC16 according to ISO/IEC 13239

#define

POLYNOMIAL8408h//

x^16 + x^12 + x^5 + 1

#define

PRESET_VALUEFFFFh

#define

CHECK_VALUEF0B8h

#define

NUMBER_OF_BYTES4// Example: 4 data bytes

#define

CALC_CRC1

#define

CHECK_CRC0

void main()

{

unsigned int

current_crc_value;

unsigned char array_of_databytes[NUMBER_OF_BYTES + 2] = {1, 2, 3,

4, 91h, 39h};

int

number_of_databytes = NUMBER_OF_BYTES;

int

calculate_or_check_crc;

int

i, j;

calculate_or_check_crc = CALC_CRC;

// calculate_or_check_crc = CHECK_CRC;// This could be an other

example

if (calculate_or_check_crc == CALC_CRC)

{

number_of_databytes = NUMBER_OF_BYTES;

Table 97.

CRC definition

CRC type

Length

Polynomial

Direction

Preset

Residue

ISO/IEC 13239

16 bits

X16 + X12 + X5 + 1 = 8408h

Backward

FFFFh

F0B8h

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