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LRI512

APPENDIX B

The CRC Error Detection Method

The Cyclic Redundancy Check (CRC) is calculat-

ed on all data contained in a message, from the

start of the Flags through to the end of Data. This

CRC is used from the VCD to the LRI512, and

from the LRI512 to the VCD.

Table 54. CRC Definition

To add extra protection against shift errors, a fur-

ther transformation on the calculated CRC is

made. The One’s Complement of the calculated

CRC is the value attached to the message for

transmission.

For checking of received messages the two CRC

bytes are often also included in the re-calculation,

for ease of use. In this case, given the expected

value for the generated CRC is the residue of

F0B8h

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

POLYNOMIAL0x8408//

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

#define

PRESET_VALUE0xFFFF

#define

CHECK_VALUE0xF0B8

#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, 0x91, 0x39};

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;

}

else

// check CRC

{

number_of_databytes = NUMBER_OF_BYTES + 2;

}

current_crc_value = PRESET_VALUE;

for (i = 0; i < number_of_databytes; i++)

{

current_crc_value = current_crc_value ^ ((unsigned int)array_of_databytes[i]);

for (j = 0; j < 8; j++)

{

if (current_crc_value & 0x0001)

{

CRC Definition

CRC Type

Length

Polynomial

Direction

Preset

Residue

ISO/IEC 13239

16 bits

X16 + X12 + X5 + 1

= Ox8408

Backward

0xFFFF

0xF0B8

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