Calculate16bitCRC (FUN)

Epec Programming and Libraries Manual

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Supported platforms: CODESYS 3.5, CODESYS 3.5 SAFETY

Calculate16bitCRC (FUN)

Description

This function calculates a 16-bit CCITT CRC according to given seed and polynomial.

The non-endianness-aware calculation algorithm is following:

Set seed value to register (16-bit)
XOR BYTE data with register’s MSB
While (bits left in BYTE) do

Shift register towards MSB by 1
If register MSb was 1 before shifting, XOR register with polynomial

If BYTEs left, get next and go to line 2
The register now contains the remainder.

This function is not typically needed by programmer, but is used by higher level checksum functions.

This function is NOT endianness-aware. Input data that consists of variables longer than a BYTE has to be converted to big-endian format before usage.

This also means that ARRAY and STRUCT members have to be converted to big-endian, and then calculated one by one.

Inputs

Input variable name	Data type	Default value	Range	Description
i_pData	POINTER TO BYTE	-	≠ 0	Pointer to data of which checksum is to be calculated.
i_Length	UDINT	-	> 0	Length of the data.
i_Seed	WORD	0	-	Checksum seed value (i.e. the CRC of previous data if several data has common CRC).
i_Polynomial	WORD	1021h	-	Polynomial value for calculation.

Outputs

Output variable name	Data type	Range	Description
o_OutputValid	BOOL		Calculated value is valid.
o_ParameterError	BOOL		Error in input parameters.

Return value

Data type	Description
WORD	Calculated checksum.

Diagnostics

Conditions	Return value	o_ValueValid	o_ParameterError
i_pData = 0 OR i_Length = 0	0	FALSE	TRUE
i_pData ok AND i_Length > 0	Calculated checksum	TRUE	FALSE

Example code

Definitions:

table: ARRAY [0..10] OF DWORD;

crc: WORD;

i: INT;

Code:

crc := WORD#0;

FOR i := 0 TO 10 DO

(* Stop if error *)

IF NOT parametererror THEN

(* Convert pointed data to big-endian *)

tempDWORD := BigEndianDWORD(i_Data := table[i]);

crc := Calculate16bitCRC(

i_pData := ADR(tempDWORD),

i_Length := LibConstants.G_SIZE_OF_U32,

i_Seed := crc,

i_Polynomial := 16#1021,

o_ParameterError => parametererror,

o_OutputValid => outputvalid

);

END_IF

END_FOR