CalcCheckSum - method of the PmBuffer object

Description:

Returns the data block checksum (hash).

Syntax:

Variant CalcCheckSum(Long nOffset, Long nSize, String sType)

Parameters:

nOffset	(Long) Specifies the position of read value in the data block. The read value must be inside the data block. The item is read either whole or not read at all. >= 0 - Index (in bytes, zero-based index) in the data block. -4 - The internal automatic position is used (see the AutoOffset property). It points behind the last read or written value.
nSize	(Long) The size of read data block. The -1 value (default) means that the data block is read from the defined starting position to the end.
sType	(String) Check sum type. "crc1" - CRC-16-CCITT, 2 bytes Cyclical Redundancy Check with CCITT polynomial [0x1021 = x16+x12+x5+1]. Cyclical Redundancy Check of the array value bytes. The method returns a value of the Integer type (i.e. 2 bytes value). This checksum is used for example in communication by the protocol PmSBUS. "crc2" - CRC-16-IBM, 2 bytes Cyclical Redundancy Check with CCITT polynomial [0xA001 = x16+x15+x2+1]. Cyclical Redundancy Check of the array value bytes. The method returns a value of the Integer type (i.e. 2 bytes value). This checksum is used for example in communication by the protocol Modbus. "crc3" - CRC-16-IBM, 2 bytes Cyclical Redundancy Check with CCITT polynomial [0x8005 = x16+x15+x2+1]. Cyclical Redundancy Check of the array value bytes. The method returns a value of the Integer type (i.e. 2 bytes value). This checksum is used for example in communication by the protocol PmABradleyDF1. "crc4" - CRC-16, 2 bytes Cyclical Redundancy Check with CCITT polynomial [x16+x13+x12+x10+x8+x6+x5+x2+1]. Cyclical Redundancy Check of the array value bytes. The method returns a value of the Integer type (i.e. 2 bytes value). This checksum is used for example in communication by the protocol DNP3. "sum1" - Byte values sum. The method returns a value of the Byte type (sum modulo 256). This checksum is used for example in communication by the protocol PmIEC8705, PmMBus, PmAdam, PmTeco, PmInmat66. "sum2" - Byte values sum. The method returns a value of the Integer type (i.e. 2 bytes value) (sum modulo 65536). "xor1" - Bit XOR of all byte values. The method returns a value of the Byte type. This checksum is used for example in communication by the protocol Pm3964. "md5" - MD5 hash of all byte values. The MD5 hash size is 16 bytes, making the resulting HexaString 32 characters long. The method returns a value of the String type. "sha256" - SHA-256 hash of all byte values. The SHA-256 hash size is 32 bytes, making the resulting HexaString 64 characters long. The method returns a value of the String type. "sha512" - SHA-512 hash of all byte values. The SHA-512 hash size is 64 bytes, making the resulting HexaString 128 characters long. The method returns a value of the String type.

Note:

This method is not functional in Web panels.

Caution: The internal automatic position (see the AutoOffset property) IS NOT CHANGED by this operation.

Example1:

Creating a data block by continuous writing of 4, 2 and 1-byte integer value (7 bytes in total) and the detecting the check sum "crc1" of the whole data block.

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var oBuf = Pm.CreatePmBuffer();

oBuf.SetInt32(-2, 0x10203040);

oBuf.SetInt16(-2, 0x5060);

oBuf.SetUint8(-2, 0x70);

var n = oBuf.CalcCheckSum(0, -1, "crc1");

Example2:

Creating a data block with a string in utf-8 encoding and then detecting the MD5 or SHA-256 hash in the form of HexaString of the whole data block.

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var sMd5, sSha256;

var oBuf = Pm.CreatePmBuffer();

oBuf.SetStringVar(-2, "Test string", 1, 1);

sMd5 = oBuf.CalcCheckSum(0, -1, "md5");

sSha256 = oBuf.CalcCheckSum(0, -1, "sha256");