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Calculating the checksum of an ASTM document

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Environment: C#, VB, C++ .NET

Using the ASTM protocol to communicate with a laboratory instrument can be an interesting project, and one of the finer points of ASTM is calculating the checksum on transmitted and received documents. Each segment (row or message) of data includes a checksum when either the laboratory instrument or the LIS (Laboratory Information System) transmits the document. The benefits are obvious in a mission (life) critical application to ensure what was sent is exactly what is received.

Following is an example of a segment of ASTM data transmitted from a laboratory instrument:

5R|2|^^^1.0000+950+1.0|15|||^5^||V||34001637|20080516153540|20080516153602|34001637 3D

The segment starts with a STX character (0x02) and ends with a ETX character (0x03). All of the characters AFTER the STX and up to and including the ETX character is the data used to calculate the checksum for this message. The following C# method calculates the checksum by adding the ASCII values of each character in the string and then performing a mod 256 calculation on the total value for all characters in the ASTM message. The checksum is then appended to the end of the message, and in this case the value is 3D. Remember that the value must be converted to it's HEX equivalent before adding it to the message.

To use this method you must include the following classes in the project:

public class T
{
    public const byte ENQ = 5;
    public const byte ACK = 6;
    public const byte NAK = 21;
    public const byte EOT = 4;
    public const byte ETX = 3;
    public const byte ETB = 23;
    public const byte STX = 2;
    public const byte NEWLINE = 10;
    public static byte[] ACK_BUFF = { ACK };
    public static byte[] ENQ_BUFF = { ENQ };
    public static byte[] NAK_BUFF = { NAK };
    public static byte[] EOT_BUFF = { EOT };
}

public class CheckSum
{
    public string value = "";
    public bool valid = false;
}

The class T above defines most of the control characters required to communicate in ASTM protocol. The CheckSum class provides the return object containing the calculated value and a valid indicator.

/******************************************************************************************
*  GetCheckSumValue()  Reads up the actual checksum from this row of data
* 
*  PARAMS  [string] row - row of data to calculate
*  PARAMS  [bool] includesChecksum - true if last 2 bytes are existing checksum digits
*  RETURNS [CheckSum] object containing the results
* ****************************************************************************************/
public CheckSum GetCheckSumValue(string row, bool includesChecksum)
{
    CheckSum result = new CheckSum();

    int lval = 0;
    //if checksum is included don't include them in measurement
    int len = includesChecksum ? row.Length - 2 : row.Length;

    for (int idx = 0; idx < len; idx++)
    {
        switch (Convert.ToInt32(row[idx]))
        {
            case T.STX:
                break;
            default:
                lval += Convert.ToInt32(row[idx]);
                break;
        }
    }

    result.valid = lval > 0;

    //hex value mod 256 is checksum, return as hex value
    result.value = Convert.ToString(lval % 256, 16).ToUpper();
    if (result.value.Length == 1)
        result.value = "0" + result.value;

    return result;
}