Calculate checksum for clinical instrument

Question

I need to calculate the Checksum for an analytical laboratory instrument. I have this example:

I am using the method taken from this post: Calculate checksum for Laboratory Information System (LIS) frames

This is the code from the example that calculates the checksum:

    private string CalculateChecksum(string dataToCalculate)
    {
        byte[] byteToCalculate = Encoding.ASCII.GetBytes(dataToCalculate);
        int checksum = 0;
        foreach (byte chData in byteToCalculate)
        {
            checksum += chData;
        }
        checksum &= 0xff;
        return checksum.ToString("X2");
    }

And this is the code I used to do the image test:

        string cStringStx = ((char)02).ToString();
        string cStringEtx = ((char)03).ToString();

        string cStx = CalculateChecksum(cStringStx);//Result: 02
        string c1 = CalculateChecksum("1");//Result: 31
        string cT = CalculateChecksum("T");//Result: 54
        string cE = CalculateChecksum("e");//Result: 65
        string cS = CalculateChecksum("s");//Result: 73
        string cT2 = CalculateChecksum("t");//Result: 74
        string cEtx = CalculateChecksum(cStringEtx);//Result: 03

        int nSum = Convert.ToInt16(cStx) +
                  Convert.ToInt16(c1) +
                  Convert.ToInt16(cT) +
                  Convert.ToInt16(cE) +
                  Convert.ToInt16(cS) +
                  Convert.ToInt16(cT2) +
                  Convert.ToInt16(cEtx);//Result: 302

        string cTot = CalculateChecksum(cStx + c1 + cT + cE + cS + cT2 + cEtx);//Result: D2

The checksum of the individual strings is correct, but the sum is not. cTot should be D4, instead it is D2. What am I doing wrong?

Answer 1

Well, it seems that you want to skip the initial [STX] character (\u0002) while summing all the others. You can do it with a help of Linq:

  using System.Linq;

  ...

  private static int CheckSum(string value) { 
    return value == null 
      ? 0
      : value.SkipWhile(c => c == 2).Sum(c => c) & 0xFF; 
  }

Demo:

 string test = "\u00021Test\u0003";

 int sum = CheckSum(test);

 Console.Write(sum.ToString("X2"));

Outcome:

 D4