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Calculating and Validating NMEA Checksums » CppBuilder » CMOS_C » The GPRMC Sentence » TurboCpp » BeginnersGuideToCurl » Curl Content Language » QbasicFAQ_Mouse » rs 485 » Euphoria » Taking Out the Garbage: NMEA Checksums

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Taking Out the Garbage: NMEA Checksums

An NMEA checksum is calculated as the XOR of bytes between (but not including) the dollar sign and asterisk. This checksum is then compared with the checksum from the sentence. If the checksums do not match, the sentence is typically discarded. This is okay to do because the GPS devices tend to repeat the same information every few seconds. With the ability to compare checksums, the interpreter is able to throw out any sentence with an invalid checksum.

[C#]

  // Returns True if a sentence's checksum matches the 
  // calculated checksum
  public bool IsValid(string sentence)
  {
    // Compare the characters after the asterisk to the calculation
    return sentence.Substring(sentence.IndexOf("*") + 1) == 
      GetChecksum(sentence);
  }
  // Calculates the checksum for a sentence
  public string GetChecksum(string sentence)
  {
    // Loop through all chars to get a checksum
    int Checksum = 0;
    foreach (char Character in sentence)
    {
      if (Character == '$')
      {
        // Ignore the dollar sign
      }
      else if (Character == '*')
      {
        // Stop processing before the asterisk
        break;
      }
      else
      {
        // Is this the first value for the checksum?
        if (Checksum == 0)
        {
          // Yes. Set the checksum to the value
          Checksum = Convert.ToByte(Character);
        }
        else
        {
          // No. XOR the checksum with this character's value
          Checksum = Checksum ^ Convert.ToByte(Character);
        }
      }
    }
    // Return the checksum formatted as a two-character hexadecimal
    return Checksum.ToString("X2");
  }

[VB.NET]

  ' Returns True if a sentence's checksum matches the calculated checksum
  Public Function IsValid(ByVal sentence As String) As Boolean
    ' Compare the characters after the asterisk to the calculation
    Return sentence.Substring(sentence.IndexOf("*") + 1) = GetChecksum(sentence)
  End Function
  ' Calculates the checksum for a sentence
  Public Function GetChecksum(ByVal sentence As String) As String
    ' Loop through all chars to get a checksum
    Dim Character As Char
    Dim Checksum As Integer
    For Each Character In sentence
      Select Case Character
        Case "$"c
          ' Ignore the dollar sign
        Case "*"c
          ' Stop processing before the asterisk
          Exit For
        Case Else
          ' Is this the first value for the checksum?
          If Checksum = 0 Then
            ' Yes. Set the checksum to the value
            Checksum = Convert.ToByte(Character)
          Else
            ' No. XOR the checksum with this character's value
            Checksum = Checksum Xor Convert.ToByte(Character)
          End If
      End Select
    Next
    ' Return the checksum formatted as a two-character hexadecimal
    Return Checksum.ToString("X2")
  End Function

Written By Jon Person (http://www.gpsdotnet.com)

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