nmea checksum in c# .net cf -

i'm trying write own nmea parser,since need info gps, , don't need interpret messages. problem have nmea message validator gives me wrong checksum. can see i'm droing wrong?

i'm using idea codepedia - calculating , validating nmea sentences.

    // returns true if sentence's checksum matches      // calculated checksum     // calculates checksum sentence     private static bool isvalid(string sentence)     {         if (sentence == "")             return false;         if (sentence.length < 14)             return false;         try         {             string[] words = getwords(sentence);             log.writetolog(words);             int checksum = 0;             string checktocompare = words[words.length - 1];             stringbuilder sb = new stringbuilder();             (int = 1; < (words.length - 2); i++)             {                 sb.append(words[i]);             }             string sentecentoparse = sb.tostring();              foreach (char chary in sentecentoparse)             {                 checksum ^= convert.tobyte(chary);             }               log.writetolog("checksum: " + checksum.tostring("x2"));             log.writetolog("obtained checksum: " + checktocompare);             return string.equals(checksum.tostring("x2"), checktocompare, stringcomparison.ordinalignorecase);         }         catch (exception exc)         {             log.writetolog("exception caused sentence:" + sentence);             log.writetolog("exception error message: " + exc.message);             return false;         }       }      // divides sentence individual words     public static string[] getwords(string sentence)     {         char[] separator = { ',', '*' };         return sentence.split(separator);     } 

run log

10:30:07 23-02-2011 opening port 10:30:08 23-02-2011 opened port 10:30:10 23-02-2011 processing data 10:30:12 23-02-2011 $gpgga 10:30:12 23-02-2011 102957.92 10:30:12 23-02-2011 4104.8569 10:30:12 23-02-2011 n 10:30:12 23-02-2011 00836.4700 10:30:12 23-02-2011 w 10:30:12 23-02-2011 1 10:30:12 23-02-2011 4 10:30:12 23-02-2011 15.100 10:30:12 23-02-2011 157.133 10:30:12 23-02-2011 m 10:30:12 23-02-2011 52.386 10:30:12 23-02-2011 m 10:30:12 23-02-2011 0 10:30:12 23-02-2011 0 10:30:13 23-02-2011 79  10:30:13 23-02-2011 checksum: 6d 10:30:13 23-02-2011 obtained checksum: 79  10:30:13 23-02-2011 invalid sentence 10:30:13 23-02-2011 $gprmc 10:30:13 23-02-2011 102957.92 10:30:13 23-02-2011 10:30:13 23-02-2011 4104.8569 10:30:13 23-02-2011 n 10:30:13 23-02-2011 00836.4700 10:30:13 23-02-2011 w 10:30:13 23-02-2011 0.000 10:30:13 23-02-2011 5.822 10:30:13 23-02-2011 230211 10:30:13 23-02-2011 0 10:30:13 23-02-2011 w 10:30:14 23-02-2011 10:30:14 23-02-2011 2b  10:30:14 23-02-2011 checksum: 4e 10:30:15 23-02-2011 obtained checksum: 2b 10:30:15 23-02-2011 invalid sentence 

if use function page linked:

private static string getchecksum(string sentence) {   //start first item   int checksum= convert.tobyte(sentence[sentence.indexof('$')+1]);   // loop through chars checksum   (int i=sentence.indexof('$')+2 ; i<sentence.indexof('*') ; i++)   {     // no. xor checksum character's value     checksum^=convert.tobyte(sentence[i]);                 }   // return checksum formatted two-character hexadecimal   return checksum.tostring("x2"); } 

and pass in string, $ start , * @ end (before checksum), , commas between words, works.

string s1 = getchecksum("$gpgga,102957.92,4104.8569,n,00836.4700,w,1,4,15.100,157.133,m,52.386,m,0,0*79"); string s2 = getchecksum("$gprmc,102957.92,a,4104.8569,n,00836.4700,w,0.000,5.822,230211,0,w,a*2b"); 

s1 = 79 , s2 = 2b hope.