The Fletcher checksum is a pretty good way to ensure data integrity….

John Fletcher on Computing at Livermore

Simple checkup calculations have drawbacks, but the fletcher checksum algorithm does a pretty good job.  Wikipedia has a good discussion of the benefits and drawbacks.

The following runs on an Arduino.  Add the following function to your code, call it with a string, and it will output the fletcher-16 checksum.

*
 * generates a Fletcher 16 bit checksum, 
 * see https://en.wikipedia.org/wiki/Fletcher%27s_checksum
 * this function contains an example to extract ascii values 
 * from a String object
 * 
 * Returns a string of the form: "SUM1,SUM2," that is 
 * meant to be prepended
 * to a data string before writing the sting to the com port
 * 
 * The receiving system is expected to decode the string and verify the
 * checksum is correct for the data string (see the function checkSumTest(string)
 * 
 */
//*********************************** generateCheckSum()
String generateCheckSum(String dataStr) {
 //Serial.println(dataStr);
 int dataStrLen = dataStr.length()+1;
 char strArray[dataStrLen];
 dataStr.toCharArray(strArray,dataStrLen); 
 byte sum1 = 0;
 byte sum2 =0; 
 for (int i=0; i<dataStrLen-1; i++) {
 int asciiVal = strArray[i];
 sum1 = (sum1 + asciiVal) % 255;
 sum2 = (sum1 + sum2) % 255;
 if (showCheckSum) {
 Serial.print("SUBD");
 Serial.print(" ");
 Serial.print(strArray[i]);
 Serial.print(" ");
 Serial.print(asciiVal);
 Serial.print(" ");
 Serial.print(sum1);
 Serial.print(" ");
 Serial.println(sum2);
 
 }
 }