Attached is a method of calculating the checksum for a string of characters.
I was tasked with simulating the data stream from a GPS receiver.
Specifically the GPGGA string, which is widely used for reporting position fix data.
Here's one definition of the NMEA message structure for GGA (Global Positioning System Fix Data).
http://www.gpsinformation.org/dale/nmea.htm
The example given here is for the string GPGGA which is used by certain manufacturers.
GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,
The VI adds the leading $ sign and appends the checksum, which starts with *.
$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47
This VI may easily be adapted to meet other requirements.
