convert char to byte representation

[Philipp H. Mohr]

Hello,

I am trying to xor the byte representation of every char in a string with
its predecessor. But I don't know how to convert a char into its byte
representation. This is to calculate the nmea checksum for gps data.

e.g. everything between $ and * needs to be xor:
	$GPGSV,3,1,10,06,79,187,39,30,59,098,40,25,51,287,00,05,25,103,44*
to get the checksum.


Thank you for you help.

Phil