For example a Garmin sentence would start with PGRM and Magellan would begin with PMGN.
Each sentence begins with a '$' and ends with a carriage return/line feed sequence and can be no longer than 80 characters of visible text (plus the line terminators).
The checksum field consists of a '*' and two hex digits representing an 8 bit exclusive OR of all characters between, but not including, the '$' and '*'. There have been several changes to the standard but for gps use the only ones that are likely to be encountered are 1.5 and 2.0 through 2.3.
These just specify some different sentence configurations which may be peculiar to the needs of a particular device thus the gps may need to be changed to match the devices being interfaced to.
The idea of NMEA is to send a line of data called a sentence that is totally self contained and independent from other sentences.
All proprietary sentences begin with the letter P and are followed with 3 letters that identifies the manufacturer controlling that sentence.All of the standard sentences have a two letter prefix that defines the device that uses that sentence type.(For gps receivers the prefix is GP.) which is followed by a three letter sequence that defines the sentence contents.For official data please contact the NMEA web site.Please see the bottom of this article for the sources of this data.