An NMEA 0183 communication is based on a transmitting device called a talker and one or more receivers called a listener. The talker sends data on a serial bus (normally RS232) in the form of string sentences in ASCII format, each starting with the character "$", ending with CR+LF and no longer than 82 characters.
After the identifier "$" the sentence has 2 characters indicating the Talker Id, which identifies the device that is providing the data. Then follow 3 characters that identify the type of data and after its values in comma-delimited format. The last value was followed by "!", followed by a two-digit check-sum.
Each judgment has different values and parameters depending on the type of data, for specific information should refer to the National Marine Electronics Association.