NMEA 0183 (or NMEA for short) - is a combined electrical and
data specification for communication between marine electronic
devices such as echo sounder, sonars, anemometer (wind speed and
direction), gyrocompass, autopilot, GPS receivers and many other
types of instruments. It has been defined by, and is controlled
by, the U.S.-based National Marine Electronics Association. The
NMEA 0183 standard uses a simple ASCII, serial communications
protocol that defines how data is transmitted in a "sentence"
from one "talker" to one "listener" at a time. Through the use
of intermediate expanders, a talker can have a unidirectional
conversation with multiple listeners, and using multiplexers,
multiple sensors can talk to a single computer port. Third-party
switches are available that can establish a primary and
secondary talker, with automatic failover if the primary fails.
At the application layer, the standard also defines the contents
of each sentence (message) type so that all listeners can parse
messages accurate