National Marine Electronics Association (NMEA) 0183 is a combined
electrical and data specification for communication between marine
electronic devices, such as echo sounders, sonars, anemometers (wind
speed and direction), gyrocompasses, autopilots, GPS receivers, and
many other types of instruments. It has been defined by, and is
controlled by, the US-based National Marine Electronics Association.
Although it is a marine-electronics protocol, it is widely used in
hand-held and mobile GPS displays and navigation systems.
The NMEA 0183 standard uses an ASCII serial communications protocol
that defines how data is transmitted in a "sentence" from one "talker"
to one "listener" at a time. Communications is typically at 4800 baud
for GPS devices. Through the use of intermediate expanders, a "talker"
can have a unidirectional conversation with multiple "listeners." 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.
Many GPS receivers have NMEA compatible serial ports on them. The Lowrance 540C and the Garmin 60C are two examples. The M7 GX
speaks the NMEA protocol to these GPSs in order to display numbered
icons on their screens at the location it receives from other GPSs.