multiplexer, also called combiner, solves the problem by offering an
intermediate storage of sentences. Every talker in the system in
fig.4 is connected to its own NMEA input on the multiplexer. The
multiplexer reads complete sentences from every connected listener
and stores them in a buffer. There is a buffer for every input,
large enough to contain several sentences. Subsequently, the
multiplexer checks every buffer in a round-robin fashion for the
presence of sentences. Each time, one sentence is taken from a
buffer and sent to the NMEA output of the multiplexer.
The diagram on this page shows a
multiplexer in a typical installation, where the NMEA data from four
instruments is combined into one stream. This stream is sent to the
connected computer over an RS-232 or USB interface, to be used for
electronic navigation. Also connected is an autopilot which receives
NMEA data from the instruments or the computer or both, depending on
the configuration of the multiplexer.