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Bluetooth Multiplexers
Miniplex 41BT

Miniplex 42BT











Wireless NMEA Multiplexers

With the NMEA-0183 protocol, information is passed in sentences that are made up of readable characters. The contents of a sentence is well defined by the standard and always starts with a '$' or a '!' character and always ends with a special code, called a LF (Line Feed). Thus, a listener always knows when and sentence starts and ends. The NMEA standard also specifies that a talker may send one ore more sentences any time it wishes, but preferably not more that once per second. An exception to this rule are gyro- and fluxgate compasses, which often transmit 10 sentences per second or more.

The NMEA standard specifies that a talker should have enough driving capability to drive up to four listeners. This means that you should be able to connect up to four instruments that receive data from one other instrument. This is very easy to achieve, just like one person telling a story to an audience of up to four people. The only requirement for the talker is to talk loud enough.

The situation gets complicated when several talkers have to send data to one listener. A typical example is where a GPS and a wind meter have to send data to an autopilot. Computer navigation is another example where several talkers (sailing instruments) must talk to one listener (the computer). The NMEA standard has no provision for these situations, so without special equipment, this is impossible. The outputs of the talkers will effectively short-circuit each other and the sentences they transmit will be corrupted since any talker can start sending at any time. The result will be like four persons telling a different story to one listener at the same time.

An NMEA multiplexer, also called combiner, solves the problem by offering an intermediate storage of sentences. Every talker in the system in 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.


Shipmodule NMEA Multiplexer
Performance Yacht Systems
919 Bay Ridge Road
Annapolis, MD  21403

Toll free: 1-877-379-2248
Local: (410) 267-8681


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