NMEA 0183 Interfacing is a common stumbling block for the do-it-yourselfer. What is it? NMEA0183 is a protocol used to share data among multiple units on your boat. It will allow you to share numeric data from one unit to another. Any proper electronics installation will make use of NMEA0183 when applicable, although sadly its often forgotten.
The purpose of my NMEA0183 explanation below is to give a common sense guide to how it all works. I will use values and scenerios that we have found to work well, explained in a way we have found our customers to comprehend. If you want a technical explanation please visit www.nmea.org
How does it work?
* NMEA0183 works by having one unit as a “talker”, and up to three other units as “listeners”. One unit can talk to up to three others, but can only “listen” from one other unit. Make sense? If not go back and re-read until it does because this is a very important piece of the puzzle.
* Each unit capable of NMEA0183 will have “Transmit” wires and “Receive” wires. Transmit wires are called any number of things in your units installation guide, some more common names are TX, Out, and Transmit. Receive wires are commonly called RX, In and Receive. So each NMEA0183 wire will either be a transmit or receive. Of these Transmit and Receive wires they are further defined by their polarity. So some are + (positive) and some are – (negative). Negative wires might also be called “gound”.
* The data that is sent between units comes in the form of “sentences”. Each bit of data will be sent as a sentence, followed by another sentence, then another very rapidly. For a list of sentences NMEA sentences search google.com for “NMEA0183 Sentences” or try this link.
What connects to what?
* There are some very basic rules to NMEA0183. If you follow these rules we officially certify you as an expert. They are:
o Maintain polarity among wires you connect – so positives connect to positives and negatives connect to negatives.
o A transmit wire always connects to a receive wire. And a receive wire always connects to a transmist wire.
* If you a paying attention and soaking this in then you are done. This is it, go off and interface happily.
* As explained before, one unit can talk to up to three others, but can only listen from one other unit. Lets use an example: If you have a gps, vhf, fishfinder and radar you can make great use of NMEA0183. The data that all four of those units can use is GPS data. The VHF will need it for DSC, the Radar will need it to have the cursor display position of a target, and the Fishfinder can use it for marking waypoints on fish targets. The best way to interface this pile of stuff is to have the GPS display talk, and the FF, Radar, and VHF listen. This will get GPS data to every device. So each transmit wire (if you are paying attention you will know there are two of them, a positive and a negative) on the GPS will be connected to three receive wires of the same polarity (one wire from each unit). Do this for both transmit wires. Ahhh, but