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:
Maintain polarity among wires you connect - so positives connect to positives and negatives connect to negatives.
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 it gets better, the GPS might be able to make use of data from one of the other units. Remember, the GPS is talking to 3 units but it can still listen from another. For our sample setup I think we will want depth to show up on our GPS display. That way when we save waypoints it might also include depth and water temp with the waypoint data. To do this we connect the Transmit wires on the FF to the Receive wires on the GPS. Done.
Some units will have multiple NMEA0183 "Channels" or "Ports". Each port follows the limitations above. So if your unit has two ports it can listen to one other NMEA0183 device on each port for a total of 2.
Do you need two or more devices to talk to one device despite the rules above? If so you need a multiplexer. Multiplexers allow for multiple units to talk to one unit. Think of it as a bottleneck on the highway going from 3 lanes down to one. Only one lane comes out of the multiplexer while multiple lanes can go into it. The multiplexer basically merges all the date just as cars would merge and puts it into your unit as if it was all coming from one other unit, thus behaving according the the guidelines above.
Tools for Interfacing
For our installations we use a number of devices depending on th esimplicity or complexity of what we are interfacing. Here is what we suggest:
3M Scotchlok Connectors - for the simplest of connections. When just connecting one device to another these are quick and easy. They offer a waterproof connecions as the connector is filled with goo.
Blue Sea Terminal Strips - These are our favorite. Why? Because typically with connecting NMEA0183 wires it usually takes a few tries to get it set up the way you want, I don't care how good you are. Terminal strips offer a very robust connection and they are easy to switch wires around. We use flanged spade connectors with them from Ancor. With these you don't need to remove the screw from the terminal strip to remove the wire.
Newmar Junction Boxes - We don't feel these are as robust as Terminal Strips, but they do conceal your NMEA0183 mess in a nice watertight box. They use a Euro Style terminal strip. This strip has holes you stick the wires in then you tighten down a screw, very easy. It is also easy to think a wire is securely connected whe it isn't, so when using this j-box for interfacing your connections will be the first place to check when troubleshooting.
My NMEA0183 Interfacing is not working?
When your units are interfaced but not sharing data we recommedn you troubleshoot in this order"
Check the connections. Give each connection a tug and make sure it is secure.
Check the wire colors, did you connect the pink/white whne you should have connected the pink?
Finally, make sure that NMEA data sharing is turned on for each unit. Some units will have NMEA0183 turned off by default, although in our experience you usually don't need to mess with this.
You've read all the mumbo jumbo above and still can't figure it out. Here are some more common pitfalls we know about, we will be happy to add others to this list of you provide them to us.
Older Gamin units - Older Garmin units like the 3000 series or 498 use the black power wire (ground) as the NMEA 0183 transmit and receive - (negative). This confuses the heck out of people. Every so often we will see other units that do this, if you unit does this just connect the - TX and RX wires from the unit you are interfacing to the sam place your Garmin black wires is connected to.
Furuno - Uhhh. Furuno seems to take what is an already complex form of interfacing and they make it confusing as heck. To this day when I am interfacing a Furuno unit it usually requires a call to them. they don't use the whole TX, RX, +, - thing in their manual. In fact some manuals are missing the directions altogether. Instead of driving yourself crazy just call them and ask the tech to convert their wire colors to the TX+, TX-, RX+, RX- deal that I explained above.