We can't promise this article will answer all of these questions in all situations, but we'll get close. Let's start with...

First, most marine electronic devices talk to each other over a pair of wires, using a standard called NMEA 183 (National Marine Electronics Association 183). There is a new NMEA standard, NMEA 2000, and a few manufacturers have developed their own proprietary standards (such as Raymarine's HSB2), but the majority of devices still use NMEA 183. This article only talks about NMEA 183.

When you hook up two NMEA 183 devices, like a GPS and a VHF radio, the communication on one set of wires is ONE WAY ONLY. That is, one device must be a "talker", the other must be a "listener" on that wire pair. So in the case of the GPS-to-VHF connection, the GPS will typically be the talker (sender, transmitter, output), and the radio will be the listener (receiver, input).

In the wire pair, one is the signal wire (+), the other is a signal ground wire (-). So for the talker, you may see the signal (+) wire identified as "NMEA Output (+)" or "NMEA Transmit (+)", and the signal ground (-) wire identified as "NMEA Output (-)" or "NMEA Transmit (-)". 

In some cases, the manufacturer does not provide a signal ground (-) wire for the NMEA line. So instead of seeing two NMEA Transmit wires (+/-), you may only see one wire, the NMEA Transmit (+). This is sometimes referred to as a "single ended" device.

IF YOU DON"T HAVE ANY NMEA OUTPUT WIRES ON A DEVICE - that device can not talk to another device. Sorry! It is a listener only.

For the listener, the same things apply. Typically, you will have two wires: "NMEA Input (+)", and "NMEA Input (-)", but you may also only see one wire, "NMEA Input (+)".

IF YOU DON"T HAVE ANY NMEA INPUT WIRES ON A DEVICE - that device can ONLY talk to another device. It can not accept any input from another NMEA 183 device.

To get a talker connected to a listener:

• If both devices have (+) and (-) wires, simply connect the OUTPUT (or TRANSMIT)  wires of the Talker to the INPUT (or RECEIVE) wires of the listener, (+) to (+), and (-) to (-). 
  
• If both devices only have (+) wires, connect the (+) wires together, and make sure the power grounds (-) for both are connected.
  
• If the talker has only a (+) wire, but the listener has two, connect the (+) wires together, and connect the listener's (-) wire to the power ground of the talker
  
• If the talker has both (+) and (-) wires, but the listener has only a (+), connect the (+) wires together, and leave the talker's (-) wire floating (not connected to anything). Make sure the talker's and listener's power grounds are connected.

If you are connecting multiple NMEA devices together:

• Make sure the talker and all of the listener (+) wires are connected together
  
• For all of the listening devices with signal ground wires (Receive (-)), make sure the signal ground wires are connected together, so that they share a common ground. Some devices also have a "common ground" wire (sometimes identified as "cground"). This wire can be connected to any convenient ground point, as long as ALL of the connected devices also connect to the same grounding circuit.
  
• If some of the listener's have only a Receive (+) wire, make sure the talker and listener's power sources share the same ground (-).
  
• If the talker is single-ended (only a Transmit (+) wire), connect the listeners' Receive (-) wires to the power ground of the talker.
  
• If the talker has both a Transmit (+) and a Transmit (-) wire, but all of the listeners are single-ended (Receive (+) wire only), just leave the talker's Transmit (-)  wire disconnected. DO NOT connect the Transmit (-) wire to the power ground of any device.

In the following two sections ("Basic (1-to-1) Hookups", and "Hooking Up 1-to-Many" ), we give you some examples to work from.

Many people connect the wires of the devices directly together. This certainly works, and if you use appropriate crimps and use heat shrink tubing to protect the connections, it will last quite well. However, it probably makes more sense to use some sort of circuit block or bus bar and ring connectors instead (click on the picture on right for more info). We originally got this solution from Randall Holman (http://www.Xlobsterman.com ). NOTE: Some people solder the wires together, and report good results. However, the American Boat and Yacht Council (ABYC) DOES NOT RECOMMEND soldering connections in marine applications   

If you go this route, it will be a simple matter to disconnect things if you ever have to separate the two devices. Also, it makes connecting multiple listeners to a single talker much easier (see "Hooking Up 1-to-Many")

(See this link: http://www.shipmodul.com/en/connections.html for a great in-depth discussion of the mechanics of connecting NMEA 183 devices. This thread was mentioned on www.thehulltruth.com, and was pointed out to us by reader Tom Sheehy)

YES! So far, all we've done is conect the two devices electrically. It turns out that you also have to make sure the two devices talk the same language, at the same speed, and that they are passing the right information. This is discussed more fully in the last section ("It Still Doesn't Work!"), but basically, you need to make sure the talker and listener are set up to use the same version the NMEA protocol, that the talker is generating the information that the listener needs in order to function, and that the communications settings (BAUD rate, parity, data bits, stop bit) match.