Connecting a Chart Plotter, VHF, AIS Receiver and Tiller Pilot using the NMEA 0183 protocol

NOTE: An updated version of this posting that includes an AIS transceiver is now available here.

Ripple's complement of communication and navigational electronics now includes:

• Standard Horizon CP390i chart plotter 
• Digital Yacht AIS receiver  
• Raymarine ST2000  tiller pilot 
• Standard Horizon VHF/DSC radio

These devices can all be networked, so installing them includes making decisions about whether and how to connect them.  As with most everything on a boat, there are tradeoffs.  The benefits of additional functionality are always at war with the unanticipated dangers of creeping elegance and the instability that arises from proliferating connections (natural failure points).

My four devices, like most modern electronics, can talk to one another using the well-established NMEA 0183 protocol.  NMEA 0183 falls short of a full-blown network, but it meets the need for point-to-point connections such as are appropriate to my configuration..  The NMEA 2000 protocol provides a full network solution that may be more reliable, and certainly more suitable when many devices are connected, but the cabling is far more costly without providing much additional benefit in my situation.

NMEA 0183 is a fairly simple protocol, explained at some length here. Each device is either a talker or a listener.  Some devices have both talker and listener channels. The communication possible among NMEA 0183 devices is limited to a defined set of sentences that have a registered meaning. Talkers can broadcast messages to as many as 4 other devices, while listeners are limited to listening to only one talker.  A listener that doesn't understand a sentence just ignores it.

A communication channel is established via a simple point-to-point wiring of two wires for each channel, to provide a circuit.  So, connections at devices are labeled as either OUT (+), OUT(-), IN(+), or IN(-).  IN and OUT refer to the direction of the communication (whether a device is a talker or listener).  (+) and (-) indicate which connection is the signal and which is for grounding the circuit. Notice that on the accompanying diagram, all (-) connections are connected to a common ground on the chart plotter, simplifying the wiring somewhat.

For my configuration, the following signal channels exist:

• Connection of a DSC VHF radio to the chart plotter enables display of DSC distress calls on the chart plotter. The VHF is the talker, the chart plotter the listener.
• The AIS receiver provides information to the chart plotter to display the location and associated data for all vessels transmitting AIS data. The AIS receiver does the talking, and the chart plotter the listening.
• Connecting the tiller pilot to the chart plotter should allow the tiller pilot to accept instructions from the chart plotter for route-following.  The Chart plotter is the talker and the tiller pilot the listener.
• The chart plotter also sends GPS sentences to the VHF, but the VHF that I have already has GPS capability.  It is unclear to me which, if either, has precedence, or if there is ever likely to be conflict between them.

Installation instructions for all of the devices to be connected are necessary (though, not necessarily sufficient).  While the NMEA specification defines wire colors for various connections, manufacturers do not necessarily abide by these recommendations: make no assumptions! I found the instructions in the Standard Horizon manuals to be confusing and ambiguous (and I suspect, incorrect in the case of the VHF manual).  Even connecting two of their own devices proved confusing.  My email request for assistance was answered within 12 hours, however.

As with any job, having the correct tools is essential.   I tried to use an inexpensive crimper to attach terminal lugs to the wires, and my success rate was about 50%.  Getting the crimping right is essential, and being able to strip the wires effectively is as well.  The 22 or 23 gauge wires are delicate but their insulation is very tough, making them hard to strip without damaging the conductor. If you don't have much experience with this, do some research on the tool options, and talk with someone whose experience you trust.  The security of your terminal connections may be the single most important factor to assure a reliable installation.

The Chart plotter comes with a cable with a connector on one end (that plugs into the device) and bare wires on the other.  I terminated each with crimped wire connectors to connect with a terminal block I installed near the chart plotter.  I situated a second terminal block near the radio equipement and connected the two with 16 gauge cabling (much larger than required, but inexpensive and far easier to crimp reliably than the small stuff).

I made mistakes that have increased the cost and complexity of my task. I bought a VHF Radio last season before I had decided to acquire an AIS receiver.  Standard Horizon makes one with an AIS receiver built in, which would simplify installation (and increase reliability). I missed that boat.  Instead, I found an inexpensive AIS receiver and ordered it online, not anticipating the problem of the AIS receiver and the radio needing to use the same antenna.  So, now I realize that i need an antenna splitter as well: another device to purchase and install -- more cabling, clutter, and connections (failure points) to worry about.  Poor planning on my part.  So it goes.  If you are interested in taking advantage of the information that an AIS receiver provides, look for one with a built-in antenna splitter, or get a VHF with AIS as part of the device.

The whole installation works fine. Having the AIS information on the chart plotter will afford an additional measure of security along the inside passage, especially in limited visibility with traffic congestion.  Given that I don't have radar, I am pleased to have this capability.