NMEA 0183 and NMEA 2000 networking

Networking

The wiring that lets your electronics share data: GPS position to the VHF and autopilot, depth and wind to the plotter, engine data to displays. Older gear usually talks NMEA 0183, one talker wired to its listeners. Newer gear uses NMEA 2000, a plug-together network on one backbone cable.

How it works

NMEA 0183 is a simple serial link: one device talks on a pair of wires and one or more others listen, usually at 4,800 baud (AIS uses 38,400). NMEA 2000 is a shared network built on the CAN bus: a single backbone cable with a 120-ohm terminator at each end, T-connectors, and short drop cables to each device. The backbone also carries 12-volt power for the devices' network connections. Gateways translate between the two.

Using it

On NMEA 0183, wire a talker's OUT (transmit) wires to a listener's IN (receive) wires. A listener input takes only one talker; to combine several talkers, use a multiplexer. One maker's plotter allows one device on its input and up to three in parallel on its output.
Set both ends of an NMEA 0183 link to the same speed: normally 4,800 baud, or 38,400 for AIS.
On NMEA 2000, keep the backbone one continuous line with no branches or loops, a terminator at each end and nowhere else, and each device on its own T-connector and drop cable. Daisy-chaining devices isn't allowed.
Power an NMEA 2000 backbone from one fused supply (or approved isolated supplies, never both). On a longer network, feeding power in near the middle splits the load between the two halves.
To mix old and new gear, use a certified gateway or converter: for example, a unit that reads NMEA 2000 GPS data and sends NMEA 0183 position to an older DSC radio. Some makers' own network cables (such as SeaTalkNG) connect to NMEA 2000 with adapter cables.
After adding a device, check it appears in the plotter's network or device list and that the data you expect is arriving.

Rules & requirements

NMEA 2000 layout limits, as given by two manufacturers following the NMEA standard: no more than 50 devices; no single drop longer than 6 m (20 ft); all drops together no more than 78 m (256 ft); and, with the common thin 'Micro' cable, no more than 100 m (328 ft) between any two points.
NMEA 2000 power (NMEA): device network interfaces must work from 9 to 16 volts; each device's draw is marked as a Load Equivalency Number (LEN), where 1 LEN is 50 mA, and a device may draw at most 1 amp (20 LEN) from the backbone.
NMEA 2000 terminators (NMEA): exactly two, one at each end of the backbone cable itself, not built into a device or placed on a drop.
These are industry standards from the National Marine Electronics Association, not government regulations. On any vessel required to carry AIS, the Coast Guard accepts installation to NMEA 0400 as 'properly installed' (33 CFR 164.46).

Looking after it

Make sure NMEA 2000 connectors are fully screwed together; loose connections are a common fault. Check them for corrosion when you're in the area.
On NMEA 0183, one manufacturer calls for 28 AWG shielded twisted-pair wire on long runs, soldered joints sealed with heat-shrink, and never connecting data wires to power ground.
When adding NMEA 2000 devices, add up their LEN numbers and check the voltage at the far end of the backbone is still at least 9 volts.
Keep device firmware up to date; makers fix network compatibility problems through updates.

Troubleshooting

NMEA 2000 devices drop off, freeze or show dashes

With the network powered down, measure resistance between the two data pins (NET-H and NET-L). About 60 ohms is right; 120 ohms means a terminator is missing; about 40 ohms means there are three.
With it powered up, check voltage between NET-S and NET-C at the far ends of the backbone; it should be at least 9 volts.
Check every connector is tight and dry.
Check for too-long drops, a branched backbone, or a device plugged in without its own T.
If it persists, split the network in half by moving a terminator, see which half still has the fault, and repeat until you find the bad device or cable.

NMEA 0183 data not getting through (for example, no GPS position at the VHF)

Check the talker's OUT wires go to the listener's IN wires, with polarity matched.
Check the baud rates match on both ends.
Check the talker is set to send the sentences the listener needs (the listener's manual will list them).
Make sure only one talker is connected to each listener input.
Check the devices share a common power ground, as one maker requires.

Buying a boat that has one

Expect a mix: many boats from 1975-2010 have NMEA 0183 wiring, a maker's proprietary network from the 1990s-2000s, and newer NMEA 2000 gear added later. Ask for or draw a wiring map.
Look for extra or missing terminators, drops longer than 6 m, and home-made splices on the backbone.
Check older proprietary instruments can still join the network through the maker's adapter or a gateway before planning an upgrade.

Sources disagree on rules: maximum NMEA 2000 backbone length — left out rather than guessed. Check your local authority.

Sources (9)

web.nmea.org · www8.garmin.com · www8.garmin.com · maretron.com · actisense.com · shipmodul.com · raymarine.com · navcen.uscg.gov · navcen.uscg.gov

General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.