20.5 NMEA 0183 & NMEA 2000 Data Networks

20.5 NMEA 0183 & NMEA 2000 Data Networks

Quick Answer: NMEA 2000 is the bi-directional, multi-transmitter, multi-receiver network. NMEA 0183 shields are grounded at the talker, unterminated at the listener. On NMEA 2000, one failed device does not interrupt the others. LEN 1 = 50 mA. Devices in one location may use an end-powered network, and 1.5 volts of drop at the last segment satisfies the cabling plan.

Sub-topic 3-N-089 (NMEA Data) is the most modern material in the Marine topic, and it is directly relevant work: a GROL technician installing a DSC VHF must get position data into it, and that is an NMEA job.

The two standards

What data language is bi-directional, multi-transmitter, multi-receiver network? NMEA 2000. The three adjectives in that question are the discriminator. NMEA 0183 is fundamentally one-way: a talker transmits and listeners listen, and a second talker on the same pair simply collides.

Shielding NMEA 0183

How should shielding be grounded on an NMEA 0183 data line? Terminated to ground at the talker and unterminated at the listener.

This is the classic single-point grounding rule, and the reasoning is worth carrying to every shielded run you install. Ground a shield at both ends and you create a ground loop: the two ends of a vessel are rarely at exactly the same potential, so current flows along the shield. That current couples noise straight into the signal pair — the shield stops being a shield and becomes an interference injector.

Grounding at one end only — the talker — gives the shield somewhere to drain the charge it intercepts without providing a return path for loop current.

NMEA 2000 fault tolerance

What might occur in NMEA 2000 network topology if one device in line should fail? There will be no interruption to all other devices.

This is the payoff of the backbone-and-drop architecture. Devices hang off the backbone on short drop cables through tee connectors; they do not sit in the data path. A failed chartplotter, a flooded transducer, an unplugged instrument — the backbone carries on.

Contrast a genuine daisy chain, where the signal passes through each device and any failure severs everything downstream. Two things do still take the whole network down, and they are worth knowing:

1. A missing or failed terminating resistor at either end of the backbone (120 Ω each) — reflections corrupt the bus.
2. A short on the backbone itself, which takes out both data and power.

Load equivalence numbers and power

In an NMEA 2000 device, a load equivalence number (LEN) of 1 is equivalent to how much current consumption? 50 mA.

LEN is a simple budgeting unit: 1 LEN = 50 mA. Every certified device is marked with its LEN, so an installer can add them up instead of hunting through data sheets:

A standard "light" backbone supports a limited total LEN; exceeding it means moving to heavy cable, adding a second power insertion point, or splitting the network with a bridge.

An NMEA 2000 system with devices in a single location may be powered using this method: an end-powered network. When every device is clustered together, feeding power in at one end of the backbone is acceptable. Spread devices along a long backbone and you insert power in the middle instead, halving the worst-case run length and therefore halving the voltage drop.

What voltage drop at the end of the last segment will satisfy NMEA 2000 network cabling plans? 1.5 volts.

That is the budget. Devices are specified to work down to about 9 V; a 12 V nominal supply less 1.5 V of drop still leaves ample margin. Because drop is I × R, the levers are total current (LEN), cable resistance (light versus heavy backbone), and run length — which is exactly why the power-insertion decision above matters.

Why this reaches Element 1

Section 4.6 and section 19.8 both turn on the same practical point: a DSC VHF needs position data, and on a modern vessel that data arrives over NMEA. If the network is mis-wired — shield grounded at both ends, a missing terminator, too much voltage drop at the far end — the DSC radio may show no position, and a distress alert then goes out without the single most valuable field it can carry.