NMEA 2000 explained for sailors

NMEA 2000 is built on CAN bus, the same bus technology used in cars, and runs at 250 kbit/s.

Unlike its predecessor NMEA 0183, every device on the network can both send and receive. Each message carries a Parameter Group Number (PGN) that says what kind of data it holds: position, heading, depth, wind and so on.

All devices hang off one continuous cable, the backbone. Each device connects to it through a T-connector and a short drop cable. You don't chain devices from one to the next, so you can remove one without cutting off the rest.

Each end of the backbone has a terminator, a 120 Ω resistor: exactly two, never more or fewer.

The cable itself has five wires. Two carry power: NET-S (+12 V, red) and NET-C (0 V, black). Two carry the data: NET-H (white) and NET-L (blue), the CAN high and CAN low pair. The fifth is the bare shield.

• Drop cables: at most 6 m each, and at most 78 m for all drops together.
• Backbone: up to 100 m with standard Micro cable, up to 250 m with heavy Mini cable.
• At most 50 devices on one network.
• Micro cable uses round 5-pin M12 plugs, often called Micro-C; heavy Mini cable has a larger connector.

The backbone also carries 12 V power for the devices. It goes in at one point, through a power tee. On a small boat that can be at one end; on a larger network, put the power tee near the middle so the current splits both ways and the voltage drop stays small.

Every device's data sheet gives its current draw as a LEN (Load Equivalency Number). Add them up and compare the total with the limit in your cable and power-tee documentation, and protect the feed with a suitably sized fuse or breaker.

Many network problems come down to the terminators. Switch the network power off and measure the resistance between the two data wires, NET-H and NET-L:

• About 60 Ω: correct. Two 120 Ω terminators in parallel make 60 Ω.
• About 120 Ω: one terminator is missing.
• About 40 Ω: there's one too many, often a device with a built-in terminator.

A typical sailboat network already carries a lot of useful data. The most common, with their PGN numbers:

• Position (129025), and course and speed over ground, COG and SOG (129026)
• Heading (127250)
• Water depth (128267)
• Speed through the water (128259)
• Wind, apparent or true (130306)
• Depending on your instruments: AIS, engine and battery data, and more

Older instruments often still use NMEA 0183: a serial link where one device sends and the others listen, usually at 4800 baud. It's slowly being replaced, but it's still on a lot of boats.

A gateway translates between NMEA 0183 and NMEA 2000, so older instruments can share their data with a 2000 network without replacing everything.

Signal K is an open-source data standard and server for boat data, built on ordinary web technology. It collects data from the systems on board, including NMEA 2000 through a gateway or interface, and passes it on to apps on a tablet or phone, logbooks and dashboards.

A Wi-Fi gateway sits on the backbone and sends the network's data over Wi-Fi to phones, tablets and laptops, often converted to NMEA 0183 so navigation apps can use it. Some models also record everything to a memory card: a data logger.