Two terminators, one drop per device. That's it.

§ 01 · What NMEA 2000 actually is

NMEA 2000 is a CAN-bus protocol — the same physical layer that runs in a heavy truck's drive train, derived from ISO 11898, and standardized for marine electronics by the National Marine Electronics Association.^[1] It is differential, two-wire, 250 kbit/s, with a separate 12 V supply pair, all bundled into a 5-conductor cable. The protocol works. The cabling is what gets boats in trouble.

§ 02 · The four cabling rules that matter

1. Single backbone, two terminators. The backbone is one cable run, with a 120 Ω terminator plug at each end. Measured across the bus with the power off and any device unpowered: 60 Ω. Anything else is wrong.
2. One drop per device, ≤ 6 m. Drops do not daisy-chain. A drop longer than 6 meters introduces reflections that crash the bus.
3. Backbone ≤ 100 m for mid-cable. For micro-cable (the smaller, more flexible variant Garmin sells), the limit drops to 80 m.
4. LEN budget ≤ 51. Each device has a Load Equivalent Number on its nameplate. Summed across the bus, they cannot exceed 51 without an external power injector. Most boats are well under, but a multi-display bridge with a few transducers is closer than you'd think.

§ 03 · Mixing brands

NMEA 2000 is brand-agnostic by design. We mix Garmin, Furuno, Simrad, Raymarine, and Maretron daily. Where the marketing literature occasionally suggests "use our cabling for our system," what they mean is "use our connector at the device." The backbone in between can be any NMEA-2000 certified cable; we standardize on Maretron mid-cable for steel hulls (heavier, more durable) and Garmin GCV for fiberglass.

The one place we have seen brand-specific issues is in proprietary PGNs — a parameter group number from one brand that another brand can't decode. The solution is rarely cabling; it's usually that the captain wanted a Furuno-only PGN on a Garmin display, and the right answer is a chart-plotter from the same brand for that data type.

§ 04 · Commissioning

The bus is energized with a Maretron N2KMeter or an Actisense NGT-1 in promiscuous mode. We capture every PGN traffic for ten minutes. We verify each device shows up with a unique address and a correct LEN. We measure the bus voltage at the most distant tee — should be 12.0–13.5 V; below 11.5 V means power-injection somewhere is needed.

Every commissioning ends with a printed wiring map — the bus, every tee, every drop, every device, every NMEA address — sleeved at the helm.

§ 05 · Cost & lead time

New backbone (5–8 devices, mid-size boat): $1.4k–$2.4k, one to two working days. Diagnosis-and-repair on an existing broken bus: $720–$1,400 plus parts, typically half a day to a day. Full electronics-suite integration on a refit: quoted as part of P-08.

Cross-references: P-02 (refrigeration) when hold-temp data is going onto the bus, P-01 (rewires) when the bus is going in clean as part of a rewire, and the August 2025 bulletin for the long-form on backbone discipline.