Both networks use the same M12 connectors, and that is precisely the trap. They may only be connected via a bridge that translates the data and galvanically isolates the networks. Direct connection is not permitted — even if it fits mechanically.
Why a bridge is necessary
NMEA 2000 and the P-Bus are two separate networks with different tasks. One transports navigation and engine data between plotters, sensors, and instruments, while the other transports the power and switching data of the on-board electrical system. They share the physical basis — CAN on twisted two-wire lines with DeviceNet connectors — but not the message formats.
In addition, there is the security level. The on-board electrical system bus system is deliberately not NMEA 2000 certified. The bridge is therefore not only a translator, but also the defined boundary between the two worlds: an error on one side does not shut down the other.
WarningPlugging a cable from the on-board electrical system bus directly into the NMEA 2000 backbone is impermissible. The connectors are identical; the networks are not.
What is transmitted
The bridge works in one direction: it reads the on-board electrical system data and makes it available as standardized NMEA 2000 messages. On the plotter, they then appear like the data of any other certified device — without additional software, in the on-board instrument pages of the plotter.
| PGN | Content |
|---|---|
| 127501 | Status of switched outputs |
| 127505 | Tank levels |
| 127506 | Detailed DC status: charge level, remaining runtime |
| 127508 | Battery status: voltage, current, temperature |
| 127744 | AC, power, and current |
| 127747 | AC, additional measurements |
| 127750 | AC, status information |
Which of these messages actually appear depends on which components are present in the on-board electrical system. Without a tank interface, there are no tank levels to transmit.
What the bridge does not do: It does not control in the opposite direction. Consumers cannot be switched from the plotter via it. For display and control, this is sufficient in most cases; if you want to operate, you do so on the system monitor or via the app.
Installation and power supply
The bridge has two connections — one to the on-board electrical system bus, one to the NMEA 2000 network — and draws its power from the NMEA network. It therefore does not need its own supply line but does load the navigation network. With a heavily utilized backbone, this must be taken into account in the power balance.
Both sides retain their own termination. The on-board electrical system bus needs its two terminating resistors, the NMEA 2000 backbone needs its own — the bridge does not replace either. It is a normal participant on both sides, connected via a T-connector, not an end of the line.
HintThe bridge is ideally located where both networks are already close to each other — usually at the navigation station. This saves a long additional cable through the entire vehicle.
Commissioning in steps
- Check both networks. On-board electrical system bus with two terminators, NMEA backbone with two terminators and functional power supply. Only when both are running independently should the bridge be added.
- Connect. Via T-connectors on both sides, with the networks unpowered.
- Register on the system monitor. The bridge is a participant like any other on the on-board electrical system side and is registered and named there.
- Search on the plotter. The plotter's device list should show the bridge. After that, the values can be placed on instrument pages.
- Cross-check values. Does the plotter show the same voltage and charge level as the system monitor? Deviations indicate an incorrect source assignment in the plotter, not a measurement error.
Multiple sources on the plotter
As soon as battery data comes from two directions — for example, from the engine via the engine interface and from the on-board electrical system via the bridge — it must be decided on the plotter which source feeds which display. Plotters usually call this data source selection or instance assignment.
The most common effect of a missed assignment: The display jumps between two values because two devices occupy the same instance number. This is not a defect of the bridge, but a configuration issue in the network. If you transmit multiple battery banks, assign instances deliberately and note them down.
Limits of the coupling
- No control from the plotter. The bridge transmits states but does not accept switching commands.
- No replacement for the system monitor. Alarms, configuration, and detailed menus remain on the on-board electrical system side.
- Only what is available. The bridge does not invent data. Without a shunt, there is no charge level to transmit.
- Display is up to the plotter. How many decimal places appear and which units are used is decided by the display device.
Typical errors
- Direct connection instead of bridge. Impermissible, even if the connector fits.
- Termination confused. The bridge is not an end of the line and does not replace a terminating resistor.
- Duplicate instances. Two sources with the same number result in a jumping display.
- Additional load overlooked. The bridge draws its current from the NMEA network.
- Expectation incorrectly set. If you want to switch from the plotter, you need another way — app or system monitor.
