P-Bus and NMEA 2000 use the same M12 connector system. However, connecting them together is not permitted — and the reason for this is not a technical formality, but a deliberate separation of two networks with different tasks.
Why not directly
For safety reasons, the P-Bus is not an NMEA 2000 certified system. It carries data that triggers switching operations in the onboard network — switching consumers, disconnecting main switches — and is therefore deliberately an independent network. A navigation network to which devices from various manufacturers are connected is not the right place for this.
The coupling therefore takes place exclusively via the CBN bridge. It reads the P-Bus data, converts it into NMEA 2000 messages, and transmits them to the navigation network — in one direction, cleanly separated.
Using NMEA 2000 cables in the P-Bus, however, is unproblematic. It's the same cable, the same connector. Only the two networks remain separate.
What data is transmitted
The bridge transmits the onboard network data as standardized NMEA 2000 messages, known as PGNs. The following are supported:
- 127501 — Status of the switch bank, i.e., which consumers are switched on.
- 127505 — Tank levels.
- 127506 — DC status.
- 127508 — Battery status.
- 127744 and 127747 — AC power and voltage, Phase A.
- 127750 — Converter status.
This makes the essential onboard network values available on the chart plotter: battery, tanks, DC and AC system.
The decisive caveat: the plotter
A transmitted message is not yet a displayed message. Whether and how a PGN is displayed is decided by the chart plotter — depending on the manufacturer and software version.
In practice, this means: Before purchasing, check which PGNs the existing device can display. Tank levels and battery status are widespread, while the status of a switch bank is significantly rarer. A disappointment at this point is not due to the bridge, but to the expectation.
NoteThe question is not "does my plotter support NMEA 2000" — they all do — but "which PGNs does it display". This list is in the plotter's manual and changes with software updates.
Connection and power supply
The bridge is connected on both sides via M12 T-cables; both are included. On one side, it is connected to the P-Bus, on the other to the NMEA 2000 network.
It draws its power from the NMEA 2000 network — a separate power supply cable is not required. This is convenient but has a consequence for planning: The bridge counts as a participant in the power budget of the navigation network, not that of the P-Bus.
When installing, the usual bus rules apply twice: Both networks each remain a single line with two terminators. The bridge is a participant in each of the two — it connects them without making them one.
What the bridge does not do
It makes onboard network data visible in the navigation network. It does not make the P-Bus operable from the plotter, and it does not bring navigation data into the P-Bus.
Anyone who wants to switch consumers continues to do so via the system monitor. This is not a limitation, but precisely the separation that makes the P-Bus an independent network.
Check after installation
- Both networks have their two terminators — the bridge does not replace a terminator.
- The bridge appears in the device directory of the plotter.
- Battery values are received and match the system monitor.
- Tank values are received — and if not, first check if the plotter displays PGN 127505.
- The power budget of the NMEA network takes the bridge into account.
Typical errors
- Direct connection of both networks. The connector fits, but it's not permitted.
- Trying to switch from the plotter. The bridge transmits data; it does not enable operation.
- PGN list of the plotter not checked. The most common reason for missing displays.
- Bridge counted as a terminator. Both networks still need two terminators each.
- Forgetting the power budget of the NMEA network. The bridge draws its power from there.
