NMEA 2000

This module checks the NMEA 2000 interface: the CAN transceiver, the bus connector and the board's ability to join a live network.

Idle

With the board powered and connected to a terminated bus that carries no traffic, CAN_H and CAN_L are read with a meter. Both must sit at the recessive level, close to the same voltage, of about 2.3 V on a Scadys board. A line pulled high or low shows a transceiver fault, a bridge on the transceiver pins or a missing termination elsewhere on the bus.

Termination

A Scadys instrument is a drop device and carries no bus termination. The inspection stage checks that no termination resistor is fitted across the pair, because a fitted one loads the bus for every other device on it.

Traffic

The board joins a live NMEA 2000 segment at 250 kbit/s. It must complete its address claim within a few seconds and then pass messages with zero bus errors. The address the board claims and its NAME are recorded. A bench data source that sends realistic traffic, from a second board or a simulator, lets the board's receive path be checked against known values.

Connector and supply lines

The NMEA 2000 connector also carries the board's supply, so the supply-range and reverse-polarity steps test the same connector. Open-line tests, in which each connector line is broken in turn through a breakout and the board must rejoin when it is restored, run after bring-up.