N2KMeter NMEA 2000® Diagnostic Tool User's Manual

11 CAN Primer

NMEA 2000 ® is based on the CAN protocol. A fundamental understanding of CAN will help you take full advantage of the N2KMeter’s features and significantly improve your ability to diagnose network problems quickly.

CAN messages are transmitted as a difference in voltage between two separate wires, NET-H

(white) and NET-L (blue). Differential transmission helps CAN and NMEA 2000 ® to operate well even with high levels of external interference (i.e., from sources like winch motors, radars, etc.) Here’s what you might see if you captured CAN signals on an oscilloscope:

Individual wires include data + noise

Differential receiver cancels noise and extracts data signal

CAN signals have two states, dominant (0) and recessive (1). The transceiver in each NMEA

2000 ® node determines whether a signal is a 1 or a 0 based on the differential voltage between

NET-H and NET-L.

Because the transceiver subtracts the NET-H and NET-L signals to determine the bit values, any noise induced in the cable (the same noise is induced in both wires) is cancelled.

Transceiver chips require NET-H and NET-L voltages to be within specific limits, otherwise a dominant (0) might be misinterpreted as a recessive (1) or vice-versa resulting in errors.

Page 14 of 28 Revision 1.1

DC common mode voltage (caused by voltage drop in the cable) is the primary cause of the voltage shift illustrated above. Noise induced in the data wires also contributes to the voltage offset.

If the combination of DC common mode voltage and induced noise causes the signal voltages to exceed the transceivers’ capabilities, bit errors are more likely to occur.

Six separate CAN voltage measurements are essential to rapid troubleshooting. The

N2KMeter accurately measures these voltages as defined in the next diagram.

Revision 1.1 Page 15 of 28