Can Primer - Maretron N2KMeter User 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
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
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.
Revision 1.3
Differential receiver cancels
noise and extracts data signal
®
to operate well
Page 15