NetAlert NetMeter User Manual page 14

CAN Primer
DeviceNet is based on the CAN protocol. A
fundamental understanding of CAN will help you
take full advantage of the NetMeter'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, CANH
(white) and CANL (blue). Differential transmission
helps CAN and DeviceNet to operate well even
with high levels of external interference (ie from
sources like motors, welders, etc.) Here's what
you might see if you captured CAN signals on an
oscilloscope:
CANH
CANL
Individual wires
include data + noise
CAN signals have two states, dominant (0) and
recessive (1). The transceiver in each DeviceNet
node determines whether a signal is a 1 or a 0
based on the differential voltage between CANH
and CANL.
CANH
CANL
Dominant
Differential
Recessive
Dominant
( D )
Recessive
Differential
Differential Signal
(CANH - CANL)
Differential receiver cancels
noise and extracts data signal
( R )
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