NetAlert NetMeter User Manual page 15

CAN Primer
Because the transceiver subtracts the CANH and
CANL 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 CANH and CANL voltages to be
within specific limits otherwise a dominant (0)
might be misinterpreted as a recessive (1) or vice-
versa resulting in errors.
10V
0V
-5V
DC common mode voltage (caused by voltage
drop in the cable, refer to page 16 and 17) 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 NetMeter
accurately measures these voltages at the
following switch settings:
10
CANH,D
CANH/L
8
Diff V, D
12
CANL,D
These out-of-range signals
could be misinterpreted
Good signal
9
CANH,R
CANL,R
CANH/L
7
Diff V, R
11
15