NetAlert NetMeter User Manual page 19

CANH/L Diff. V (R)
Thresholds:
Diff R Voltage High Fault
Diff R Voltage Low Fault
What is "recessive?"
DeviceNet messages consist of 1's and 0's. The
recessive state of the bus (where the differential voltage
between CANH and CANL is close to zero) represents
the logical value 1 (opposite from what you might
expect). The bus is always in the recessive state except
when a node is transmitting.
NetMeter measures the difference between CANH and
CANL for recessive bits on the network, and if the
difference is not close to 0 volts, NetMeter indicates a
warning or a fault.
For CANH/L differential recessive voltage
levels we suggest:
!
Check for shorts and opens on the CANH and
CANL wires
!
Check for a missing or bad terminator (should
be two 120 ohm terminators from CANH to
CANL - one at each end of the network)
!
Disconnect nodes one at a time or temporarily
split the network to check for faulty transceivers
or cables.
0.08V
-0.18V
CANH/L Diff. V deals
with these two wires
(CANL & CANH)
7
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