NetAlert NetMeter User Manual page 17

Common Mode V
Thresholds:
CMV High Fault
CMV High Warning
Notes About Common Mode Voltage (CMV)
!
NetMeter's CMV measurements are not affected by
where on the network you take the measurement.
!
DC CMV problems are typically caused when devices
draw more current than expected or when cable
resistance is higher than expected (too much cable)
or a combination of both.
!
CMV problems can be intermittent since few devices
have a constant load current.
For CMV levels we suggest:
! Check for devices that might be drawing more
current than you expect
! Verify your network design with special
attention to cable length and load current
calculations
!
Move the power supply toward the middle of the
network, or toward the nodes that draw the
most current
!
Investigate adding another power supply to
reduce voltage offset in V-
!
High levels of external interference may require
you to reduce the DC CMV limit below the 5V
recommended in the DeviceNet Specification to
keep total CMV below 6.5V
6.50V
5.00V
Common mode
voltage deals with
these three wires
(CANL, CANH,
and V-)
6
17