Common Mode V
NetMeter measures the worst-case total Common Mode
Voltage for your network (including DC CMV and noise).
What is Common Mode Voltage?
Common Mode Voltage is an incidental voltage that is
common to both signal conductors in a differential
transmission system. CMV manifests itself as an shift in
signal voltage without any change in differential voltage.
Excessive CMV may cause signal voltages to exceed
the capabilities of transceiver chips, ultimately resulting
in communication errors.
The primary component of CMV is voltage drop in the
power conductors. The cable resistance causes the V+
voltage to drop from 24 VDC with increasing distance
from the power supply. Similarly, the V- voltage
increases proportionally from 0VDC at the power supply.
This offset results in each station having a slightly
different reference point (V-) which causes an apparent
shift in signal voltages from each node's point of view
(the signal voltages don't really change, but the
difference in V- makes it look that way).
Intermittent factors such as external interference,
variations in network load current and electrical noise
internal to nodes also contribute to CMV. Collectively,
these intermittent factors are called "noise".
Total CMV is critical to network health. NetMeter
measures total CMV over time and records the
maximum. To measure worst-case total CMV, leave the
NetMeter connected to the network for an appropriate
period of time (one machine cycle, one shift, one day).
Display
What it means
Worst-case total CMV
recorded since the
NetMeter was plugged in
or reset is 3.33V.
V
250
6
16
Common Mode Voltage
can not be determined
because there is no bus
activity.
