Common Mode Voltage (Switch Position 6); What Is Common Mode Voltage - Maretron N2KMeter User Manual

12 Common Mode Voltage (Switch Position 6)

The N2KMeter measures the worst-case total Common Mode Voltage (CMV) for your network
(including DC CMV and noise).
Display

12.1 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 a 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 NET-S voltage to drop from 12 VDC with increasing distance from the power supply.
Similarly, the NET-C voltage increases proportionally from 0 VDC at the power supply. This
offset results in each node having a slightly different reference point (NET-C), 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 NET-C 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. N2KMeter measures total CMV over time and records
the maximum. To measure worst-case total CMV, leave the N2KMeter connected to the network
for an appropriate period of time (one voyage, one watch, one day).
Thresholds
CMV High Fault
CMV High Warning
Revision 1.3
What it means
Worst-case total CMV recorded since
the N2KMeter was plugged in or reset
is 1.33V.
Common Mode Voltage cannot be
determined because there is no bus
activity.
5.5V
2.5V
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