Verify network voltages – Rockwell Automation DeviceNet Media Design Installation Guide User Manual
Page 112
Publication DNET-UM072C-EN-P - July 2004
5-8 Correct and Prevent Network Problems
When choosing a power supply, keep the following tips in mind:
•
The thin wire trunk line is rated to 3A current flow. The thick wire trunk
line is rated to 8A current flow. In North America, however, current is
limited to 4A. You can install multiple power supplies on a DeviceNet
network, but no section of cable should allow more current flow than
that for which the trunk line is rated.
•
If you install multiple power supplies on the network, break the red V+
wire between the power supplies to isolate one power supply from the
other.
•
On DeviceNet networks with multiple devices or extra long trunk lines,
which result in drawing large currents at longer distances, common
mode voltage can affect network operation.
Verify Network Voltages
The DeviceNet network communicates using a three-wire signal voltage
differential among the CAN_H (white), CAN_L (blue), and V- (black) wires.
DeviceNet messages consists of ones and zeros. A one is recessive, meaning
that the difference in voltage between CAN_H and CAN_L should be as close
to 0V as possible. A zero is dominant, meaning that the difference in voltage
between CAN_H and CAN_L must be within certain limits when the zero bit
is set.
Out-of-range differential voltages can be caused by such factors as:
•
opens or shorts in signal wires
•
faulty devices (particularly transceivers)
•
severe interference
•
incorrect media
To check for proper voltage, use a voltmeter in dc mode. Measure voltages at
the DeviceNet scanner.
IMPORTANT
We recommend that the DeviceNet power supply be used
to power only the DeviceNet network.
if voltage
then
so you should
on the black V- wire differs from one point
of the network to another by more than
4.65V
communications problems could result
•
move an existing power supply
closer to the heavier current loads
•
add an additional power supply
between the red V+ wire and the black V-
wire drops below 15V