Rockwell Automation DeviceNet Media Design Installation Guide - Manual (Page 117)

1 Publication DNET-UM072C-EN-P - July 2004
Appendix
B
Power Output Devices
Use DeviceNet Power
Supplies to Operate Output
Devices You can power some output devices on the DeviceNet network. The
application must allow the voltage to remain within the DeviceNet
specification limits of 11-25V dc. Because most actuators usually require more
power than is practically available from the DeviceNet network, they must be
powered by a separate power supply. Also, the large voltage variation of
11-25V that DeviceNet allows is typically beyond the range over which most
available actuators or output devices can safely operate.
You can use DeviceNet power to operate output devices such as hydraulic and
pneumatic solenoid valves, pilot and stack lights, and motor starter coils with
the following caution:
The DeviceNet common mode drop voltage specification limit of 10 volts, 5
volts in each power supply V+ and V- conductor, will never be a concern. This
is because in the design process we start with a 24V dc power supply and allow
for the 4% stack-up tolerance which leaves 23V dc to work with. From here
we consider the output device’s minimum required operating voltage of 19.2
volts. This gives 23V dc-19.2V dc = 3.8V dc for the common mode voltage or
1.9V dc in each conductor. This is far more restrictive than the 5 volts of the
DeviceNet specification and will result in shorter allowable distances for the
installation.
ATTENTION
Do not let DeviceNet voltage at the relevant node exceed
the output device’s acceptable voltage range. Output
devices rated 24V dc rarely are specified to operate below
19.2V dc or -20% of their 24V dc rating. Many only operate
down to 20.4V dc or -15% of the rated voltage. This means
that the DeviceNet network design must not allow the
available voltage to drop below 19.2 volts, for example,
instead of the 11 volts that the DeviceNet specification
allows. This higher lower voltage limit which is within the
DeviceNet specification will actually restrict the distance of
the DeviceNet network from what would be possible if
actuators were not utilizing the DeviceNet power
IMPORTANT
Design your network to make sure that sufficient voltage is
available to operate the output device wherever it is
installed. This is especially important when it is connected
at the farthest location from the power supply.