January 5, 2012 - Vision Systems GmbH - Data communication specialist - delivers
Serial Device Servers, powered over the Ethernet connection.
Three models are available since February 29th 2012.
The NetCom 111 PoE, 113 PoE and 211 PoE
are upgraded variants of the well known Serial Device Servers
NetCom 113 and
As those they provide the exact same functions and handling as the long existing
products. But they add the feature of getting powered over the Ethernet connection.
If this option is used the need for an extra DC power adapter is removed. In such an installation
an Ethernet switching Hub with PoE option inserts the supply currents onto the network cable,
the NetCom Servers use this to operate. If the external power adapter is installed in parallel,
this operates as an auxiliary power supply.
The source of electric power is completely transparent to any software, application
or configuration tool. The new models maybe used as drop-in replacements of existing devices in
already running installations.
Using a supply via the LAN cable eliminates the need to create an extra AC or DC power line to the
location of the device. Of course in all installations there is some kind of supply at the location,
at least to run the connected serial equipment. But this may be a low power supply, run from battery,
or also the distance to the Serial Device Server may be significant when using RS422.
Further the serial device might draw power from the serial port (RS232 or RS422), this also is an
ideal occasion to use Power over Ethernet.
Power over Ethernet: Such functions have been in development since a long time.
The wish to use one cable only instead of separate data and power cables resulted in some
proprietary solutions. In 2003 the Internet Engineering Task Force created the first official
version of a standard, now known as IEEE-802.3af-2003. This provides roughly 13W of electric power
from a Power Sourcing Equipment (PSE) to a Powered Device (PD).
The power is applied after sensing whether a 802.3af compliant device is connected to the LAN cable.
The PSE usually is an Ethernet Switch with added capabilities for PoE function,
but sometimes special Power Inserters are installed.
In the same way, the power may be extracted by a so-called PoE Splitter at the device location.
"Power over Ethernet" is nicknamed as PoE, and in general also refers to the old and
proprietary methods as well. But today without a special reference it is used for the IEEE standard
only. Those PoE sources provide roughly 13W to a PD, and in some installations the PD may use up to
15W of electric power.
In 2009 the newer standard named IEEE-802.3at-2009 was finished. This new standard is also
referenced as "PoE plus" or "PoE+" for short. There is no known ambiguity with
proprietary standards, so this nickname only refers to the 802.3at version.
A PD can declare itself as a member of a certain power class, defined as 'Very Low power' (<3.9W),
'Low power' (<6.5W), 'Mid power' (<13W) and 'High power' (<25W).
The last class is only available in PoE+ installations.
Announcing the power class to the PSE (the Switch) allows the PSE to optimize the power budget
for all Ethernet ports and the connected PDs.
Power Class: The new NetCom PoE Serial Device Servers are Class 2 (Low Power) devices.
The maximum consumption is up to 3W in NetCom 211 PoE, while for Class 2 the PSE allows up to 6.5W of electric power.
The NetCom 111 PoE and 113 PoE use even less power. In principle they are Class 1 (Very Low Power) devices.
The Class 2 has been choosen to cover difficult connections (long cables, higher resistance, …), where the
power draw may be slightly above the maximum 3.85W for Class 1.
In most installations it is save to apply a Class 1 limit at the PSE (Ethernet Switch).
The supply via the Ethernet cable has precedence over an external power adapter, users may install
an additional local power adapter as auxiliary or backup supply. However if the PSE fails to power the NetCom,
it is likely the Ethernet connection fails as well. So this may be an option for test environments,
but not used in productive installations.