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Introduction
Overview
Power Through the Cable
A standard CAT5 Ethernet cable has four twisted pairs, but only two of these
pairs are used for 10Base-T and 100Base-TX data. The specification allows
two options for using these cables for power:
■ The spare pairs are used. The pair on pins 4 and 5 are connected
together and form the positive supply, and the pair on pins 7 and 8 are
connected and form the negative supply.
■ The data pairs are used. Since Ethernet pairs are transformer coupled
at each end, it is possible to apply DC power to the center tap of the
isolation transformer without upsetting the data transfer. In this mode of
operation the pair on pins 3 and 6 and the pair on pins 1 and 2 can be of
either polarity.
The standard does not allow both pairs (spare and data) to be used - a choice
must be made. The Power Sourcing Equipment (PSE) applies power to either
set of wires. Hewlett-Packard has chosen to supply PoE power over the data
pair. The Powered Device (PD) must be able to accept power from both
options.
An obvious requirement of the specification is to prevent damage to existing
Ethernet equipment. A discovery process, run from the PSE, examines the
Ethernet cables, looking for devices that comply with the specification. It does
this by applying a small current-limited voltage to the cable and checks for the
presence of a 25k ohm resistor in the remote device. Only if the resistor is
present, will the full wattage be applied, but this is still current-limited to
prevent damage to cables and equipment in fault conditions.
The Powered Device must continue to draw a minimum current. If it does not
(for example, when the device is unplugged) then the PSE removes the power
and the discovery process begins again.
PoE Capabilities of the Products
The HP ProCurve PoE switch devices are multiport switches that can be used
to build high-performance switched workgroup networks with PoE. These
switches are store-and-forward devices that offers low latency for high-speed
networking. The HP ProCurve PoE switch devices are designed to support
Redundant Power Supply and Power over Ethernet (PoE) technologies.
The 2650-PWR and 2626-PWR switches have 48 and 24 auto-sensing 10/
100Base-TX RJ-45 ports, respectively, and two dual-personality ports—either
auto-sensing 10/100/1000Base-T RJ-45, or mini-GBIC. The dual-personality
ports do not support PoE.
Introduction
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