Introduction
Overview
Power Through the Cable
A standard CAT5 Ethernet cable has four twisted pairs, but only two of these pairs are used for
■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.
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
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
The
Introduction