The supply of power for terminal devices in Ethernet networks has so far mostly been realized with the aid of switched power packs, which means that a 230 V socket outlet is required in addition to each network socket. A new solution to this problem has for some time been available with the IEEE standard Power over Ethernet (IEEE 802.3af).

PoE uses existing network wiring (Twisted Pair Cat. 3/4/5/7) to power the terminal devices. However, the physical limits of these cables must be maintained, with the result that the current must not exceed 350 mA and the voltage equals 48V (tolerance range 44 to 57 V), in order to meet with the requirements for safety extra-low voltage. This allows twisted-pair cables to carry up to approx. 15 Watt.
The standard describes two possibilities for supplying power via a 10/100 Base-T Ethernet cable. The Spare Pairs method is normally used: this method uses the spare wire pairs 4/5 and 7/8.

The Signal-Pairs method (phantom powering) is an alternative method which uses the wire pairs 1/2 and 3/6 which also transmit data. However, it must be noted in this context that the polarity is not defined and is additionally crossed in a cross-over cable. Polarity must therefore be detected by a suitable input circuit in the powered device.

A process known as "Resistive Power Discovery" is used by the PoE power sourcing device to detect whether the powered device is PoE compatible. For this purpose, the internal resistance of the consumer is determined with the aid of a test circuit. The power supply is activated if this resistance is between 19 and 26.5 kohm with a maximum capacitance of 10 µF. The power class is then determined in a second discovery phase.

Maxim/Dallas supplies PoE-compliant products MAX5940, MAX5941 and MAX5942 for powering Ethernet devices. The functional scope of module MAX5940 is particularly large, as it is equipped with all the components needed for detecting, classifying and powering a PoE-compatible consumer.