Banish Those "Wall Warts" With Power Over Ethernet

Endpoints And Midspans
The 802.3af standard defines two types of PSE. A typical PSE combines 802.3af power-sourcing abilities with the data terminal equipment (DTE) or repeater capabilities of Ethernet switches and hubs in the field today. These PSEs, called endpoints, exist at the end of a network connection. Endpoints typically send power over the data pairs, even though the standard allows them to alternatively use the spare pairs, because these wires definitely connect to the PD.

A second type of PSE fits in-line between the data switch and the PD. This type, called a midspan, applies power only to the spare pairs while the data pairs pass straight through. With a midspan-based network, the PD receives data from an existing non-802.3af switch but receives power from the midspan.

It is possible to connect an endpoint and a midspan in series so power is available on both the spare and data pairs. To prevent unwanted interactions, a powered PD must not present a 25-kΩ signature on the other pairs. The diode ORing circuit shown in Figure 2 prevents this. Additionally, a midspan must wait at least two seconds after a failed detection before attempting another detection. This delay gives the endpoint a head start to detect and power the PD before the midspan detects again.

Several IC vendors are producing chips that handle the complexities of an 802.3af PSE. Some of these solutions are microcontroller peripherals that provide an interface to the PoE port but rely on the controller software to do much of the work. Devices with more capability detect and classify valid PDs autonomously and manage overcurrent and disconnect with minimal software overhead. These devices may need only the system software to decide whether there is enough power remaining to meet the PD’s demands.

For example, the LTC4259 can control four 802.3af-compliant ports with little or no software intervention. In auto mode, the device assumes there is 15.4 W available for every port. Alternatively, the LTC4259 can alert the controller when it detects a new PD, letting the controller decide to power the port or not. The device’s analog circuitry implements 802.3af-compliant detection, classification, I_LIM, and I_CUT limits, as well as ac and dc disconnect with a minimum of external components. The device uses standard 1/8-W surface-mount resistors to sense the port current, and its ac-disconnect circuitry measures the PD impedance without being fooled by long cables or stray capacitance. In large systems, up to 16 of the ICs can coexist on the same I²C or SMBus, controlling up to 64 Ethernet ports. Programmable interrupts and push-button registers keep software complexity to a minimum.