Power Over Ethernet Interoperability Guide

Some Practical Detection Techniques

In the inset in Fig. 3.3, we have shown the port voltage during the initial sequence. Region B is detection. The port voltage starts from zero (cable not energized) toward fully applied “48 V.” In this simplest case of detection, just two steps of voltage or current are being applied, and they are applied on the “charging” curve (ascending staircase). As per the IEEE standard, two steps (or samples) are a minimum, since we need them to eliminate diode offset from the bridge, as discussed previously. The standard also asks we ensure that the voltage levels we use for our calculations are more than 1 V apart. Using (V₂ - V₁)/(I₂ - I₁), we thus get R. This may sound very basic, and it is, but it is a viable detection ...