Power Over Ethernet Interoperability Guide

Type 2 PD Delay Timer

To keep things simple, inrush requirements are classless. They apply to any class. It is only when Power-on starts that the PSE adjusts its current limits based on the reported class. As indicated on the left side of Fig. 5.7, the AT standard requires that a Type 2 PSE await the end of the inrush timer before it raises its set current limit to Class 4 requirements (typically 850 mA nominal). This can take up to 75 ms, which we know is the max value of the inrush timer. Hence, to avoid inadvertent port shutdown, the standard correspondingly asks the PD not to ask for higher than Class 0/3 currents before 80 ms. This is the timer T_DELAY. We have chosen a nominal value of 90 ms for the curves in Fig. 5.7. All the timings are ...