Keeping the Port Alive
The AT standard defines a low-current threshold IHOLD with a min of 5 mA and a max of 10 mA. What that means is that a current above 10 mA definitely implies the presence of a PD, whereas less than 5 mA definitely implies no PD. Between 5 and 10 mA is a “don’t care” region. Therefore, a typical PSE controller-design puts a nominal IHOLD threshold centered between 5 and 10 mA, say at 7.5 mA. Now the PSE will consider any port current above 7.5 mA as a PD, and any current below 7.5 mA as no PD.
There are two timers to know about here:
1. TMPS: This is the maintain-power signature timer. It starts the moment the port exceeds the nominal IHOLD threshold current, and stops when it falls below that threshold. Table 33-11 ...