resistive power discovery
With Power over Ethernet( PoE), there is the Power Sourcing Equipment( PSE), which is the power-sourcing unit that supplies power to the connected Powered Devices( PD). To ensure that the connected end devices support Power over Ethernet (PoE) and to prevent damage to the devices, there is a process called Resistive Power Discovery that checks and ensures the compatibility of the devices.
Power over Ethernet is standardized in IEEE standards802.3af and 802.3at. These standards, also known as PoE and PoE+, have different power classes that are used by the PoE supply unit to sense the power requirements of the powered device. The PSE unit adjusts its supply power to match the power class of the powered device. In the Resistive Power Discovery process, the PSE unit applies a minimum voltage to the live wires. This verifies that the PD unit has an input resistance between 19 kOhm and 26.5 kOhm. Then the input capacitance is checked, which must be less than 150 nF. The load is then supplied with an increasing voltage, signaling to which of the four power classes defined in the 802.3af standard it belongs. This is followed by supplying the correct power.
Since the supply powers of 802.3af are up to 15.4 W and thus exclude more powerful consumers, the IEEE 802.3at standard was specified, which supplies output powers of up to 25.5 W.