[FAQ] How is the PoE handshake done between PSE and PD (detection, classification and inrush)?

Before ports turn on, PSE and PD are required to complete the handshake procedure (detection, classification and inrush) to ensure a valid PD is connected.

PSE starts this procedure by sending detection current signal and calculates the detect resistance based on measured voltage. The detect resistance is considered valid if it falls in the range of 19kΩ to 26.5kΩ. After successful detection, PSE starts classification by sending voltage signal and at the same time, PD removes detect resistor on the port and presents class resistor. If PD determines the classification signal is valid, PD will turn on the internal LDO which applies 2.5V across the class resistor. PSE measures the current going through the class resistor and recognize class level of the PD.

After a valid classification result (not overcurrent or class mismatch), PSE will turn on the port and start monitoring inrush current. At the same time, PD takes the control to limit the inrush current (~140mA). If the current keeps below PSE’s inrush current limit (~425mA) during Tstart (maximum allowed overcurrent time during inrush) period, PD will pull full load and the port starts normal operating. More information can be found in the TPS23861 datasheet.

Additional Resources

• Check out TI's PoE landing page: www.TI.com/PoE

• Learn about the IEEE802.3bt standard: Power over Ethernet: Getting Started with the New Standard