Line Detection - Interlogix GE-DS-242-PoE User Manual

Chapter 8: RJ-45 Pin Assignment

Line Detection

Before power is applied, safety dictates that it must first be ensured that a valid
PD is connected to the PSE's output. This process is referred to as "line
detection", and involves the PSE seeking a specific, 25 KO signature resistor.
Detection of this signature indicates that a valid PD is connected, and that
provision of power to the device may commence.
The signature resistor lies in the PD's PoE front-end, isolated from the rest of the
the PD's circuitries till detection is certified.
Classification
Once a PD is detected, the PSE may optionally perform classification, to
determine the maximal power a PD is to consume. The PSE induces 15.5-20.5
VDC, limited to 100 mA, for a period of 10 to 75 ms responded by a certain
current consumption by the PD, indicating its power class.
The PD is assigned to one of 5 classes: 0 (default class) indicates that full 15.4
watts should be provided, 1-3 indicate various required power levels and 4 is
reserved for future use. PDs that do not support classification are assigned to
class 0. Special care must be employed in the definition of class thresholds, as
classification may be affected by cable losses.
Classifying a PD according to its power consumption may assist a PoE system in
optimizing its power distribution. Such a system typically suffers from lack of
power resources, so that efficient power management based on classification
results may reduce total system costs.
Start-up
Once line detection and optional classification stages are completed, the PSE
must switch from low voltage to its full voltage capacity (44-57 Volts) over a
minimal amount of time (above 15 microseconds).
A gradual startup is required, as a sudden rise in voltage (reaching high
frequencies) would introduce noise on the data lines.
Once provision of power is initiated, it is common for inrush current to be
experienced at the PSE port, due to the PD's input capacitance. A PD must be
designed to cease inrush current consumption (of over 350 mA) within 50 ms of
power provision startup.
GE-DS-242-PoE Managed Ethernet Switch User Manual 205