The Poe Provision Process; Line Detection - Planet Switch User Manual

While adding PoE support to networked devices is relatively painless, it should be realized that power cannot
simply be transferred over existing Cat5e cables. Without proper preparation, doing so may result in damage
to devices that are not designed to support provision of power over their network interfaces.
The PSE is the manager of the PoE process. At the beginning, only small voltage level is induced on the
port's output till a valid PD is detected during the Detection period. The PSE may choose to perform
classification to estimate the amount of power to be consumed by this PD. After a time-controlled start-up, the
PSE begins supplying the 52V DC level to the PD till it is physically or electrically disconnected. Upon
disconnection, voltage and power are shut down.
Since the PSE is responsible for the PoE process timing, it is the one generating the probing signals prior to
operating the PD and monitoring the various scenarios that may occur during operation.
All probing is done using voltage induction and current measurement in return.
Stages of powering up a PoE link
Stage Action
Measure whether powered device has
Detection the correct signature resistance of 15–
33 kΩ
Measure which power level class the
Classification
resistor indicates
Where the powered device will start up
Startup
Normal operation Supply power to device

6.1 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 KΩ
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 PD's circuitries till detection
is certified.
Industrial Renewable Energy 4-Port 10/100/1000T

6. THE POE PROVISION PROCESS

75
802.3at PoE+ Managed Ethernet Switch/Router
Volts specified
802.3af 802.3at
2.7-10.0
14.5-20.5
>42 >37.2
44-57 50-57
BSP-360
Volts managed
by chipset
1.8–10.0
12.5–25.5
>38
25.0–60.0