Industrial Renewable Energy 4-Port 10/100/1000T
802.3at PoE+ Managed Ethernet Switch/Router
BSP-360
75
6. THE POE PROVISION PROCESS
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
Volts specified
Volts managed
by chipset
802.3af 802.3at
Detection
Measure whether powered device has
the correct signature resistance of 15–
33 kΩ
2.7-10.0 1.8–10.0
Classification
Measure which power level class the
resistor indicates
14.5-20.5 12.5–25.5
Startup
Where the powered device will start up >42 >37.2 >38
Normal operation
Supply power to device 44-57 50-57 25.0–60.0
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.
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