Hi every body!
I need some detailed link about how the PSE detrmine the class of PD
The following link was forwarded to me on net pro did not address the following issues.
Here is some excerpt from the link.
" During classification,PSE presents a voltage between 14.5 to 20.5 v. At this time PD presents a load current as determined by the resistance on the Rclass pin."
The questions are:
1) PD is not "ON" yet, how can it offer load current?
2) what is Rclass pin? just dropped it in the link without any explantion?
"After PSE measures the load current and determines it can deliver the requested power, it moves into "ON" state by raising the voltage above 42 V"
the question is does switch has to allocate this much voltage (above 42 v) per port, then it can add up as switch could many poE ports.
I really appreciate if someone help me understand this.
thanks a lot!
How this rclass resistance helps PSE determine the class of PD?. Impedance of PSE= impedance of PD, otherwise , the power will not be transferred fully
This resistance has value of
When the classification option is not required, simply leave the RCLASS pin open to set the PD to the default Class 0 state.
During the classification mode, the voltage is between 12V and 23.5V. In this voltage
range, the class resistor RCLASS is engaged.
When volatege rises above 23.5V, RCLASS resistor will disengage.
1) to measure a resistance the PD device doesn't need to be "ON" is enough to apply a voltage and to measure the current that flows.
This comes from electrical theory and practice.
A resistance is a passive component
For example in CIPT6.0 vol. I says:
The PSE (Cisco Catalyst switch) detects a powered device by applying a voltage in the range of
-2.8V to -10V on the cable and then looks for a 25kOhm signature resistor. Compliant
powered devices must support this resistance method. If the appropriate resistance is found, the
Cisco Catalyst switch delivers power
2) rclass pin means the resistance value.
when the switch decides to power on the port it applies the full voltage 42V.
With an higher voltage a lower current is needed to provide the same power
see the following show from one of our access switch
sh power inline
Available:1400(w) Used:1013(w) Remaining:387(w)
Interface Admin Oper Power(Watts) Device Class
From PS To Device
--------- ------ ---------- ---------- ---------- ------------------- -----
Gi2/1 auto on 5.6 5.0 IP Phone 7911 n/a
Gi2/2 auto off 0.0 0.0 n/a n/a
Gi2/3 auto on 16.7 14.9 IP Phone 7971 3
Gi2/4 auto on 14.5 12.9 IP Phone 7961 3
Gi2/5 auto on 5.6 5.0 IP Phone 7911 n/a
Gi2/6 auto on 14.5 12.9 IP Phone 7961 3
notice 7911 doesn't support the signature (n/a)
the more powerful the phone the more power it requires
That high voltage may come from the telephony world (-48V in central office)
Hope to help