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PoE

PoE

(OP)
Which pair on the PoE Power Data Out port supplies the 48V DC? Does it have to be plugged into the device before it puts out the power?

RE: PoE

There are 2 POE standars , the older standard used the non data pairs
The newer standard allows POE inline on the data pairs

I dont believe a port will supply power to nothing?
From Wikepedia
The original IEEE 802.3af-2003[2] PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA[3][4]) to each device.[5] Only 12.95 W is assured to be available at the powered device as some power is dissipated in the cable.[6]

The updated IEEE 802.3at-2009[7] PoE standard also known as PoE+ or PoE plus, provides up to 25.5 W of power.[8] The 2009 standard prohibits a powered device from using all four pairs for power.[9] Some vendors have announced products that claim to be compatible with the 802.3at standard and offer up to 51 W of power over a single cable by utilizing all four pairs in the Category 5 cable.[10]

Powering devices

Two modes, A and B, are available. Mode A delivers power on the data pairs of 100BASE-TX or 10BASE-T. Mode B delivers power on the spare pairs. PoE can also be used on 1000BASE-T Ethernet in which case, there are no spare pairs and all power is delivered using the phantom technique.

Mode A has two alternate configurations (MDI and MDI-X), using the same pairs but with different polarities. In mode A, pins 1 and 2 (pair #2 in T568B wiring) form one side of the 48 V DC, and pins 3 and 6 (pair #3 in T568B) form the other side. These are the same two pairs used for data transmission in 10BASE-T and 100BASE-TX, allowing the provision of both power and data over only two pairs in such networks. The free polarity allows PoE to accommodate for crossover cables, patch cables and auto-MDIX.

In mode B, pins 4–5 (pair #1 in both T568A and T568B) form one side of the DC supply and pins 7–8 (pair #4 in both T568A and T568B) provide the return; these are the "spare" pairs in 10BASE-T and 100BASE-TX. Mode B, therefore, requires a 4-pair cable.

The PSE, not the powered device (PD), decides whether power mode A or B shall be used. PDs that implement only Mode A or Mode B are disallowed by the standard.

The PSE can implement mode A or B or both. A PD indicates that it is standards-compliant by placing a 25 kΩ resistor between the powered pairs. If the PSE detects a resistance that is too high or too low (including a short circuit), no power is applied. This protects devices that do not support PoE. An optional "power class" feature allows the PD to indicate its power requirements by changing the sense resistance at higher voltages. To stay powered, the PD must continuously use 5–10 mA for at least 60 ms with no more than 400 ms since last use or else it will be unpowered by the PSE.[21]

There are two types of PSEs: endspans and midspans. Endspans are Ethernet switches that include the power over Ethernet transmission circuitry. Endspans are commonly called PoE switches. Midspans are power injectors that stand between a regular Ethernet switch and the powered device, injecting power without affecting the data.

Endspans are normally used on new installations or when the switch has to be replaced for other reasons (such as moving from 10/100 Mbit/s to 1 Gbit/s or adding security protocols), which makes it convenient to add the PoE capability. Midspans are used when there is no desire to replace and configure a new Ethernet switch, and only PoE needs to be added to the network.

If I never did anything I'd never done before , I'd never do anything.....

RE: PoE

(OP)
Thanks Bill... That is a big help!

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