Telephone system:Cisco PoE hack

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Inspired from info at, found it confusing, so I started testing myself..

This part gives some quick information on converting/applying 802.3af PoE for usage on the 79x0 series pre-standard PoE PDs.

For a type B PSE uses the spare pairs (4,5 and 7,8) with being 802.3af: undetermined polarity. Test power by placing a 25kohm resistor between pins 4 and 7. Now, make sure the pins 4 and 5 contain the positive (+48) voltage, while pins 7 and 8 have the negative (GND) voltage. You might need to swap pairs 4+5 with 7+8 (blue with brown)

For a type A PSE (like NetGear FS108P), make sure the switch supports auto-sensing MDI/MDIX Test power by placing a 25kohm resistor between pins 1 and 3. Grab a crossover cable and make sure the voltage polarity is correct.

the stuff tested

  • Netgear GS110TP which turned out to be 802.3af type A (power on the data pairs)
  • Cisco CP-7940 and CP-7960: known to be pre-standard PoE.
  • 22k resistor

what you need to do to build one

  • Build an adapter cable that has a regular connector wired type B, and an outlet with the cables crimped on it the same (regular 'extension' cable, but can be very short)
  • undo the outlet's cap carefully, keeping the wires in place
  • use S-39 on the 'forks' for pin 1 and 3 (white-orange and white-green).
    careful that you have the ordering correct: outlets order pins by color pairs.
    the S-39 is for the forks to quickly get tinned without destroying the insulation
  • be sure the leads of the resistor are slightly shortened, so that it will just pop out of the outlet at the back.
  • solder it!
  • carefully place the cap back with the resistor leads joining the other wires

connecting it

Now, if you connect everything, the phone won't boot. It seems to be a kind of protection.

  • leave the phone disconnected
  • connect a regular network cable to the switch of your desired length
  • connect the adapter's outlet to the cable and wait (at most 5 seconds)
  • when the PoE led lights up, plug it in to the CP-79x0 phone's switch socket: it should get power and start the boot sequence


When at first it didn't work, I tried to make a cross cable (at least I think I did, I have to verify the spare pair ordering), but that didn't seem to work either: it triggered power on the switch, but the phone did nothing (probably thanks to a reversed polarity protection diode).

Then, when I plugged in the straight cable without phone, it booted; so I took a gamble and plugged the powered rj45, and it worked.

After testing, the actual network seems to work fine (tested with 3m cable)