Powering a 79xx Cisco Phone with a Standard POE

A VoiP Phone Cisco 7040G arrived to my home (on another post I’ll explain why/how/etc) and the big surprise was that it has to be fed with an special POE from Cisco. It’s called pre-standard POE. I have a standard POE (802.3af) that fed properly my previous VoIP phone… and after connecting it to the 7940, the phone won’t boot up…A bit annoyed I bought the power supply of the damned 7940G (extra 15€ to the trash) which is called CP-PWR-CUBE-3. It arrived and the phone finally booted up.

BUT I always go beyond things and don’t want to have extra cables around the VoIP phone (if previous was fed by a unique cable, why this damned Cisco has to have extra cables annoying on the living room?). So after a bit of research I found this interesting information: Cisco POE from voipinfo.org.

It turns out that the 79xx series of Cisco Phones (Pre-Standard POE) can be fed with a standard 802.3af POE but with changed polarity. I tested it and voilà: It works! I can re-use my infrastucture (POE hidden – and the VoIP phone fed only by a single cable).

If the cable is crimped following 568b wiring, the scheme would be:

In standard 802.3af POE, pairs 4-5 are negative and pairs 7-8 are positive. So you have to change it to send the reverse polarity to the Cisco 79xx and voilà!! Phone is fed with a single ethernet cable 🙂

Cisco 7940 AnigweiFinally… If you don’t dare to try this and want to fed a Cisco 79xx… you could be interested in the AC adapter that I’m selling at ebay 🙂

4 Replies to “Powering a 79xx Cisco Phone with a Standard POE”

  1. I figured it out over the weekend. Posting here for anyone else that may want the solution.
    Use the following for 658a
    POE end.
    1. Grw
    2. G
    3. Orw
    4 Brw
    5 Br
    6 Or
    7 Bl
    8 Blw

  2. hi! i have the exact same phone (cisco 7940) and tried out your custom RJ45, but my phone doesn’t power up. I’m using a Netgear GS108PP switch. Do you think there is something else i could try, apart from getting an old cisco catalyst poe switch?

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