Cisco PoE

During the build up of the VoIP PBX I grabbed a couple of Cisco PoE phones, I wasn’t to fussed on PoE from the switch as it was going to increase the purchase price of the switch for what was going to be a very small number of PoE devices, so a few cheap POE (power over Ethernet) injectors where purchased of eBay. There was a warning on the listing “this item may not be compatible with some older Cisco Models that use Pre-Standard power and not the IEEE 802.3af Standard” So a bit of digging around and I discovered the following;

What I found was Standard POE injectors use PINS 4&5 as the Positive (V+) line and PINS 7&8 as the Negative (V-) line. (again confirmed on the listing)
Cisco for some reason use the connection the opposite way round, PINS 4&5 as the Negative (V-) line and PINS 7&8 as the Positive (V+) line.

Cisco 7940G IP phones must have diode protection to prevent damage from a reverse polarity connection, as plugging things just to see what happens did not damage either of the two phones I purchased 😉

I cut the end of a patch cable, and swapped over the blue and brown cables and crimped a new plug on (you only need to do this one end) and the “standard” cheep PoE injector now works with my old Cisco phones 🙂 The cable will allow you to pass 100Mb/s data but not 1000Mb/s as you have just crossed over a pair. So I got a couple of “orange” cables and made the adjustment now knowing that if a socket on the patch panel was feed with an “orange” cable I was only ever going to get 100Mb/s out of the wall port.


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