I had pretty much forgotten about improving on the Power over Ethernet progress I mentioned previously. A couple of weeks ago I bought another 7970 that I successfully converted to SIP to run in the study, and I was considering buying a few 7961s or Linksys PoE-capable phones to use in other places. However, I got an e-mail from a reader whose success at using a hacked cable with his 7960G prompted me to have another go.
While I did a heap of research about PoE and IEEE 802.3af, the hints I got about using a hacked cable with a standard 802.3af switch to power a Cisco phone came almost exclusively from the voip-info.org wiki. Everything I’d seen about this trick relied on the use of a crossover cable to fix the problem where the phone using the Cisco pre-standard expects the power in the opposite polarity to that delivered by 802.3af.
When I’d had a go previously, the info I had told me that I had to get power onto the spare pairs in the Ethernet cable, because the Cisco pre-standard used the spare pairs for power and not the data pairs. This was a problem as my switch provided Type A PoE, which is power-over-data-pairs. In the end I figured that I’d have to come up with some kind of electronics to get the power off the data pairs and onto the spare pairs.
My friendly reader informed me, however, that Cisco pre-standard phones take power from the data pairs as well as the spare pairs! Nothing I’d seen indicated that this was a possibility. So I pulled out the hacked-up cable I’d used previously and gave it another try… but it didn’t work.
I tried a bunch of alternatives that I probably tried before as well. I tried putting the sense resistor across the spare pairs instead of the data pairs, I tried switching the spare pairs around. But, since others had only ever reported success with a crossover cable, it had never occurred to me to try a straight cable instead. A bit of resoldering later, another try, and it worked!
Tried it with all my 7960s and it worked fine. So it looks like some 802.3af switches put power on the pairs in the opposite polarity to others (which is not a problem usually, as 802.3af devices have a bridge rectifier that allows them to handle either polarity).
Thanks to my friendly reader, I now have a way to power all my Cisco phones via PoE! Yay! The only caveat (one that I’ve only seen briefly mentioned anywhere) is the extra load placed on the cable by the 25kohm sense resistor — doubt it’s significant, but over a few phones it might add up.
Crossed Wires 
Did you come up with a clean way to install the 25kohm resistor?I read a post where someone proposed adding this resistor to the switch port. It seems to me you would want to add the resistor in the cat5 cable that goes between your phone and your RJ-45 wall outlet. In this way, if you unplugged your phone and plugged a different device into the same RJ-45 wall port, you would not send power to the port any longer and risk destroying a device not meant to take power over ethernet.
LikeLike
Sorry for the delay… I took a hint from a colleague who designed a way to make a portable Ethernet crossover out of a short Cat5 with a plug at one end and a socket at the other. For mine, I simply added the resistor across the terminals of the RJ45 socket. Result is a short (10cm) extension that I plug into the back of the phone and then plug the patch lead into the extension.
That was the plan, although I found that the extension has to be plugged onto the patch lead first because the resistance of the phone doesn’t allow the switch to detect the resistor properly.
What did you end up using?
LikeLike
Had another thought… You could use an RJ45 joiner to house the resistor. Might be a bit fiddly to get it back together again, but would be more flexible than my first method.
One more note: I bought a Linksys (now Cisco Small Business) switch with PoE, and found that it actually did the Cisco pre-standard PoE as well as 802.3af! It’s model is SRW224G4P, and if you can get one with recent firmware (I had some early stability issues) it might be a better way to run those old phones.
Regards,
Vic
LikeLike