Re: Passive PoE?

TommyPedersen69598 · ‎08-03-2020

We would like to use passive PoE for Cisco CP-7821 IP phones in a special application.

Edit: By passive PoE I mean applying voltage without the standardized protocol that determines device class etc before the application of 48V.

1. Does the phone support passive PoE?

2. What voltage should we use? Is 48V ok? What is the preferable voltage?

3. If we would like to support other Cisco phone models, e.g. 8811, do these support passive PoE?

Thanks in advance for answers!

Nithin Eluvathingal · ‎08-03-2020

You will get the values from Prodcut Data sheets

Leo Laohoo · ‎08-03-2020

All Cisco phones support PoE.

TommyPedersen69598 · ‎08-03-2020

The question is if passive POE is supported. I.e. without the standardized protocol to determine device class etc before applying the 48V. Or is this a supported case in the standard?

Nithin Eluvathingal · ‎08-03-2020

Passive PoE refers to any devices using PoE that is not 802.3af or 802.3at. BUT Cisco phones support 802.3af or 802.3at. Cisco phone negotiate the power and in Passive PoE power negotiation doesn't happen. so you cannot use. Screen Shot 2020-08-04 at 9.47.27 AM.png

Leo Laohoo · ‎08-04-2020

Wait ... Passive PoE means Power Injectors.

Nithin Eluvathingal · ‎08-04-2020

He is trying to connect a Phone to 48V port which doesn't support any negotiations.in this case I hope the extra power will destroy the device.

As per my understanding , for POE injectors also we have passive and active.

Dr Alban · ‎08-04-2020

Not true, the SPA303 for example doesn't.

Leo Laohoo · ‎08-04-2020

@TommyPedersen69598 wrote:

Edit: By passive PoE I mean applying voltage without the standardized protocol that determines device class etc before the application of 48V.

So you're looking for a Cisco phone that powers up without using LLDP or CDP to negotiate power?

Better yet, why not get a power brick instead? Less configuration and less hassle.

TommyPedersen69598 · ‎08-04-2020

The power brick is too large and too expensive. We are (based on customer requirements) designing a small box that will sit on the back of the phone. The box will have 230VAC and Optical fiber as inputs. Our box will provide media conversion 100Base-FX to 100Base-TX and provide power supply plus a few other features that are proprietary to our business. As it will be an integrated product there will be no need for negotiation. Integrating a chip for negotiation will require development resources, money and time. We would rather just add 48V to the two unused pairs. Building the box out of COTS products such as media converter and power brick would be too expensive (purchasing, manufacturing of box, and handling RMA cases) and provide too many points that could fail.

As I said, we would rather just add 48V to the two unused pairs in the ethernet cable.

By the way, the Cisco power injector CP-PWR-INJ conforms to 802.3af.

Nithin Eluvathingal · ‎08-04-2020

What I recommend is to use power cube or Cisco power injector.

Power should be in watts I am not sure what the power in your case. You have mentioned it as 48v.

i think it will damage ur phone.

Leo Laohoo · ‎08-04-2020

Ok, you want Passive PoE injectors, try Phihong USA.

TommyPedersen69598 · ‎08-05-2020

Thanks, but No, I don’t want to buy passive injectors. We are developing our own PCB. My question is simply if I can add the 48V to the two unused pairs. If I were to use a COTS power supply I would use the ordinary power bricks, but those are not appropriate in our case, see my other answers for some of the reasons.

Leo Laohoo · ‎08-05-2020

I think you need to engage your Cisco AM/SE.

Nithin Eluvathingal · ‎08-06-2020

you have mentioned you will use "UNUSED PIN" u cannot just use any unused pin for powering up the device. just go through all the links and still u fell confident its up to you.