[time-nuts] Embedded NTP server ideas and feature requests

[Michael Tharp]

On 02/10/2013 11:22 AM, Chris Albertson wrote:
> Yes there is a standard but there are also may non-standard implementations.
>
> I was a little worried about the comment regarding "isolation" being
> hard to implement.   The standard for Ethernet required galvanic
> isolation on all Ethernet ports by use of a transformer.  So POE
> simply buts a DC bias on the wire.  I think it can provide 12 Watts of
> power

Of course the Ethernet data is isolated, as is required by the spec and 
also pretty much needed for anything to work at all. Many transceivers 
won't even work without a transformer to drive into.

I was referring to *power* isolation. Regardless of whether it's the 
more common "unused pairs" type or using center taps as you describe, 
passive adapters don't provide any galvanic isolation between the Cat5 
line and the power input to the device. 802.3af requires this, which 
means a 48V, isolated SMPS is required.

The other advantage to 802.3af is that the powered device advertises its 
readiness to accept power to the switch, and the switch absolutely will 
not turn on the power until it sees that the device can accept it. So no 
surprise 48V on your laptop.

But I suspect this is quite enough PoE talk; I don't want to tire the 
list with non-time-related matters.

-- m. tharp