[ntp:questions] SNTP server + ntpd 4.2.4 client

David Woolley david at ex.djwhome.demon.co.uk.invalid
Tue Mar 18 18:28:22 UTC 2008


Unruh wrote:

> Just looked it up. A bit bizarre-- power over the ethernet? The ethernet
> has no power supply capability. Do you mean that you have to supply the
> device with 60V running on one of the unused ethernet cable lines? Sounds
> noisy to me. 
> 
I believe it is a relatively new, but very real, standard.  The power is 
transmitted as a phantom between two pairs.  In one variation, they are 
the pairs used for normal, 10baseT.  I gather one reason is that there 
are exemptions in electrical codes for ELV power feeds as part of 
datacommunication systems, whereas a normal feed would require a 
formally qualified (not just competent) electrician.

The feed is 48V DC.  I'm not 100% sure that counts as ELV, but it is the 
same as most analogue telephone systems.

The apparent source specification is IEE 802.3-2005, although I haven't 
gone to source.

As the power is common mode with respect to the signals, the noise 
should not be excessive.




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