[ntp:questions] Re: PPS working or not? FreeBSD 5.4

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid
Thu Feb 2 10:17:55 UTC 2006


Eugen COCA wrote:
> For sure, FreeBSD does not have PPS_SYNC enabled by default.
>
> NTP developers say that the ntpd service does not need PPS_SYNC
> enabled as you activate PPS in ntp.conf (enable pps). But, as we may
> see in the graphs, there is an enormous difference between the two
> configurations.
>
> All my three servers are "web enabled" and the configurations are
> visible if you put the mouse over the "hardware" word in the page.
>
> The main problem (as I think) with your configuration is that the GPS
> receiver does not see the whole sky. I have 2 GPS receivers (one of
> them connected with ntp1 and the other with ntp2 and ntp3 - YES, it
> works) mounted on the top of the roof (100% clear sky). Your error is
> almost the same at almost the same time. Look at the graphs !
>
> You may test all your servers with only one GPS receiver, at the same
> time. I made a test and it works - one GPS receiver and three servers.

I'm hoping that one of the NTP developers will come back on this - my 
understanding now is that there is a "software PPS", treated just like any 
other sync source, and a "kernel PPS" which is treated differently and 
rather than be part of a phase-locked loop simply forces correct time.  As 
I've not looked at the sources, it is this latter "kernel PPS" I am unsure 
about.

Your PCs are all somewhat better specified than my old 133MHz/48MB 
Pentium, but the timekeeping is similar as you say.  The experimenter in 
me want to get down from 10us jitter to 1us jitter, but I really don't 
have the time to do this.

I have just 50% sky coverage in the present arrangement (I added a photo 
to my Web page), so I will see how well that continues to work.  As I run 
Windows on everything else here, there seems little point in splitting the 
GPS receiver feed, but it sounds an excellent idea.

Cheers,
David 





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