[ntp:questions] How to determin hardware latency for PPS offset given simple tools.

David Taylor david-taylor at blueyonder.co.uk.invalid
Mon May 27 13:43:53 UTC 2013


On 27/05/2013 04:52, Paul G wrote:
> On Saturday, May 25, 2013 3:05:13 AM UTC-4, David Taylor wrote:
>> What happens with the fudge set to zero?
>> Please show the table for each machine.
>
> Showing 192.168.0.2 and .244 with time1 = 0 as compared to the first post where
> time1 was ~100 microsec.
>
> These two have gigabit interfaces. 0.2 is driven by a Garmin 18x and 0.244 is
> driven by a Firefly (I).
>
> 0.2
>       remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> o127.127.22.0    .PPS.            0 l    1    8  377    0.000   -0.001   0.002
> +192.168.0.244   .PPS.            1 u    -    8  377    0.075   -0.128   0.001
> *192.168.0.210   .GPS.            1 u    7    8  377    0.519    0.117   0.098
>
> 0.244
>       remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> o127.127.22.0    .PPS.            0 l    5    8  377    0.000    0.000   0.002
> +192.168.0.2     .PPS.            1 u    4    8  377    0.079    0.132   0.003
> +192.168.0.210   .GPS.            1 u    4    8  377    0.547    0.253   0.135
> *192.168.0.192   .PPS.            1 u    3    8  377    0.475    0.262   0.036

Well, that's interesting!  Many thanks.  128/132 microseconds is way 
more than I would expect a GPS/PPS signal to be off - it should be 
nearer 1 microsecond (unless you're doing something unusual like a long 
line driver, or triggering off the wrong edge of a 130 microsecond 
pulse, perhaps?).  I can't see a spec for the pulse width from the 
Firefly, but I do see it has a programmeable offset.  I presume that's 
set to zero?

Purely for comparison, on the systems here:

- my FreeBSD/Intel Atom server sees my Windows XP system as -0.004, and 
my Win-7 system as -0.077.

- my three Raspberry Pi cards see the FreeBSD server as -0.030 to +0.100.

- my three Raspberry Pi cards see the Windows XP system as -0.040 to + 
0.070, and my Win-7 system as -0.020 to + 0.130.  One RasPi is connected 
via Wi-Fi.

All of these are triggered from PPS signals which are well within 0.1 
microseconds of each other when viewed on a digital storage scope.  The 
18x LVC signal to the FreeBSD node may be a little delayed because of 
the way it's being processed.  I guess it says something about the 
internal delays in each PC, and perhaps something about the difference 
between network and serial/DCD processing.  Each PC's clock is /not/ 
being set to UTC, but to some offset from UTC, which NTP measures as 
near zero.

Cheers,
David
-- 
Cheers,
David
Web: http://www.satsignal.eu



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