[time-nuts] Need Time Help

David J Taylor david-taylor at blueyonder.co.uk
Tue Oct 4 05:04:25 EDT 2016


From: Hal Murray

david-taylor at blueyonder.co.uk said:
> You can use a GPS puck or equivalent with PPS output to get the sort of
> accuracy you need.  ..:

PPS output is only half the battle.  You also have to get it into the system
that needs it.

A modem control signal on an old fashioned serial port (not USB) is the
classic approach.  Unfortunately, many modern systems, especially laptops,
don't have serial ports.

GPIO pins on things like a Raspberry Pi also work well.

PPS over USB adds the USB polling interval to the error budget.  It will
probably work well enough if the goal is under 15 ms.
====================

Hal,

Agreed, PPS over USB isn't ideal, but possibly the extra jitter in the 
context of a Windows system isn't too bad.  On desktop systems I've been 
able to add a 3rd-party serial PCIe card and get excellent results.

On laptops I've usually been able to get well under a millisecond over 
Wi-Fi:

  http://www.satsignal.eu/mrtg/performance_ntp.php#wifi

apart from one rather cheap-and-nasty system which I won as a prize.  I 
wonder why they were giving that unit away!!  The secret is, perhaps, the 
new API introduced in Windows-8 (with some pushing from Dave Hart, I 
suspect) which allows much more precise retrieval of the Windows time, and 
is used by NTP when possible.

Cheers,
David
-- 
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor at blueyonder.co.uk
Twitter: @gm8arv 



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