[ntp:questions] bad time with a laptop on windows 7

Rob nomail at example.com
Sun Aug 3 08:07:08 UTC 2014


Greg Hennessy <greg.hennessy at cox.net> wrote:
>>> Well, I chose the Trible Accutime just to have access to a PPS
>>
>> Your Accutime does provide a TTL PPS signal (by the way, PPS capable
>> GPS receivers are quite inexpensive and common these days) but USB
>> isn't the right way to import the signal.
>
> Since the hardware has a USB connector, what is the right way to
> import the signal if not USB?

Well, the hardware actually has an RS422 connector, on which a very
accurate PPS signal is present.  Your problem is that your PC does
not have an RS422 input and probably not even an RS232 input, and the
manufacturer recognizes that problem and provided an RS422 to USB
converter with the unit.

Now, USB is more or less a network protocol.  The very accurate timing
signal from the GPS is converted to a network message over the USB bus
that is transferred when time permits, and the moment the message
arrives in the PC is much less accurately defined than the timing of
the original hardware signal.

Not that this is a problem in your situation.  The people here are
usually time nuts that are looking at accuracies that are much better
than what you need and what you are achieving now, and at that point
the USB really becomes a problem.
(e.g. on a project I am working on, it is required that the time is
accurate to within a couple of microseconds.   that is 1000 times better
than what you have)

The USB connection causes inaccuracy, but not to the point where you
would not be able to achieve 10ms accuracy.  Don't be misled by people
who claim that USB is your problem.

What might be your problem is that the combination of software and OS
does not handle the PPS timing and only relies on the serial messages
transferred over the same USB bus.  Those messages contain status info
(like which satellites are in view) and the current time and date.
For your unit they can be using the NMEA or the TSIP protocol.
The timing accuracy of such messages is in the range of 5..50ms typically,
and it would explain your problems when those are the only ones
evaluated and the PPS signal being ignored.  At least select a config
where the TSIP protocol is used, as that will be better than NMEA.

I don't know the exact capabilities and limitations of your setup
(Windows 7 plus Meinberg-installed ntpd) as I am using Linux myself.
I do use Windows at work and I do know that it is hard to synchronize
its time to anywhere near what can be done on Linux.

You probably have other reasons why you have to stick to Windows.
So it is best to investigate what Windows can do with PPS today.



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