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

Paul tik-tok at bodosom.net
Tue Aug 5 01:03:42 UTC 2014


On Sun, Aug 3, 2014 at 4:07 AM, Rob <nomail at example.com> wrote:
> 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.

Actually the problem isn't really USB induced latency.  The problem is
getting a USB to RS-NNN interface that will usefully provide DCD (or
possibly CTS) to NTPD.  For instance in the OP's case there may be an
Acutime Starter involved.  Yes the provided UIM converts RS-422 to USB
but it doesn't propagate PPS.

I've read that people use USB to transport PPS but the details always
seem a bit sketchy.  The spec sheets for some FTDI parts say yes but
others say no.  Even if the part supports it the pin(s) may not be
connected.  The only interfaces I'd have any confidence in are the
ones with proper specs from the microcontroller hobbyists like Pololu,
Adafruit or Tindie.

E.g. http://www.pololu.com/product/391


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