[Pool] External device sync / PPS / USB

Daniel Norton daniel at danielnorton.com
Wed Mar 2 19:11:02 UTC 2011


On 3/2/2011 11:00 AM, Anssi Johansson wrote:
> Hi, using USB (either via serial port adapters or otherwise) for time
> synchronization is most probably just a waste of your time and money.
> So, in short, don't do it.
>
> A longer answer to this question can be found from
> http://gpsd.berlios.de/faq.html#time , for example.
Thought so. The hidden point not mentioned at that link is that for many
systems (most notably, laptops), serial/RS232 ports are implemented over
the USB bus, so it's not safe to say that a serial/RS232 port is
sufficient. It must be one that is implemented over an appropriate
underlying bus.

On 3/2/2011 11:32 AM, David J Taylor wrote:
> Using a Sitecom USB hub which includes a serial port, I made a
> comparison here, Daniel:
>
>  http://www.satsignal.eu/ntp/NTP-on-Windows-serial-port.html#usb
>
> Yes, it does increase the jitter.  Whilst certainly not as good as a
> direct connection, it's considerably better than a LAN-only connection.
Well, in your setup that might be true (and with the shorter polling
interval), but it's probably dependent on the other USB traffic on that
same hub (and on up to the main bus).

So I guess it's /possible/, but probably inadvisable.

--
Daniel

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