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Re: [questions] Re: GPS+PPS vs NTP server, why a huge offset ?




> On 17 Jun 2022, at 12:52, Jim Pennino <jimp@xxxxxxxxxxxxxxxxx> wrote:
> Daniel O'Connor <darius@xxxxxxxxxxx> wrote:
>> 
>> 
>>> On 17 Jun 2022, at 00:07, David Taylor <david-taylor@xxxxxxxxxxxxxxxx.invalid> wrote:
>>> 
>>> On 16/06/2022 10:00, Thiebaud HUMBERT wrote:
>>>> To do the inversion, I just changed the "Pulse Mode" parameter to "Falling edge" from "Rising edge".
>>>> The offset induced by the "pulse length" has disappeared.
>>>> But there is still an offset of around 10.3ms, which I think is induced by USB as explained in this article about other chipsets (https://lists.freebsd.org/pipermail/freebsd-usb/2019- August/016078.html)
>>> 
>>> Yes, Thiebaud, USB is not good enough for PPS signals!
>> 
>> This is absolutely false.
>> 
>> If you are using it for NTP then GPS+PPS over USB is quite adequate (from personal experience).
> 
> As USB is a two wire interface, there is no such thing as PPS over USB.

The fact USB only has 2 data lines is irrelevant to wether you can send PPS over USB.

> You of course can get the ASCII data over USB, but to get a PPS signal
> you in general have to hack a USB GPS and add a signal wire for PPS then
> hack some interface on the computer to accept PPS.

This is absolutely not true in any meaningful sense.

> If all you need is accuracy in the 2 millisecond range, most recent USB
> GNSS dongles will achieve that without PPS.

You can easily do better than that with GPS/PPS over USB.

It is very easy to setup, readily accessible and cheaply done.

--
Daniel O'Connor
"The nice thing about standards is that there
are so many of them to choose from."
-- Andrew Tanenbaum
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