[time-nuts] Raspberry Pi NTP server

[Steven Sommars]

 I don't want to hijack Andrew's thread.  Just wanted to add to Achim's
comments about jitter and offset.

USB2 devices should accept polls every 125 microseconds.  [My USB knowledge
is limited.]

I have two devices.  One is the Navisys GR701 which I suspect you're
familiar with; it is an integrated GPS + Serial->USB.  The other is a
Garmin LVC18x connected to a Sabrent USB 2 to serial cable.
Both devices report having a PL2303.  Both poll at 1 msec.  The GR701 has a
thin cable.  The Sabrent USB->Serial has a thick cable.







On Wed, Jul 8, 2020 at 6:53 PM Hal Murray <hmurray at megapathdsl.net> wrote:

>
> stevesommarsntp at gmail.com said:
> > My RPi4 (Raspbian Buster) has a GPS+PPS/USB.  Serial->USB uses Prolific
> > PL2303, which supports USB 2.0
>
> > which means 1 msec polling of the PPS signal.   I've been unable to poll
> more
> > frequently
>
> As far as I know, the PL2303, 067b:2303, is an old/slow chip.  (I forget
> the
> right magic USB terms)  Why do you expect it to go faster than 1 ms?
>
> It and the FTDI chip(s) are popular and widely known to be well supported
> on
> Linux.  I'll be very surprised if it goes faster.
>
> What sort of device are you using?  One way to tell if it is likely to go
> faster than 1 ms is the thickness of the wire.  Faster speeds need more
> shielding for EMI reduction (or something like that) which turns into
> fatter
> cables.  It's pretty easy to tell if you have samples of both in front of
> you.
>  I think you can only use the thinner cable if t runs at 1 ms and you hard
> wire the chip to the end of the cable as is typical of a GPS mouse.
>
>
>
> --
> These are my opinions.  I hate spam.
>
>
>
>
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