[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs. PPS decoding cards

Thomas Petig thomas at petig.eu
Fri Feb 17 22:58:29 UTC 2017


Hi,

I was wondering whether there is some data/information available on the
claimed +/- 100 ns jitter?

Regarding the PPS -> USB (using the CTS line of a FTDI FT232R), I
plotted, using some lines of Python, the time offset as attached. Just
to get an overview how it is 'worst case', i.e., user program, python,
etc. The 1PPS signal comes from a GPS rx.
Looks like a standard deviation of around 150 us.
y-axis:  measured pps offset from full second (computer time) in us,
x-axis pps pulse number.

On the long term it looks interesting (while measuring I played with the
NTP server on this computer)
Until ca. second 10000: ntpd synchronization via internet
Until ca. second 17000: made an additional LAN NTP server (GPS) available
Until the end: replaced ntpd with chrony (still using internet and local
servers)

Interesting points:
-It looks surprisingly bad with using the normal ntpd (especially, there
is not really an improvement having an local GPS based server
available, did I do something wrong? Only the offset changes by ca. 3
ms.)
-It looks surprisingly good with chrony. But there are continuously
outliers of up to 4500 us, is this a result of the chrony control loop?
The time is wandering around with ntpd, but has less jitter.

Conclusion:
Despite the 150 us stddev, the using PPS over USB gives some interesting
inside of what the local ntp server is actually doing. It looks to me
like it would be an improvement to use it when using ntpd, but not when
using chrony.

Best regards,
   Thomas
   DK6KD
   SA6CID

PS:
Raw data is here, if you want to zoom in: (1.7 MiB, one row per PPS
offset in us)
http://petig.eu/pps-usb.txt

On Tue, Feb 14, 2017 at 07:26:23AM -0500, Bob Camp wrote:
> Hi
>
> A direct port might be a +/- 100 ns sort of thing most of the time and a +/-10 us
> thing every so often under some OS’s. Most desktop operating systems are not
> designed to prioritize random pin interrupts. A dirt cheap MCU coded with a few
> (hundred) lines of assembly code may be a better option than a typical desktop.
> Complicating this further is the degree to which some OS’s can be directly or
> indirectly optimized. Install *this* package and it all goes nuts. Install that package
>  and not much happens ….
>
> Bob
>
> > On Feb 13, 2017, at 11:07 AM, Ruslan Nabioullin <rnabioullin at gmail.com> wrote:
> >
> > Hi, generally speaking, what are the performance differences between the following: 1. direct RS-232 (i.e., what I believe is a standard PCI card offering RS-232---essentially UARTs interfaced more-or-less directly to the PCI bus); 2. RS-232 via USB; 3. PPS decoding PCI cards (which might also have an IRIG input or even an onboard GNSS receiver).
> >
> > Thanks in advance,
> > Ruslan
> > _______________________________________________
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