Steve, I'm curious: in the absence of a PPS or other electrical output from your "computer", what is the nature of the time "output "? If it's just visual via a display, why not view the light output of the seconds digit with a photocell. Most of the transitions should produce an easily-detectable step or glitch in the light.
Dana On Mon, Jul 8, 2019 at 9:03 AM Steve Summit <s...@eskimo.com> wrote: > This is a different sort of question than what seems to be > discussed here usually, and I apologize if it's wholly off-topic, > but I'm guessing there are some time nuts here who might be able > to give me some pointers. > > If I have two boxes with clocks that are supposed to be perfectly > synchronized, and I need to verify this, and they're physically > right next to each other, and they both have PPS outputs, one > way to verify the synchronization would be to compare the two PPS > outputs with a 'scope. (I'm guessing there's probably a dedicated > instrument out there specialized for the task of comparing two > such PPS signals for frequency, phase and jitter, and if I were > a proper time nut myself I guess I'd know that.) > > My specific question concerns the case that one of the boxes is > a computer, for example, a Linux box with time kept using ntpd > or chrony (perhaps also listening to a PPS signal coming in on a > serial port). But I've never seen a computer with a PPS *output*. > > Is this a reasonable thing to be thinking about, or am I going > about it wrong? How would *you* positively verify synchronization > of such a system? Me, I'm pursuing this idea because too much > of the time (at least in my own, perhaps parochial experience) > synchronization seems to be "verified" either by saying "Yes, > we configured it properly", or by observing a status output > from ntpd or chrony claiming the requisite synchronization > has been achieved, but in neither case by making a definitive, > independent, external, empirical determination. > > (Perhaps PPS is overkill for this situation; perhaps it's the > case that given the inherent inaccuracies of software-based > timekeeping, a software-based query mechanism -- perhaps using > ordinary NTP or PTP protocols -- would be sufficient for > externally assessing synchronization.) > > I guess it's not a *completely* unreasonable thing to be thinking > about, because I've found a couple of web pages (for example > https://www.raspberrypi.org/forums/viewtopic.php?t=115554 and > https://github.com/jsln/pps-gen-gpio) describing how to implement > a PPS output under Linux. And I do realize that trying to > minimize the jitter and latency in this situation (given that > the principal drivers for the hypothetical output are inherently > software-based) presents considerable difficulties. But taking > all of that into consideration, I'm wondering what others think > of the approach. Thanks for any comments. > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
