On 2019-07-16, Andreas Mattheiss <please.post@publicly.invalid> wrote: > Hello, > > I have a cheap GPS hooked onto my PC via a MAX232 level converter - the > PPS goes directly into the DCD pin of a native serial port. > > Everything is fine; after some time ntpd locks onto the PPS signal, and > any other servers from the net show a small offset in ntpq -p. > > What is the correct way of dealing with this? We are talking about 3 ms > here. Is it appropriate to use a fudge on the PPS to bring it in line with > the mean of a handfull of other servers from the net, or is is more > appropriate to use the PPS's pings face value, not using any fudge?
A couple of possibiliites-- your connection to the net is highly assymetric, so that it send path is 6 ms longer than the receive path.That will make the ntp software think that everything outside is later than it is. Or, you are triggering pps on the falling rather than the rising edge of the pulse. Oh, a third possibility-- your level converter is really really really slow. GPS PPS (even cheap ones) should be good to 1 usec, not 3 ms. > > Thanks, regards > Andreas > _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
