[ntp:questions] LOCL clock reachability not 377?
martin.burnicki at meinberg.de
Thu Jul 31 08:43:20 UTC 2014
> A C <agcarver+ntp at acarver.net> wrote:
>> ATOM always stops when the prefer peers die even if there are other
>> peers available but not marked prefer. I'm still running an older
>> development version (4.2.7p270) that I modified to remove all the prefer
>> code so that the selection algorithm continues to run normally without
>> shutting down the ATOM driver as peers come and go. If all the peers
>> die then ATOM will stop, too since there's no more selected clock.
> Yes this is where the problem is. The ATOM clock needs to have some
> way of determining that the PPS pulses are valid, and the kludge to use
> the existing "prefer" keyword to indicate a clock that is to be used
> for that is causing all the trouble.
> The designer probably believed that any clock that provides PPS would
> also output some serial stream with less accurate time and also status,
> and that this would then be marked prefer. When that clock is OK, the
> PPS is also OK.
> However, that is broken. Not only do you probably not want to mark
> that clock prefer (external references are often more accurate than the
> serial NMEA time, for example), but also you may have two or more ATOM
> PPS clocks, each with their own status, and there is no way to do that
> with this method.
I've already proposed some times ago that another way of assigning PPS
signal(s) to other time source(s) would be more versatile:
This would also provide a simple way to declare a PPS signal "reliable",
e.g. if it is derived from a Rubidium or so, in which case it could
continue to be accepted even though other time sources become unreachable.
On the other hand, if a PPS input signal is associated to a particular
time source the PPS signal could be discarded if the associated time
source becomes unreachable.
> Also, you may have (I do have) a GPSDO that provides PPS and status info,
> but no time. So I know when the PPS is valid, but I don't have an NMEA
> or TSIP or whatever stream to feed to a reference clock driver.
I'm sure there would also be a way to configure such specific setup.
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