[ntp:questions] Re: ntpq times out if NMEA refclock configured?
Richard B. Gilbert
rgilbert88 at comcast.net
Sun May 14 02:53:41 UTC 2006
R Jenkins wrote:
> I'm trying to add a GPS refclock to my server.
> After total failure with a basic Trimble TSIP output GPS plus the parse
> clock, I'm now using a Garmin GPS25 and the NMEA refclock.
> I'm getting a very strange effect:
> If the NMEA refclock is enabled in my ntp.conf, ntpq stops working, it just
> times out.
> ntptime still works any gives a something like reasonable display.
> I have three other servers listed in ntp.conf & simply commenting out the
> NMEA refclock lines allows ntpq to work again.
> I've tried commenting out all the restrict lines, this does not change the
> This is what I'm adding in ntp.conf - I've also tried it with & without the
> 127.127.22.0 PPS driver & 'enable pps' command.
> # NMEA Clock using Garmin GPS
> server 127.127.20.0 prefer
> fudge 127.127.20.0 refid GPS flag3 1 time1 0.042
> ntptime produces this:
> ntp_gettime() returns code 0 (OK)
> time c810bfbb.0cb40000 Sat, May 13 2006 21:27:39.049, (.049622),
> maximum error 192016 us, estimated error 16 us
> ntp_adjtime() returns code 0 (OK)
> modes 0x0 (),
> offset 0.000 us, frequency -495.911 ppm, interval 4 s,
> maximum error 192016 us, estimated error 16 us,
> status 0x1 (PLL),
> time constant 0, precision 1.000 us, tolerance 496 ppm,
> pps frequency -495.911 ppm, stability 0.000 ppm, jitter 0.000 us,
> intervals 0, jitter exceeded 0, stability exceeded 0, errors 0.
> but ntpq does this after about 10 seconds:
> ntpq -c peers
> localhost.localdomain: timed out, nothing received
> ***Request timed out
> I've previously added the ppskit-lite patch to the kernel, which is 126.96.36.199
> on Centos 4.3 x86-64, Athlon 64 CPU.
> I have udev configured to link /dev/gps0 to /dev/ttyS0 which I believe is
> what the NMEA refclock expects (& also to /dev/pps0 for the pps clock).
> The GPS is on ttyS0 with the PPS signal converted to +/- 12V on pin 1.
> I've tried the standard Centos RPM for NTP & I'm now using one built on the
> machine from the Redhat source rpm (ntp-4.2.0.a.20040617-4.src.rpm).
> This seems to be configured as standard to enable all refclocks & it appears
> to be recognising the kernel PPS capability during the configure stage.
> Any ideas appreciated!
> Robert Jenkins.
After rereading a little more carefully, I notice that your frequency
correct of -495.9 PPM is on the ragged edge of the 500 PPM limit. It is
unusual for a clock to have a freqency error this large; most are below
50 PPM in absolute value.
Does your system have a kernel parameter called "HZ"? Is it set to a
value greater than 100? I believe I have seen references to values of
both 250 and 1000; neither value works well with NTPD. The system seems
to lose clock interrupts when HZ is greater than 100. YMMV but if you
are not using 100, give it a try.
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