[ntp:questions] How to debug GPS PPS?

clayj at nwlink.com clayj at nwlink.com
Mon Dec 28 16:27:05 UTC 2009


Pardon me for jumping in here - I have exactly the same config Tom
mentions (Oncore GPS, TAPR interface board); but have been using the NMEA
driver and trying to hack the Linux kernel (Ubuntu) to do PPS.

This seems like perhaps an easier way - Tom, can you post the details on
the OnCore driver?

Thanks in advance!

Clay
N7QNM


> On 2009-12-28, xyz-2041 <xyz2041 at gmail.com> wrote:
>> Plugged in the GPS unit's serial cable into a Windows
>> computer running WinOncore12 v1.0 (Build 37):
>>
>>   http://www.google.com/#q=WinOncore12Installation.exe
>>
>> Seemed to work without any problems.  Generated
>> all sort of graphs and charts.  Let it run and it
>> told me exact latitude, longitude and height above
>> sea level.
>>
>> Used an analog volt meter and from pin 5 (supposed
>> to be ground), I only noticed voltage on pins 2 and 3.
>> Pin 2 was going erratically negative once a second.
>> I believe that this must be received data as per
>> standards.  Pin 3 was +5 volts, but dropping to
>> -5 volts once per second.
>>
> You did not mention how your Oncore receiver was being interfaced to
> your computer or it's model number.  I use one of the TAPR boards and
> Oncore UT+ combination.  The NTP refclock driver for the Oncore includes
> the code for receiving the PPS signal on the computer DCD pin.
>
>> I've changed my configs a bit, /var/log/ntp.log and
>> /var/log/ntpd.log don't show any errors.
>>
>> However, "ntpq -c pe" still doesn't show any response:
>>
>>  GPS_NMEA(0)     .GPS.            0 l    -   16    0    0.000
>> 0.000   0.001
>>  PPS(0)          .GPS.            0 l    -   16    0    0.000
>> 0.000   0.001
>>  GPS_ONCORE(0)   .GPS-.           0 l    -   16    0    0.000
>> 0.000   0.001
>>
> You should not be using GPS_NMEA or PPS.  The type 30 refclock
> communicates with the receiver using Motorola Binary protocol.  You may
> need to use WinOncore to set your receiver communications to use the
> binary protocol or even better, reset the receiver to factory defaults.
> Your config files already have your position defined, so there is no
> need to have anything in your receiver at startup.  The almanac will be
> received while NTP is settling down after startup.  You will need to
> configure at least 4 other NTP servers to speed up the initial startup.
> The refclock code wants to have a synchronized NTP server before adding
> itself to the peer selection.  I find about 30 minutes after a cold
> start my PPS LED will start to flash and I observe that my Oncore gets
> selected.
>
>> - Create symbolic links:
>>
>>     ln -s  /dev/cuad0  /dev/oncore.pps.0
>>     ln -s  /dev/cuad0  /dev/oncore.serial.0
>>     ln -s  /dev/cuad0  /dev/gps0
>>     ln -s  /dev/cuad0  /dev/pps0d
>>
>> - Create /etc/devfs.conf links:
>>
>>     link cuad0 pps0
>>     link cuad0 gps0
>>     link cuad0 oncore.pps.0
>>     link cuad0 oncore.serial.0
>>
>>
> I think that you will find that the symbolic links in /dev are not
> required and
> only the /etc/devfs.conf entries are all you will need.  You won't need
> either pps0 or gps0 entries for anything.  Only the oncore* stuff is
> needed for the refclock type 30.
>
> Tom
>
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