[ntp:questions] Befuddled and confused, again...

unruh unruh at wormhole.physics.ubc.ca
Sun Feb 14 23:41:49 UTC 2010

On 2010-02-14, David Lord <snews at lordynet.org> wrote:
> unruh wrote:
>> On 2010-02-14, Dave Baxter <spam at goes.nowhere.com> wrote:
>>> In article <_6ednYXJW52xiOXWnZ2dnUVZ_u2dnZ2d at supernews.com>, 
>>> lausts at acm.org says...
>>>> On 2010-02-14, Dave Baxter <spam at goes.nowhere.com> wrote:
>>>>> Hi All.
>>>>> Been trying (again) to setup a FreeBSD GPSDNTP server, as I want to use 
>>>>> the currently used Win2k (With the Meinberg binary etc) box for 
>>>>> something else, having found that running anything else "significant" on 
>>>>> it, "distrubes" the NTPD system somewhat.  Nothing wrong with the 
>>>>> Meinberg system (far from it in fact) it's just that I need that machine 
>>>>> to replace an older PC that has finaly expired, so as I have another 
>>>>> with no OS on it, I thought I'd go the FreeBSD route this time.
>>>> A good move!
>>>> Trimmed things a bit...
>>> Hi Tom.
>>> All that I'm trying now, I have done successfully before, but for 
>>> whatever reason it doesnt want to fly now.  Not sure if my notes are 
>>> bad, DT's website, or the F-BSD docs are different now.
>>> I *Need* PPS support, as I have an app that needs mS accruacy.  See:-
>>> http://www.dxatlas.com/Faros/  for details of what I have running, that 
>>> needs such data.   Plus, for whatever reason, my ISP (Demon/Thus/C&W) 
>>> seem unable to provide a stable NTP service these days.
>> You do not need PPS support. Just use something like gpsd, and the shm
>> refclock and you will get usec accuracy (not ms accuracy). 
> That might be a Linux specific feature of gpsd.
> Man page mentions Linux version of gpsd detecting PPS on
> serial DCD line otherwise best accuracy around 10ms.
> On NetBSD I've only tried gpsd + shm with GlobalSat BR-304
> and that doesn't have PPS out and couldn't get anything near
> 10ms from it having tried with large mindist and various
> fudge times. It's on my todo list to try with the Garmin
> gps-18x-LVC in case the PPS DCD support is working on NetBSD
> to compare with using NMEA/PPS and NMEA + ATOM drivers.

There is also the shmpps program, which detects pps either on the serial
or the parallel port and uses the shm driver to deliver the time offset.
Something else is needed to determine the "seconds" while shmpps
determines the usec using the serial or parallel port interrupt drivers. 

I do not know if shmpps works out the box on Freebsd, but it should not
be hard to adapt, assuming bsd supports the shm refclock driver.

> David
>>> I have at present, a seperate Win2k machine, running the Meinberg port 
>>> of NTPD and Dave Hart's serialPPS hack.  It works very well, even 
>>> surviving Windows updates and a reboots etc!
>>> Trouble is, I want that physical PC for something else, and as I had a 
>>> "spare and empty" PC, I decided to revisit the FreeBSD version, after 
>>> all, it only took me 2 days last time to get it going.
>>> I've been at it (trying to get FreeBSD working again) this time for over 
>>> 3 weeks!  And am about to lob the thing out of the window.
>> Install a prepackaged Linux from one of the distros. don't install X or
>> all of the other stuff (openoffice, firefox, etc) to keep the size down.
>> Then on either Linux or bsd, you could use chrony instead, and get even
>> better accuracy and much faster convergence, but since you already only
>> want ms accuracy, that might be overkill except for the faster
>> convergence. 

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