[ntp:questions] Problem troubleshooting gps/pps on freebsd 8.1

Peter S pst at telia.com
Thu Dec 16 15:19:34 UTC 2010

> Hi..
> I have successfully done that with FreeBSD V8.0, and following the
> instructions "To the letter" it works fine, all on an old Gateway small
> format desktop PC.  The GPS (Garmin GPS16LVS) is powered by the
> reduntant mouse port too.

Hi Dave B.
The instruction didn't work 100% as when I installed 8.1 as it
installed another ntpd but an older one in another directory which
took me a while to catch because the version numbers didn't match.
Also my serial ports are named /dev/cuau0 and /dev/cuau1.

> You do have to configure the GPS RX to only spit out 1 posibly 2
> sentences (I went for GPRMC only.)   What GPS RX are you using?   Have
> you made sure with a 'scope for example, that the PPS signal is getting
> to the DCD pin, and is still enough volts, and wide enough to trigger it
> (he does suggest using a pulse of 200ms, but it's the leading edge
> that's used, not the entire pulse as such.)
> Some COM ports are a little reluctant to work relaibly with just a 0..5V
> signal, so it may need a buffer.

I have some electronics between the GPS and com port.
The scope looks like this:
(DCD on channel one and Rx on channel two)

I think it the voltage levels are OK.
I hope I use the correct flank on the DCD???

> One thing I did notice...   Your ref clock server addresses  
> Is that right?   Should be ???
> Assuming you got the symlink OK.
> link  cuau0 gps1
> From that blog page...  It does say:-
> It has the special IP address 127.127.20.x where x is the number of the
> GPS unit in /dev/gpsx.
> And illustrates that with:-
> server    mode 0  minpoll 4 maxpoll 4   prefer
> fudge    flag1 1  flag3 1 refid GPS
> Doing all that "*Exactly*" as he describes does work.  I know, if I can
> make it work, the instructions must be OK! :-)

I used this instead:
link	cuau0 gps0

I just used gps0 instead of gps1 since it seemed more logical. So that
why it's

and I have "mode 16" because I use 9600 baud instead of 4800.

> Unless something is significantly different between FreeBSD V8.0 and 8.1
> And of course, you did recompile the Kernel, to support PPS?  It has to
> be asked...
> Best Regards.
> Dave B.

Yes I did and I think the correct why to check if it works is to do:
[root@ ~]# ntpdc -c kern

And then look for this:
pps frequency:        25.103 ppm
pps stability:        1.537 ppm
pps jitter:           2.367e-06 s
calibration interval: 256 s
calibration cycles:   188
jitter exceeded:      24
stability exceeded:   0
calibration errors:   1

Thank for your help. It was the ntpd bug that was the primary problem
but there may still be other problems.


More information about the questions mailing list