[ntp:questions] Fwd: Re: NetBSD GPS/PPS using 4.2.6p3

David Lord snews at lordynet.org
Wed Aug 24 11:43:12 UTC 2011

A C wrote:
> On 8/23/2011 21:50, Mischanko, Edward T wrote:
>>> -----Original Message-----
>>> From: questions-
>>> bounces+edward.mischanko=arcelormittal.com at lists.ntp.org
>>> [mailto:questions-
>>> bounces+edward.mischanko=arcelormittal.com at lists.ntp.org] On
>>> Behalf Of A C
>>> Sent: Tuesday, August 23, 2011 8:25 PM
>>> To: questions at lists.ntp.org
>>> Subject: Re: [ntp:questions] Fwd: Re: NetBSD GPS/PPS using
>>> 4.2.6p3
>>> On 8/23/2011 15:27, unruh wrote:
>>>> On 2011-08-23, Uwe Klein<uwe at klein-habertwedt.de>   wrote:
>>>>> unruh wrote:
>>>>>> But from his test, his system is labelling both edges.
>>>>> so he has bounces on the line?
>>>>> either that or rise/falltime is so low and noise so high
>>>>> that receiver hysteris is not sufficient to  supress multiple
>>>>> HL/LH changes?
>>>> No, his test shows that the line changes and then 100ms later
>>> it changes
>>>> back and 900ms later it changes again (ie once per second it
>>> rises and
>>>> falls.) Ie, it is behaving exactly as it should if it were
>>> detecting a
>>>> pulse 100ms long.It is detecting both the rising and falling
>>> edge.
>>> I just checked and the pulse is almost exactly 100 ms low going
>>> and 900
>>> ms high (within about 1 ms) so it's 90% duty cycle high most of
>>> the time
>>> with a swing low.  The signal itself is clean down to microvolt
>>> levels.
>>>    The total voltage swing is about 12 volts (which would stand
>>> to reason
>>> since I'm feeding the TTL level PPS output of the GPS board
>>> through one
>>> channel of a MAX232 level shifter).
>>> Therefore the machine is receiving a nice, clean PPS signal on
>>> DCD (DCD
>>> pin was also verified yet again and is correct by hardware
>>> specifications).
>> [Mischanko, Edward T]
>> Seems like your PPS pulse is inverted?  You may want to add
>> Flag2 1 to your fudge line.
>> server
>> fudge   flag2 0 flag3 1 refid PPS
> I did try that before but I'll try it again and see what happens. 
> However, a low going voltage should be an assert under RS232.  So this 
> signal is swinging to low voltage during the pulse.

If you've had pps working then you probably have flag2 correct
as it's unlikely all your other ntp sources are 100ms out.

"PPS Clock Discipline" from html directory of ntpd-4.2.6p3
has 'flag2 1' = falling (clear) edge as default
and 'flag2 0' = rising (assert) edge


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