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List:       freebsd-hackers
Subject:    Re: system time instability
From:       Hrant Dadivanyan <hrant () dadivanyan ! net>
Date:       2016-12-13 19:52:07
Message-ID: E1cGt7P-0006W1-8E () pandora ! amnic ! net
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> > The server did run for almost a day without PPS and looks stable. I
> > start
> > to believe, to my shame, that I did a mistake when testing this
> > previously.
> > Then the whole post is wrong and cable seems to be most suspected
> > part again.
> > Even now it's hard to understand this wrong behaviour, but anyway ...
> > 
> > Just replaced the cable with shielded one where each pair has
> > separate
> > shield, used dedicated pair for PPS and ground; grounded the shields.
> > 
> > Thank you Konstantin, thank you Ian !
> > Hrant
> > 
> 
> A bad PPS signal could definitely lead to frequency trouble, if the way
> the signal is bad involves ringing, or the electrical level floating
> around the cutoff points for detecting low vs. high level -- you'd get
> false pulses, and some of them would be close enough to the time of the
> real pulse that they would make it through the spike/median filters in
> ntpd.  An early or late pulse looks like a phase step, and several
> consistant-enough phase steps in the same polling period looks like a
> frequency step.
> 
> You mentioned using a 74LS245 bus driver... that can lead to ringing if
> the load is light, maybe the rs232 port on this new hardware has a much
> higher input impedance than your old system.  It might be worth adding
> a series resistor at the computer end to soak up reflections, something
> in the 30-100 ohm range should work.
> 

Wow, thank you, will try !

> -- Ian

-- 
Hrant Dadivanyan (aka Ran d'Adi)		hrant(at)dadivanyan.net
/* "Feci quod potui, faciant meliora potentes." */       ran(at)psg.com
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