Date: Tue, 13 Dec 2016 23:52:07 +0400 (AMT) From: Hrant Dadivanyan <hrant@dadivanyan.net> To: Ian Lepore <ian@freebsd.org> Cc: Konstantin Belousov <kostikbel@gmail.com>, freebsd-hackers@freebsd.org Subject: Re: system time instability Message-ID: <E1cGt7P-0006W1-8E@pandora.amnic.net> In-Reply-To: <1481654211.1889.346.camel@freebsd.org>
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[ Charset ISO-8859-1 converted... ] > > 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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