[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs. PPS decoding cards

Bob Camp kb8tq at n1k.org
Wed Feb 15 07:33:04 EST 2017


> On Feb 14, 2017, at 9:23 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Tue, Feb 14, 2017 at 7:31 AM, MLewis <mlewis000 at rogers.com> wrote:
>> - a dedicated machine/box for unencumbered acceptance of PPS, and
>> - for systems with a business need, a dedicated NTP server/box disciplined
>> by the PPS source (with dedicated communication), while maintaining
>> internet NTP sources as backup for when the PPS source fails?
>> Is there a better way?
>> Other considerations?
> Don't ever think about "backup servers".  NTP will always select the "best"
> reference clocks.   The best ones are defined as the subset of references
> that track each other.
> Best practice today is to have two independent NTP servers and two GPS
> receivers.  

I would argue that you need at least three servers (and more like five). When given only two 
servers NTP simply dithers back and forth between them. It does not have 
a way to figure out which of two clocks is wrong. It will detect a missing clock, but 
not one that is simply off time by a bit.


> It is best if these are independent as you can make them,
> different buildings if you can.   I would even use different brands of
> hardware to protect against a bug.   Then throughout your company all your
> PCs are configured to look at both NTP servers
> Each server is configured to use the GPS reference clock, the other "twin"
> NTP server as well as about five Internet "pool" servers.
> If your location does not have an Internet connection. ( YES this can
> happen.  I've worked on computers that process classified information and
> these computers never have Internet access.)  You can configure them so
> they run in "orphan mode" that is they all use each other as reference
> clocks.  Then when GPS is lost thenoormal NTP clock selection algorithm
> will select the subset of PCs that all agree on what the time is.   The
> outliers tent to get ignored.    When GPS comes back up the system makes a
> gradual and graceful recovery.
> Chris Albertson
> Redondo Beach, California
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