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

Chris Albertson albertson.chris at gmail.com
Tue Feb 14 21:23:42 EST 2017

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.   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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