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

MLewis mlewis000 at rogers.com
Wed Feb 15 00:38:12 EST 2017


On 14/02/2017 9:23 PM, Chris Albertson 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.   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
That dual set model is new to me. Interesting to see its fall-back on 
failures. And the offline model.

It's the poor-man's version of that model that I was aiming for (and 
one, not two sets of receiver-with-server):
- A small box as "GPS receiver" with NTP, receiving the PPS from a GPS 
timing module.
- That box as a source to an NTP Server that also looks at six internet 
pool sources (pools are the 'backup' if GPS/receiver-box fails).
- My systems (two boxes) look to the NTP Server for their time reference.

With all everyone has responded with, as a novice to this I have a lot 
of reading to do...
then some choices to make.

Thanks to all

Michael





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