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Why does "NTP" indicate "Not Valid" in the NTP Reference Status table

Question

Why does "NTP" indicate "Not Valid" in the NTP Reference Status table whether or not there are available NTP Peers?

Answer

This is an often-asked question about the web browser.

With the newer (black and charcoal background) web browser, available in software versions 5.1.0 and above:
The Reference Status table (Home page of the browser, as shown below ) reports the validity of the input references (not the unit's outputs) that the time server can sync with. NTP output status is indicated on the left side of the Management-> NTP Setup page of the browser.

User-added image



With the "classic interface" (white background) web browser:
The Reference Status table (Status -> Time and Frequency page of the browser, as shown below ) reports the validity of the input references (not the unit's outputs) that the time server can sync with. NTP output status is indicated at the top of the Status -> NTP page of the browser. 

NTP Not Valid
User-added image


Additional information for all software versions
 NTP is a unique input reference in that unlike GPS and other continuous inputs, NTP is only a periodically present input reference (for one second each minute). NTP is only declared "valid" when it’s the selected reference (peers and servers have been configured and NTP is the highest priority valid reference).  Otherwise, if NTP peers have been added, but NTP is not the selected reference, NTP input is not declared valid.
 
When NTP is not the selected input reference (GPS is selected for instance), the NTP “Time” and “1PPS” indicators will show NTP as “Not Valid”, except for one second each minute, it will blink on.  But if NTP becomes the selected input reference, both of the NTP indicators should turn and remain solid green, as long as NTP is selected. If another higher priority reference becomes present and valid (NTP is no longer the selected reference),  both of the NTP indicators will go back to indicating NTP is “Not Valid” again

While watching this page of the browser refresh, if it refreshes at the same moment that the LED blinks green (NTP not selected), you will see it momentarily be lit green.  Otherwise, it will appear to always be "Not valid". 

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