Spectracom SecureSync User Reference Manual page 166

3.6 OTHER Setup Pages
When to use the "User" reference
The "User" reference is designed to fulfill the following use cases:
a.
Temporarily, no external references are available (e.g., during system setup, or for test-
ing/simulation purposes), or
b.
No external references are required (e.g. if SecureSync is used solely to synchronize com-
puters on a network, with no need for precise timing.)
This is why the "User[x]" reference becomes invalid once SecureSync is reset or reboots, or once
the Holdover Time expires (whichever occurs first), and needs to be manually set again ( Edit Sys-
tem Time > Manual Time Set ).
In order to allow use case (b.) above, two additional steps are required to override SecureSync's
default settings intended to avoid outputting an illegitimate System Time as a valid reference:
i.
In the Edit System Time window, the checkbox Synchronize to Battery Backed Time on
Startup must be checked. This will override SecureSync's default setting to declare the
System Time valid only if it is synchronized to an external reference (or a Local System
time, see "Input Reference "Local System"" below.)
ii.
In the Oscillator Settings window, the Holdover Timeout ought to be set to a duration that
meets your accuracy requirements (up to 5 years): Once this Holdover Timeout window
expires, SecureSync goes out of sync, and you need to manually set a new System Time .
Using the "User" reference with other reference priorities
If the "User[x]" Reference is used in conjunction with other, external reference priorities (such as
GNSS or IRIG), the System Time should be set as accurately as possible:
Otherwise, the large time correction that needs to be bridged when switching from a lost reference
to a valid reference, or from a valid reference to a higher-priority reference that has become avail-
able again, will cause NTP to exit synchronization. If the difference is under 1000 seconds, NTP
will remain in sync and will "slew" (over a period of time) to the new reference time.
3.6.5.2 Input Reference "Local System"
The Local System Reference is a unique input reference in that it can be used as either the Time
reference or the PPS reference, but never both.
When the Time reference is configured as Local System , the Time that SecureSync powers up
with is considered valid, as long as the PPS input reference is valid. The same applies the other
way round, i.e. with a Local System PPS reference.
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Caution: Operating SecureSync with a manually set "User" time bears the risk of
inadvertently outputting an illegitimate System Time thought to be a valid reference
time.
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3 SecureSync User Reference Guide Rev. 18