iPump 6420 User’s Manual
www.wegener.com 800070-01 Rev B Chapter 3, Page 91
3.6.4. Time Management
Master Time Reference
The iPump6420 makes use of a real-time clock (RTC) for many different functions, not just
elapsed time. In most missions, the i6420 RTC must be synchronized to some other reference.
In the majority of missions, this is the Compel clock. The way this is done is to make use of the
Compel time-of-day messages (also called “keep alives”, see Section 3.1.1). These are messages
sent to “all” devices which convey a UTC time/date stamp. The i6420 uses these messages to
implement a low-bandwidth timebase-tracking loop. Therefore, the i6420 must see at least one
such message each second for the most effective time tracking, that is to make guarantees on
time tracking for the Local Scheduler and for premium features (see Section 3.7). Of course, if
the Compel system itself has its clock synchronized by NTP to a precision local or internet
source, then the field i6420s, due to the satellite transit delay seen by the Compel stream (~ 250
mS), will see their current time value lag behind those same NTP sources by the same amount.
As an alternate to using the in-channel Compel command stream for synchronizing the
i6420 RTC, the unit may also be set to use NTP direct to a specified source. This is
recommended for the non-satellite-based installations, which will receive Compel control over
the internet and will execute timed events based on a Local Scheduler and its RTC.
The relevant user controls are:
1. Time source control (Compel or NTP)
2. NTP time source, by IP address only
3. Set time (if not using Compel or NTP, generally a test scenario only)
Time Zone management
All timed events loaded into the Local Scheduler show the time of the event as it was set in
the Compel system. To set a local Scheduler line, Compel must tell the iPump6420 what time, in
local Compel time, that the event must occur. Along with this, Compel communicates its own
“timezone rule” to the listening i6420. In turn, the i6420 must translate this command to setup
its Scheduler to show the command execution time in terms of the i6420’s own time. This is
done by translating the Compel time to UTC using Compel’s timezone rule, then translating
UTC to its own time, using its own timezone rule. The “timezone rule” is a logical
concatenation of a time zone offset from GMT, a logical Boolean stating whether a daylight
savings is observed, and the legal dates of that observance in that locality. For instance, the
eastern US uses the America/New_York timezone rule, which implies that the offset from GMT
is -5, and that DST is observed between the 2nd Sunday in March and the 1st Sunday in
November.*
* The set of possible rules are also called the Olson database strings.
Since the local web interface displays the time in terms of the i6420’s own timezone rules,
users monitoring the Scheduler must be aware of the opportunity for confusion. In the event that
the web user’s browser is not running on a PC in the same time zone as the target iPump6420,
then the time values shown in the Scheduler may appear to be in error to the observing user.
When checking the Scheduler, the user is cautioned to check both the time and the timezone rule
setting of the i6420.
The relevant user controls are:
1. Set timezone rule (web or front-panel)