Rockwell Automation FactoryTalk Historian SE 3.0 UniInt Interface User Guide - Manual (Page 13)

UniInt Interface User Manual 7
Parameter Description
an offset of 5 seconds, and the second scan class has a
scanning frequency of 7 seconds with no offset.
When no offset is specified, the scan class will be scheduled
for immediate execution. That is, the interface will not wait for
a well-defined moment in time before scanning when no offset
is specified.
When an offset is specified, the scans occur at well-defined
moments in time according to the formula:
scan times = (reference time) + n(frequency) + offset
where n is an integer and the “reference time” is midnight on
the day that the interface was started. In the above example,
“frequency” is 60 seconds and “offset” is 5 seconds for the
first scan class. This means that if the interface was started at
05:06:06, the first scan would be at 05:07:05, the second scan
would be at 05:08:05, and so on.
There is no guarantee that a scan will occur at the interval
defined by the scan class. If the interface is under a large
load, then some scans may occur late or be skipped entirely.
See the section called “Performance Summaries” for more
information on skipped or missed scans, page 27.
Sub-second Scan Classes
One can also specify sub-second scan classes on the
command-line such as
/f=0.5 /f=0.1
where the scanning frequency associated with the first scan
class is 0.5 seconds and the scanning frequency associated
with the second scan class is 0.1 seconds.
Similarly, sub-second scan classes with sub-second offsets
can be defined, such as
/f=0.5,0.2 /f=1,0
Wall Clock Scheduling
Wall-clock scheduling means that the scans will occur at the
same time every day, even across daylight savings time
transitions. The interface implicitly assumes that certain scan
classes should be scheduled according to a wall clock. For
example, the interface implicitly assumes that
/f=24:00:00,08:00:00 means that the scan should
occur once a day at 8 AM. The period of the scan is actually
25 hours during daylight saving time to standard time
transitions and is 23 hours during standard time to daylight
savings time transitions.
Wall-clock scheduling is assumed only when an offset is
explicitly defined for a scan class. For example,
/f=24:00:00 means that the scan should occur once
every 24 hours even across daylight savings time transitions.
Even when an offset is explicitly defined, wall-clock scheduling
is implicitly assumed only if the scan would ordinarily occur at
the same time every day. For example, the scans associated
with
/f=23:00:00,08:00:00 would occur at 8 AM the
first day, 7 AM the next day, and so on. The interface will
perform the scan once every 23 hours even across daylight
savings time transitions since there is no implicit reason to