Rockwell-automation FactoryTalk Historian SE 3.0 UniInt Interface User Instrukcja Użytkownika Pobierz pdf (Strona 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

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