Scan Classes

The Scan Classes list defines the time period between interface scans in terms of hours (HH), minutes (MM), and seconds (SS). The scans can be scheduled to occur at discrete moments in time with an optional time offset specified in terms of hours (hh), minutes (mm), and seconds (ss). If HH and MM are omitted, then the time period is assumed to be in seconds.

• To create a new scan class, click the new scan class button, enter a scan class value in the field provided, and press Enter.

• To delete a scan class, select it from the list and click the delete button .

• To reorder a scan class, select the scan class to be moved and click the appropriate arrow key to adjust it up or done in the order of scan classes.

  Note: Deleting scan classes or changing their order can adversely affect the operation of existing PI points, which are closely related to scan rates. Scan classes should be adjusted only by PI System Managers who are fully aware of the PI System configuration and the effects of any such changes.

• To modify a scan class, right-click on a scan class and select Edit Scan Class. This opens the Scan Frequency field for editing. Click the Save settings icon in the upper navigation bar, or enter Ctrl+S, to save a modification.

There is no limit to the number of scan classes that can be defined. PI points are associated with a particular scan class via the Location4 PI point attribute. For example, all PI points that have Location4 set to 1 will receive input values at the frequency defined by the first scan class. Similarly, all points that have Location4 set to 2 will receive input values at the frequency specified by the second scan class, and so on.

Two scan classes are defined in the following example:

• Scan class 1: 00:01:00,00:00:05

• Scan class 2: 00:00:07

or, equivalently:

• Scan class 1: 60,5

• Scan class 2: 7

The first scan class has a scanning frequency of 1 minute with an offset of 5 seconds, and the second scan class has a scanning frequency of 7 seconds. When an offset is specified, the scans occur at discrete 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. Since no offset is specified for the second scan class, the absolute scan times are undefined.

The definition of a scan class does not guarantee that the associated points are scanned at the given frequency. If the interface is under a large load, then some scans may occur late or be skipped entirely. For more information on skipped or missed scans, see the "Hit, skipped, or missed scans" topic in the PI Universal Interface (UniInt) User Guide.