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The PI Tag Attribute Location4 refers to the scan class # of the particular interface it is associated with via the Point Source. So for example on the interface node in PI ICU, Scan Class 1 had a value of 1 (second), Scan Class 2 had a value of 10 (seconds), then the corresponding scan class number you set in Location4 is how often the interface will scan/check for a new value (Location4 = 2 which is a scan rate of every 10 seconds). Example of interface config below.
Full information of Location4 is in the respective interface user guide. Snip from the OPC Interface user guide below.
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Sometimes it is difficult to login to interface node to see these settings. In that case, I use module database.
PI SMT can show that.
PI SMT > Operation > Module Database
PI DAServer Name > Modules > %OSI > Interfaces > Interface machine name > interface name > Properties > Arguments
It contains /f parameter which is scan frequency.
For showing values, right click > Show > Alias and Property values can be used. It is handy.
As others have mentioned you can see the frequency of scan classes by looking at the Interface Configuration Utility and the Module Database. The specific documentation needs to be reviewed though for each interface that will be used to understand the nuances of each. For example Scan Class 1 on OPC interfaces is reserved for Advise tags and Location3 can change the behavior of other Scan Classes. The PI-to-PI interface uses Scan Class 1 for exception data and others for archive data only. For the UFL interface scan classes can be configured in ICU, but it doesn't use them at all.
Please do note that interfaces can behave differently wrt scan classes. So be sure to always check the configuration of the specific interface to fully understand the impact of a scan class setting.
Please refer OPC DA user guide for more details about location parameters.