Item Types
Items are the core elements of a group. They are executed, and the values are then used by the group for logic purposes, by other items, and to write to the database. They can be written to from the database or from other items.
Type of Item |
Description |
|
OPC Item |
Directly subscribed to an OPC server at the rate of the group. Executed by the group, so alarms are evaluated when the group is executed. These items are executed even when the trigger isn't active. |
|
Run-Always Expression |
Much like an expression tag, can be either a static value, an expression, or a database query. Run-Always expression items are evaluated at each group interval, before the trigger state is evaluated. |
|
Triggered Expression Item |
Same as Run-Always expression items, except that they are only executed after the trigger has been evaluated and is active. |
|
Tag Reference |
A reference to a Tag. Allows a Tag to be used in a group like any other item type, except that the tag is evaluated by its scan class instead of by the group. For more information, see Tag References vs. OPC Items. |
Execution Order
Items generally aren't executed in a reliable order, with the exception of Expression items. Expression items can be ordered using the up and down arrows located to the right of the list where the items are displayed. This can be crucial for performing complex operations that require a specific sequence.