Overview

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 alerts are evaluated when the group is executed. These items are executed even when the trigger isn't active.
Run-Always Expression Item	Much like an expression SQLTag, 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.
SQLTag Reference	A reference to a SQLTag. Allows a SQLTag 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. See SQLTags vs. OPC Items below for more information.

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.

It is easy to confuse the definition and purpose of SQLTags and OPC items in transaction groups, though they have distinct benefits.

SQLTags may be referenced inside of groups, however it is critical to remember that they are executed by the Ignition gateway, according to their scan classes, and independently of the group. Adding a SQLTag into a group is like creating a shortcut to that tag. However, once in the group, the item can be used like any other item. That is, it may be mapped bi-directionally to the database, used as a trigger, be the target of another item, etc. It is even possible to create an hour meter out of the item. Core properties of the tag such as alerting and scaling, however, are defined in the actual SQLTag, not in the group.

OPC Items in groups (as well as expression items in groups), however, are completely executed by the group. They do not exist outside of the group in which they are defined. They are subscribed and evaluated according to the rate of the group.

Generally speaking, it is most common to create OPC items in groups, even if a particular point might already exist in SQLTags. This leads to more understandable group execution, as all evaluation occurs in the group according to the timer and trigger settings. SQLTag references are useful when it is necessary to have a single value in multiple groups, for example, as a trigger in order to coordinate execution.