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Items are the backbone of a Transaction Group. They represent a link between a source value (derived from either an OPC value or an expression) and a cell in a database table. 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. Below is a listing of each type of item.

Tag References and OPC Items

It is easy to confuse the definition and purpose of Tag reference items and direct OPC items in Transaction Groups.

Tags may be referenced inside of Transaction Groups through a Tag Reference Item. Since the source of the Tag reference item exists outside of the Transaction Group, they have their own rules and configurations that determine when their value changes. Thus Tag reference items can have their value update according to their own execution (commonly, a Tag Group). Adding a Tag 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. Tag 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.

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.

Refer to the Properties Table at the bottom of this page to see the properties for both Tag and OPC Items.

Expression Items

Expression Items are items not driven by a PLC. Instead they allow you to configure a static value, or use some other means to automatically set a value, such as a query. They are useful for executing comparisons, simple math, and looking up values from other database tables.

Much like OPC Items, Expression Items can have alarms configured, as well as numeric scaling applied directly to the item. 

Scope

It is important to understand that an Expression Item only exists within its group, and can not be referenced by items in other Transaction Groups, Tags, and any components on a window. The only exception is to use an Expression Item to store/retrieve a value with storeVariable() and getVariable() functions. These functions store values in a space that is accessible by all Transaction Groups.

Execution Order

All Expression items will evaluate in order from top to bottom. This means referencing an Expression Item above will pull the new value, but referencing an Expression Item below will give you the value from the last group execution.

Expression Type

How an Expression Item determines its value depends heavily on its type. 

Refer to the Property Table at the bottom of this page to see the Expression Item properties.

Run Always vs. Triggered Items

Expression Item can be configured in two different evaluation states:

• Triggered: The Expression Item executes only when the Transaction Group is triggered. However if the group is not configured to execute on a trigger, then the item will evaluate every time the group executes (similar to how the Run-Always state works). This is the default evaluation state new Expression Items use. 
• Run-Always: The Expression Items will run before the group trigger is checked, so it always executes at the group's rate. This allows your expression to always evaluate regardless of the trigger in the group. Additionally, this state allows you to use the Expression Item as the trigger for the group. We advise that you never have a Target for a Run-Always Expression item because it always runs.
  
  

Changing the Evaluation State

Toggling between the two modes can be accomplished by dragging and dropping the Expression Item to either the Run-Always Expression Items table or the Triggered Expression Items table. Alternatively, the evaluation state can be changed by editing the Expression Item and toggling the Run-always (ignore trigger) checkbox

SQL Queries and Expressions

Expression items can use SQL statements and Ignition's Expression language to automatically determine the value of an Expression Item. This is useful in scenarios where you want to use a value from the database as the trigger for the Transaction Group, or aggregate several other items in the group into a single value. 

Expressions and queries on an Expression Item can reference the value of other items in the group or Tags in the system by clicking on the Tag  icon.

There are several Expression functions available that exist only for Transaction Groups. You can find them in the Store and Forward and Variables sections of the f(x) function list.