Subscription Items Tab

Use

On the Subscription Items tab, you can subscribe to tags for an agent instance, as well as change and delete the subscriptions. You thereby specify the tags that you want the source system to transfer in the notification process. In the case of static notifications, the subscription items are valid and active for the entire agent instance as soon as the agent instance has been started.

Procedure

If the agent instance contains versioned notifications, you can select via a tree on the left-hand side whether you want to maintain subscription items for static notifications or for an individual notification version of a versioned notification. Select the relevant node. If your agent instance only contains static notifications, the tree is hidden.

You can choose Browse for Tags to browse the namespace of the source system and select one or multiple tags. Alternatively, you can choose Add New Subscription Item Manually to add an individual tag specifically. (See also: Adding Tags.) The selected tags are then displayed in the table of subscription items with their attributes:

Field	Description
Name	Specifies the tag name.
Source	Specifies the name of the tag in the namespace of the source system, for example, Channel1.DataTypes.Boolean.
Deadband	Here you can specify an absolute value for the deadband. The deadband respresents a limit value for filtering out tag values. The deadband specifies the value by which the tag value must change at least so that PCo sends a notification message to the destination system. Example Example 1 You have specified the value `1` as the deadband here. The last measured tag value was 33. The subsequent tag value is 34.5. The difference is 1.5. Result: 1.5 > 1 In this case, PCo sends a notification message to the destination system. Example 2 The last measured tag value was 33. The subsequent tag value is 33.5. The difference is 0.5. Result: 0.5 < 1 In this case, PCo does not send a notification message to the destination system.
Only Changes	You use this indicator to specify that you only want PCo to send a notification message to the destination system if the tag value has changed in comparison to its predecessor. If the value remains the same, no notification message is sent. This allows you to avoid the sending of superfluous notification messages. Note You can use this indicator in particular for nonnumerical tags, for example, tags with the data type String. It is recommended that you use the deadband for numerical tags since, due to measurement fluctuations, it is very unlikely that the value of a tag is the same in two consecutive notifications.
Data Type	Specifies the data type of the selected tag. The data type is determined automatically, for example, during browsing. The available data types might be restricted for technical reasons and in addition be limited by the data types that are available in the .Net Framework. If a data type is not supported, the tags are not offered for selection during browsing. They are only displayed but cannot be adopted into the list and need to be added manually. (See also: Unsupported Data Types for OPC UA Applications.)

To change the attributes of a subscribed tag, select the row of that tag and choose Edit Subscription Item.

The system displays the Edit Subscription Item dialog box with all the attributes of the selected tag. All the attributes can be changed.

If you are using static notifications, the name and data source need to be unique within all static notifications of an agent instance. Therefore, it is not possible to use a tag name or a source twice within the same static notification or in different static notifications of the same agent instance.

If you are using versioned notifications, the name and data source only need to be unique within a notification version. Therefore, within an agent instance in different versioned notifications and even in different versions of the same notification, you can use tag names and data sources multiple times.

Caution

The expression for the data source cannot be checked by the source system when the subscription items are maintained manually. Any incorrect spellings are registered when the agent instance starts and are traced in the log for the agent instance.
To delete subscription items, select one or more rows and choose Delete Selected Subscription Items.
Click on the title row of the table to sort the list of subscription items by the content of the selected column.