Data Change Events and Subscription

Introduction to Data Change Events

Using the read and readasync functions described in Read Data from an Item, you can obtain information about OPC server item values upon request. The OPC Data Access specification provides another mechanism for clients to get information on server item values. This mechanism allows the OPC server to notify a client when a server item value or quality has updated. This mechanism is called a data change event. OPC Toolbox™ software supports data change event notification by executing a MATLAB^® function when a data change event is received from a connected OPC server. This section describes how to use the data change event notification.

Configure OPC Toolbox Objects for Data Change Events

A data change event occurs at the dagroup object level. Using dagroup object properties, you can control whether a data change event is generated for a particular group, the minimum time between successive events, and the MATLAB function to run when the event notification is received and processed by OPC Toolbox software. You can also control which items in a particular group should be monitored for data changes. In this way, you can control the number and frequency of data change events that MATLAB has to process. On a busy OPC server, you can also turn off data change notification for groups that you are not currently interested in.

The following sections describe how to control data change notification.

Control Data Change Notification for a Group describes how to turn off data change notification for a dagroup object.
Temporarily Disable Items in a Group describes how to control which items in a group must be monitored for data changes.
Customize the Data Change Event Response provided information on how to configure the MATLAB function to run when a data change event occurs.

Control Data Change Notification for a Group

The following properties of a dagroup object control whether a server notifies the group of data changes on items in that group:

UpdateRate: The UpdateRate property defines the rate at which an OPC server must monitor server item values and generate data change events. Even if a server item's value changes more frequently than the update rate, the OPC server will only generate a data change at the interval specified by the update rate.
Subscription: The Subscription property defines whether the OPC server will generate a data change event for the group. When you create a dagroup object, the Subscription property is set to 'on'. When you set the Subscription property to 'off', you tell the OPC server not to generate data change events for that group.
Active: The Active property must be 'on' for data change events to be generated. When you create a dagroup object, the Active property is set to 'on'. When you set the Active property to 'off', you remove any ability to read data from the group, whether through read operations or data change events.

A summary of group read, write, and data change behavior for the Active and Subscription properties is given in the following table.

Active	Subscription	Read	Write	Data Change
`'on'`	`'on'`	Yes	Yes	Yes
`'on'`	`'off'`	Yes	Yes	No
`'off'`	`'on'`	No	No	No
`'off'`	`'off'`	No	No	No

Temporarily Disable Items in a Group

You can temporarily disable items in a group without deleting the item from the group. When you disable a daitem object, the OPC server no longer monitors changes in the associated server item's value, and will therefore not generate data change events when the value of that server item changes.

You can disable a daitem object by setting that object's Active property to 'off'. You can reenable the daitem object by setting the Active property to 'on'.

Force a Data Change Event

You can force an OPC server to generate a data change event for all active items in a group by using the refresh function with the dagroup object as the first argument. The OPC server will generate a data change event containing information for every active item in the group.

You can pass an optional second argument to the refresh function to instruct the OPC server where to source the data values that are sent back in the data change event. By specifying a source of 'device', you instruct the OPC server to update the values from the device. By specifying a source of 'cache' (the default) you instruct the OPC server to return values from the OPC server's cache.

How OPC Toolbox Software Processes Data Change Events

OPC Toolbox software uses data change events for a number of tasks. The following activities take place when a data change event occurs:

The Value, Quality, and TimeStamp properties of the daitem object are automatically updated. For more information on these properties, see OPC Data: Value, Quality, and TimeStamp.
If the dagroup object is logging, the data change event is logged to memory and/or disk as a record. For information on logging, see Log OPC Server Data.
If the dagroup object's DataChangeFcn property is not empty, that function is called with the data change event information. By default, this property is empty, since data change events occur frequently. You can customize the behavior of the toolbox by setting this property to call a function that you choose. For information on the data change event, see the reference page for the DataChangeFcn property.
Note
If you disable data change events by setting the Subscription property to 'off' or the Active property to 'off', none of the activities listed above can take place. You cannot change the Active or Subscription properties while a dagroup object is logging, otherwise the logging task may never complete.

Customize the Data Change Event Response

One of the activities that occurs when OPC Toolbox software receives a data change event from the OPC server is the running of the function defined in the DataChangeFcn property. By setting this property to a the name of a function that you have written, you can fully customize the data change event behavior of the toolbox. For example, you may configure a dagroup object to monitor a server item that is updated from an operator interface. By pushing a button on the operator interface, the server item value will change, initiating a data change event on that group. By configuring the DataChangeFcn property to run a MATLAB function that performs control loop optimization, you can allow an operator to initiate a control loop performance test on all critical control loops in the plant.