Data Access deals with the representation and use of automation data in Servers.
Automation data can be located inside the Server or on I/O cards directly connected to the Server. It can also be located in sub-servers or on other devices such as controllers and input/output modules, connected by serial links via field buses or other communication links. OPC UA Data Access Servers provide one or more OPC UA Data Access Clients with transparent access to their automation data.
The links to automation data instances are called DataItems. Which categories of automation data are provided is completely vendor-specific. Figure 1 illustrates how the AddressSpace of a Server might consist of a broad range of different DataItems.
Figure 1 – OPC DataItems are linked to automation data
Clients may read or write DataItems, or monitor them for value changes. The Services needed for these operations are specified in OPC 10000-4. Changes are defined as a change in status (quality) or a change in value that exceeds a client-defined range called a Deadband. To detect the value change, the difference between the current value and the last reported value is compared to the Deadband.