Address Space

The DataAccess namespace of the Servers’s address space models the objects and items configured in ACM. When the Server is started, the DataAccess address space is empty. Object nodes and item Data Variable nodes are added as monitored items are requested by the client. This allows the address space to only contain nodes that clients are requesting, rather than modeling the entire SCADA system.

Custom Views of the DataAccess namespace is not supported.

Node Ids and Item Names

The DataAccess namespace is index 2 in the Server. All items requested from ACM must be created in this namespace. All NodeIds in the DataAccess namespace are of type string and have the following format:

ns=2;s=acmObject.itemname

· acmObject – the device, server, or connection object defined in ACM.

· Itemname – the item name as defined in the ACM user manuals

Examples: ns=2;s=MyRtu.40001, ns=2;s=MyRtu.Demand

Object names and Item names in the DataAccess namespace are case sensitive, but within ACM they are not. All connecting client applications should use the same case when defining a monitored item so that multiple nodes are not created in the address space. Example, “myRtu.40001” and “MyRtu.40001” refer to the same item in ACM but will result in two nodes being created in the DataAccess Namespace. Another example is “MyRtu.Demand” and “MyRtu.demand”.

Browsing

The OPC UA Server supports browsing of the address space by a client application. The browsing functionality is done in a virtual manner because the entire ACM configuration is not modeled in the address space; only the objects and items that have an associated Monitored Item are. As a client browses, objects being browsed, such as folders, devices, and connections, are created in the address space if they do not already exist there. Data items associated with those objects can be browsed but are not created in the address space automatically. Item nodes are only created when a Monitored Item for it is requested by a client so at browse time, not all data items in the browse list will exist in the address space.

If a monitored item has been created by a client, any client can browse to that item’s node and do an attribute read of the item. If a monitored item for the node has not been created by a client, browsing to the node will return a BadNodeIdUnknown error when trying to read the attributes.

Monitored items can be directly created from any browse list item using the functionality of the client, usually a drag-drop sequence.

Because browsing is done in a virtual manner by the Server, it does not support browse continuation points.

Browsing of custom Views is not supported.

Monitored Items

Monitored items are used to obtain real time data from the ACM system by OPC UA clients. Nodes representing ACM items do not exist in the DataAccess namespace until a client requests a monitored item for it.

When a monitored item is requested by a client, the item name used is validated by ACM. Refer to the section Node Ids and Item Names for correct node id and item name formats. If the item name is not valid, the Server will return “Bad NodeId Unknown” to the request. If the item name is valid, a Data Variable Node for the item is created in the DataAccess namespace if it does not already exist. Once successfully created, the monitored item is available to publish data to the client.