HowTo: Access Non-Browsable COM DA Server Items

Introduction

When aggregating OPC COM DA servers, UaGateway constructs OPC UA NodeIds for COM DA items, thus making it possible for OPC UA clients to use these items for Read, Write and MonitoredItems. To be used for OPC UA Services, COM DA items do not need to be browsable. If ItemIds are known, the corresponding OPC UA NodeIds can be manually composed and used in OPC UA clients.

Construct NodeId from Namespace Index and ItemId

When aggregating OPC COM DA servers, an OPC UA NodeId for OPC COM DA items is composed of the COM DA server’s namespace index in UaGateway’s address space and the COM DA ItemId as string identifier.

Example:
For our example, we added Kepware’s KepServerEx to UaGateway as described in HowTo: Add and Connect to a COM Server. By reading UaGateway’s namespace array, we find that the KEPServerEx has received the namespace index 4 (see screenshot).

KepServerEx in UaGateway’s Namespace Array

Together with the ItemId of the item we’re interested in (e.g. Channel_1.Device_1.Tag_3), we can compose the OPC UA NodeId:

Namespace Index: 4
String Identifier: Channel_1.Device_1.Tag_3

Masking “Slash” Character

Special care has to be taken if the ItemId contains a forward slash character. This character is masked with “//” by UaGateway, so it has to be masked as well when entered manually. The ItemId MyItem.A/B, for instance, has to be replaced with MyItem.A//B when entered manually.

Using the Constructed NodeId in UaExpert

How a NodeId is to be entered manually in an OPC UA client is product specific. In UaExpert, you can manually enter a NodeId for reading by clicking the “+” symbol at the top right corner of the Attributes window (see screenshot).

Open Dialog Window

A dialog window for entering the NodeId will open. Choose “4” as namespace index, “String” as identifier type and enter the ItemId to the input field for the identifier part of the NodeId.

Enter a NodeId Manually in UaExpert

Then click “OK” to read the item. The information returned from the server will then be shown in the attribute window (see screenshot).

Custom Node in Attribute Window of UaExpert

Subscribing to an item works similarly: Right-click in the Data Access View and select “Add custom node...” from the context menu (see screenshot).

Open Dialog Window in Data Access View

The same dialog window for entering the NodeId will open. Enter the required information as shown above and click on “OK” to subscribe to the item.