Chapter 12. Calling OPC UA Methods from DataHub Scripts

OPC UA defines a node type called a Method that allows a client application to synchronously execute a predefined function on the server. The client calls the OPC UA method with zero or more arguments and waits for it to complete. OPC UA servers may use methods as an alternative to executing their internal functions in response to a data value change.

OPC UA methods are synchronous, and normally cannot be executed through a DataHub tunnel, because data flow through a tunnel is always asynchronous. You can, however, use a Gamma script to convert an asynchronous data change into a synchronous OPC UA method call.

Method calls are provided through a class called OpcUaSupport. You can include this in your script by adding the following line to the top of your script:

require ("OpcUaSupport");

The OpcUaSupport.g file is located in the require folder of your DataHub installation.

In your script, you can now create an instance of the OpcUaSupport class:

opcua = new OpcUaSupport(self, "UaClientConnectionLabel");

You must supply a reference to an instance of a class that derives from Application, and the label of an active OPC UA client connection on which the method will be called. This label is configured in the DataHub OPC UA properties option. The instance reference is commonly self, referring to your script instance itself.

This instance of OpcUaSupport can now be used to attach method calls to data point changes:

opcua.AddMethod(triggerPointName, feedbackPointName, parentNode, 
                methodName, triggerType, arguments…);

where:

You can add additional trigger methods by adding your own method to OpcUaTriggeredMethod, like this:

method OpcUaTriggeredMethod.myTriggerFunction(args)
{
    // Implement your logic here.  Somewhere in this code you should 
    // call .uaMethod.call(args);
    // The following variables are defined automatically 
    // in this method:
    // this – the name of the data point that triggered this method
    // value – the value of the data point that triggered this method
    // previous – the previous value of the data point that 
    //            triggered this method
}

You can add your trigger method in your own script, so long as the call to require("OpcUaSupport") is made before your trigger definition. Do not alter OpcUaSupport.g, since it will be overwritten when you upgrade the DataHub application. Look at the definitions of risingTrigger and toggleTrigger in OpcUaSupport.g for examples of how your code can be written.

Example:

The following code will call a method called OpenCloseDoor on the OPC UA node Objects.Refrigerators.Refrigerator #1 whenever the point default:fridge changes value. The feedback will be written to default:fridge_fb. The trigger function will be toggleTrigger, a method defined in OpcUaSupport.g. The value of default:fridge will be passed to the OPC UA method as the only argument.

method MyApp.constructor()
{
local opcua = new OpcUaSupport(self, "UaClientConnectionLabel");
opcua.AddMethod("default:fridge", "default:fridge_fb", 
                "Objects.Refrigerators.Refrigerator #1", 
                "OpenCloseDoor", #toggleTrigger, #eval(value));
}