Introduction

The OPC XML-DA standard and the currently widely used OPC DA V2 standard address the same tasks but the server interface is quite different, due to their different base, web services respectively DCOM. Developers of OPC client applications have to be aware of these differences. The sample client applications show how the Read and Subscribe/Poll functions are used from different kinds of applications. The VB.NET and C# samples use the .NET web services support, the HTML/ASP scripts use the MSSOAP toolkit.

Background

Read the XMLDA.NET White Paper for a description of how web services differ from DCOM and how this influences the design of client applications. Evaluation versions of .NET based OPC XML DA servers a clients can be downloaded.

Using the Code

VB and C# applications typically use the .NET features to access web services. Desktop and ASP.NET applications can access OPC XML-DA servers in the same way. XML-DA has a stateless design that fits well into ASP.NET applications. The code snippet shown below uses the Read function to periodically read the value of two items from the server. The XML-DA Read function is self contained and not dependent on any previous calls.

  ' create the server object and set the call options
  Dim Srv As localhost.OpcXmlDA = New localhost.OpcXmlDA
  Srv.Url = "http://advosol.us/XMLDADemo/ts_sim/OpcDaGateway.asmx"
  Dim ReqOptions As localhost.RequestOptions = New localhost.RequestOptions
  ReqOptions.ClientRequestHandle = "test"
  ReqOptions.ReturnErrorText = True
  ReqOptions.ReturnItemName = True

  ' Data structures for an XML-DA Read call, initialized to read 2 items
  Dim ReadReqList As localhost.ReadRequestItemList = New ReadRequestItemList
  ReDim ReadReqList.Items(1)    ' Item request list for 2 items
  ReadReqList.Items(0) = New localhost.ReadRequestItem       ' item 1
  ReadReqList.Items(0).ItemName = "Test/Integer"
  ReadReqList.Items(1) = New localhost.ReadRequestItem       ' item 2
  ReadReqList.Items(1).ItemName = "Test/Float"

  ' Data structures for the read reply data
  Dim reply As localhost.ReplyBase
  Dim ReadReplyList As localhost.ReplyItemList
  Dim Errors() As localhost.OPCError

  While 1 = 1            ' periodically read the items
    Try                  ' access the server
      reply = Srv.Read(ReqOptions, ReadReqList, ReadReplyList, Errors)  

      '  display the result
      Console.WriteLine()
      Dim i As Int32
      For i = 0 To ReadReplyList.Items.Length - 1
        If ReadReplyList.Items(i).ResultID Is Nothing Then  ' display value
          Console.WriteLine(ReadReplyList.Items(i).ItemName + " = " + _
          Convert.ToString(ReadReplyList.Items(i).Value))
        Else                               ' display error code
          Console.WriteLine(ReadReplyList.Items(i).ItemName + " = " + _
          ReadReplyList.Items(i).ResultID.Name)
        End If
      Next
    Catch ex As Exception
            Console.WriteLine("Error:  " + ex.Message)
    End Try

    Threading.Thread.Sleep(2000)
  End While

Optional parameters are a source for common mistakes. The .NET web services interface has an 'enable' parameter with each optional value type parameter. If this parameter is not set to 'true', then setting the optional parameter has no effect.

      subscription.EnableBuffering = True
      subscription.EnableBufferingSpecified = True

The usage of the XML-DA functions is quite simple but initializing the request lists with the required definitions for hundreds of items need quite a bit of code. In applications with many items, it is better to define the request lists in an XML file and in the application only call a helper method that initializes the request list object from the XML file. Advosol's XML-DA Client Component offers such support. The code snippet below shows how a read call is done using the SDK helper classes. Download the XML-DA Client SDK evaluation version to explore this further.

   Private Sub btnRead_Click_1(ByVal sender As System.Object, _
              ByVal e As System.EventArgs) Handles btnRead.Click
      Dim srv As XmlServer = New XmlServer(cbURL.Text)   ' server object
      Dim readItemList As ReadRequestItemList
      readItemList = embeddedList.GetReadItemList("OCSTKSimulatedData")
      Dim reply As ReplyBase
      Dim options As RequestOptions = New RequestOptions
      options.ReturnErrorText = True
      options.ReturnItemName = True
      Dim rslt As ReplyItemList
      Dim err As OPCError()
      Try
         reply = srv.Read(options, readItemList, rslt, err)
         If rslt Is Nothing Then
            MessageBox.Show(err(0).Text, "Error at Read")
         Else
            Dim txt As String = ""
            Dim iv As ItemValue
            For Each iv In rslt.Items
               txt += iv.ItemName
               If iv.ResultID Is Nothing Then ' success
                  txt += "   = " + iv.Value.ToString() + Environment.NewLine
               Else
                  txt += " : Error: " + iv.ResultID.Name + Environment.NewLine
               End If
            Next
            tbItems.Text = txt
         End If
      Catch ex As Exception
         MessageBox.Show(ex.Message, "Exception in Read")
         Close()
      End Try
   End Sub

HTML and ASP applications need to make the OPC XML-DA server access from client-side respectively server-side scripts. The source code download contains samples with JavaScripts and VB scripts. They show how to use the Read and the more complex but also more efficient Subscribe/Poll functions. The XML-DA web service call is made by building the SOAP request message using the MSSOAP toolkit. The code snippet shows how a Read request message is created.

function CreateXMLDAReadRequest(ServerURL)
{
    NSxmlda = "http://opcfoundation.org/webservices/XMLDA/1.0/";
    var mobjWS = new ActiveXObject("MSSOAP.SoapClient30");
    ErrText = "";
    if( ! mobjWS )
        ErrText = "SoapClient NOT created";

    mobjWS.ClientProperty("ServerHTTPRequest") = "true";
    mobjWS.MSSoapInit( ServerURL + "?wsdl", "OpcXmlDaSrv", "OpcXmlDaSrvSoap" );

    //-------------------- We are going to build the SOAP request message
    var oTempXml;
    var strXML;
    DocXml = new ActiveXObject("msxml2.domdocument.4.0");
    if( ! DocXml )
        ErrText = "DomDocument NOT created" ;

    strXML = "";
    DocXml.loadXML( strXML );
    oTempXml = DocXml.createElement("Read");
    oTempXml.setAttribute( "xmlns", NSxmlda );
    // create Options Node
    DocXml.documentElement.childNodes(0).appendChild( oTempXml );
    oTempXml = DocXml.createNode(1, "Options", NSxmlda );
    oTempXml.setAttribute( "ReturnErrorText", "true" );
    oTempXml.setAttribute( "ReturnItemTime", "true" );
    oTempXml.setAttribute( "ReturnItemName", "true" );
    oTempXml.setAttribute( "LocaleID", "en" );
    DocXml.selectSingleNode("//Read").appendChild(oTempXml);
    // create ItemList node
    ItemListNode= DocXml.createNode(1, "ItemList", NSxmlda);
    DocXml.selectSingleNode("//Read").appendChild( ItemListNode );
    
    DocXml.setProperty("SelectionNamespaces", "xmlns:xda='" + NSxmlda + "'");
    return DocXml ;
}

License

This article has no explicit license attached to it, but may contain usage terms in the article text or the download files themselves. If in doubt, please contact the author via the discussion board below.

A list of licenses authors might use can be found here.

OPC XML-DA Sample Clients