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Please use our Code Center to obtain source-code and example projects/solutions:
http://codecenter.softwaretoolbox.com
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 Simple Read Example
The following example builds upon the Tutorial - using with VB.NET.
This paper shows you how to read:
- one-time read of one tag
- one-time read of multiple tags
- subscription reads/exception-based
 Reading a Single Tag:
Simply copy & paste the code below into the Form load event:
'
'the following constants are used here to help make the code more readable
'from an educational perspective.
'Feel free to modify the contents of the values below.
Const strPCName = ""
Const strOPCServer = "
OPCLabs.KitServer.2"
Const strTag = "Simulation.Random"
Const strAccessPath = ""
Const iDataType = 0
Try
'
'define our OPC Web Client
Dim objOWC As New OPCLabs.EasyOPCDANet.EasyDACOM
'
'define the object to receive the value, quality and timestamp
Dim objTagVTQ As OPCLabs.EasyOPCDANet.IVTQNet
'
'now to make the call to read the tag
objTagVTQ = objOWC.ReadItem( _
strPCName, _
strOPCServer, _
strTag, _
strAccessPath, _
iDataType)
' 'now to show the value to the screen, but lets do the right thing here and 'check the QUALITY of the TAG first!!!! '
'0 = bad '192 = good
If Not objTagVTQ Is Nothing Then If objTagVTQ.Quality >= 192 Then MessageBox.Show("Value: " & objTagVTQ.Value.ToString)
Else MessageBox.Show("Value: " & objTagVTQ.Value.ToString _
& " is NOT GOOD quality, quality : " _ & objTagVTQ.Quality.ToString)
End If
End If
'
'now to destroy the objects
objTagVTQ = Nothing
objOWC = Nothing
'
Catch ex As Exception
'
'display an exception to screen
MessageBox.Show(ex.Message)
End Try
Reading Multiple Tags within a single transaction:
Try
'
'create ourselves an opc web client object
Dim EasyDA As New OPCLabs.EasyOPCDANet.EasyDACOM
'
'now create an array to receive our values into
Dim Values As Array
'
'now request the read of multiple items
'I have actually broken this single instruction over multiple lines
'to try and make it more readable.
'You will notice that there are essentially 3 parameters:
' (1) the computer name for the opc server, which is blank in this case
' (2) the opc server
' (3) the array of items to read
Values = EasyDA.ReadMultipleItemValues _
( _
"", _
"OPCLabs.KitServer.2", _
New Object() {"Simulation.Random", _
"Trends.Ramp (1 min)
", _
"Trends.Sine (1 min)", _
"Simulation.Register_I4"} _
)
'
'now to take a look at what we received
If Not Values Is Nothing Then
'
'iterate thru the array
For I As Integer = LBound(Values) To UBound(Values)
'
'lets check to make sure that the value is not an error code type
If VarType(Values(I)) <> VariantType.Error Then
'
'no error here, so lets display the value to a textbox
TextBox1.Text += "Element " & I & ": " & Values(I) & vbCrLf
Else
'
'lets call the "raiseError" method and raise the error returned in
'the value
EasyDA
.RaiseError(Values(I))
'
'display an error to the textbox
TextBox1.Text += vbCrLf & "** " _
& Err.Source _
& ": " & Err.Description _
& " (0x" & Hex(Err.Number) & ")" _
& " **"
End If
Next
End If
Catch ex As Exception
'
'display any exceptions to screen
MessageBox.Show("Error: " & ex.Message)
End Try
Subscription Reads (Exception-based reads)
Subscription reads require that your application run in a Multi-threaded apartment. To enable this in VB.NET you must define a "Sub Main" as your applications entry-point, and apply the "MTAThread" attribute to it.
IMPORTANT: You MUST read the following FAQ - Threading Apartments.
Create a new module and then define a Sub MAIN as follows. Within the Sub Main be sure to open your form/application.
Module Module1
<MTAThread()> Public Sub main()
End Sub
End Module
Now, the code to setup the subscription and receive the event notifications, which may exist anywhere within your application.
In this example the "oItem" is the object that will be "bound" to a Tag subscription. This item needs to be kept alive to keep the subscription alive.
Public oItem As IItemNet 'defined in a global location
Private oSvr As New OPCLabs.EasyOPCDANet.EasyDACOM
Public Sub New()
oItem = oSvr.RequestItem("", "SWToolbox.TOPServer", _
"Channel_0_User_Defined.Ramp.Ramp1")
AddHandler oItem.MeasurementChange, AddressOf itemchanged
End Sub
Private Sub itemchanged(ByVal sender As Object, ByVal e As EventArgs)
Dim theTag As OPCLabs.EasyOPCDANet.IItemNet
theTag = DirectCast(sender, OPCLabs.EasyOPCDANet.IItemNet)
Me.ListBox1.Items.Add(theTag.VTQ.Value.ToString)
End Sub
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