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Step 3: Reading and writing values

reading

 
There are several ways to read values from an OPC server.

 

 

In the sample, you will select the method from a combo box when connecting.

Both synchronous and asynchronous reads are performed at regular intervals using a timer.

 

 

 

■ Synchronous reading

 

Synchronous reading is performed by issuing the SyncRead method to the OPC group object.

Private Sub timSyncRead_Tick(ByVal sender As Object, ByVal e As System.EventArgs) Handles timSyncRead.Tick
    Try
        Dim Errors As Array = Nothing
        Dim ItemValues As Array = Nothing
        Dim Qualities As Array = Nothing
        Dim TimeStamps As Array = Nothing
        mvGroup.SyncRead(OPCAutomation.OPCDataSource.OPCDevice, mcItemCount, mvSHandles, ItemValues, Errors, Qualities, TimeStamps)
        SbDrawData(mcItemCount, mvCHandles, ItemValues, Qualities, TimeStamps)

 

 

■ Asynchronous reading

 

Asynchronous reading is performed by issuing the AsyncRead method to the OPC group object.

Private Sub timAsyncRead_Tick(ByVal sender As Object, ByVal e As System.EventArgs) Handles timAsyncRead.Tick
    Try
        Dim Errors As Array = Nothing
        mvReadTxId = mvReadTxId + 1
        mvGroup.AsyncRead(mcItemCount, mvSHandles, Errors, mvReadTxId, mvReadCancelId)

 

The result will be received in the AsyncReadComplete event.

Private Sub Group_AsyncReadComplete(ByVal TransactionID As Integer, ByVal NumItems As Integer, ByRef ClientHandles As Array, ByRef ItemValues As Array, ByRef Qualities As Array, ByRef TimeStamps As Array, ByRef vErrors As Array) Handles mvGroup.AsyncReadComplete
    SbDrawData(NumItems, ClientHandles, ItemValues, Qualities, TimeStamps)

 

 

■ Change notification

 

When you set the IsActive property of a OPC group object to True, the group becomes active.

    Select Case CmbType.SelectedIndex
        Case 0  'SyncRead
            timSyncRead.Enabled = True
        Case 1  'AsyncRead
            timAsyncRead.Enabled = True
        Case 2  'Subscription
            mvGroup.IsActive = True
    End Select

 

When a change occurs in the data value or quality flag of an item in the group at the UpdateRate period, the Datachange event is generated.

Private Sub Group_DataChange(ByVal vTransactionID As Integer, ByVal NumItems As Integer, ByRef ClientHandles As Array, ByRef ItemValues As Array, ByRef Qualities As Array, ByRef TimeStamps As Array) Handles mvGroup.DataChange
    SbDrawData(NumItems, ClientHandles, ItemValues, Qualities, TimeStamps)
End Sub

 

 

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write

 

There are several ways to write values to the OPC server.

 

 

 

■ Synchronous writing

 

Synchronous writing is performed by issuing the SyncWrite method to the OPC group object.

Private Sub btnSyncWrite_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSyncWrite.Click
    Dim vServerHandles(mcItemCount) As Integer
    Dim vItemValues(mcItemCount) As Object
    Dim vNumItems = FnGetInputData(vServerHandles, vItemValues)
    If vNumItems > 0 Then
        Dim Errors As Array = Nothing
        mvGroup.SyncWrite(vNumItems, vServerHandles, vItemValues, Errors)
    End If
End Sub

 

 

■ Asynchronous writing

 

Asynchronous writing is performed by issuing the AsyncWrite method to the OPC group object.

Private Sub BtnASyncWrite_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnASyncWrite.Click
    Dim vServerHandles(mcItemCount) As Integer
    Dim vItemValues(mcItemCount) As Object
    Dim vNumItems = FnGetInputData(vServerHandles, vItemValues)
    If vNumItems > 0 Then
        Dim Errors As Array = Nothing
        mvWriteTxId = mvWriteTxId + 1
        mvGroup.AsyncWrite(vNumItems, vServerHandles, vItemValues, Errors, mvWriteTxId, mvWriteCancelId)
    End If
End Sub

 

If you want to receive the write result, define AsyncWriteComplete and receive it.