Add items

Just as you add groups to the server's OPCGroup object, you add items to a group object. Items must be valid fields in the DeltaV database. The ItemID is the DeltaV path to the item (area name is not included). An example path note "OPCSAMPLE/STEP_COUNT.CV". The same item may exist in multiple groups. There are a few settings that are changed at the item level. They are:

RequestedDataType - This is the data type of the item that the client application would like to receive. For example, to request the String representation of a Named Set use a requested data type of string (VT_BSTR) instead an integer value (VT_I4).

ClientHandle - This value is any unsigned 32-bit integer. The DeltaV OPC Data Access Server will provide this handle when returning data in an asynchronous connection. This allows the client application to identify the item to which the data refers.

IsActive - The client application can change the active state of an item at any time using the SetActiveState command. If an item is not active the DeltaV OPC Data Access Server will not keep its value up to date. The client application will not be notified of changes. Read requests to cache will return successfully however, the data value will be stale and the quality status will be "bad" (for a detailed description of statuses, refer to Appendix B). Read requests to "device" always return the current value if no errors are encountered.

Note

AccessPaths and Blobs are defined in the OPC specification but are not required for the DeltaV system. These variables are ignored.

The DeltaV OPC Data Access Server returns the following information about the item:

Canonical Data Type - This is the native data type of the item.

Server Handle - This is an unsigned 32-bit integer. Clients give this handle to the DeltaV OPC Data Access Server to identify an item. The DeltaV OPC Data Access Server uses the group server handle with the item server handle to identify the item.

AccessRights - This is the access rights allowed to a given item. The return values are Read only, Write only or Read/Write.

Note

The DeltaV OPC Data Access Server assumes that all items are read/write. If a client application attempts to write to a parameter that is in fact read only in the DeltaV system, the write will fail, and the DeltaV OPC Data Access Server will return an error to the client.

Add two functions to the OPCServer class, AddItem and RemoveItem. They should be public functions and should return an integer.

Add the following parameters to the AddItem function:

Public Function AddItem(GroupName As String, ItemID As String, DataType 
As Variant) As Long

In the AddItem function, add the following code:

    Dim anOPCGroupHandle As Long
    Dim anOPCItemCollection As OPCItems
    
    Dim AddItemCount As Long
    Dim anOPCClientHandles() As Long
    Dim anOPCItemIDs() As String
    Dim anOPCItemServerHandles() As Long
    Dim anOPCItemServerErrors() As Long
    Dim anOPCRequestedDataTypes() As Variant
    Dim anOPCAccessPathss() As Variant
    
    Dim Msg As String
    
    AddItem = 0
    On Error GoTo ErrorHandler
    
    'Look up group
    anOPCGroupHandle = LookUpGroup(GroupName)
    If anOPCGroupHandle <= 0 Then
        MsgBox "Error - Group Not Found"
        Exit Function
    End If
    
    'Link item collection to group
    Set anOPCItemCollection = MyGroups(GroupName).OPCItems
    anOPCItemCollection.DefaultAccessPath = ""
    
    AddItemCount = 1
    ReDim anOPCClientHandles(AddItemCount)
    ReDim anOPCItemIDs(AddItemCount)
    ReDim anOPCItemServerHandles(AddItemCount)
    ReDim anOPCItemServerErrors(AddItemCount)
    ReDim anOPCRequestedDataTypes(AddItemCount)
    ReDim anOPCAccessPathss(AddItemCount)
    
    anOPCClientHandles(1) = anOPCItemCollection.Count + 1
    anOPCItemIDs(1) = ItemID
    anOPCRequestedDataTypes(1) = DataType
    anOPCAccessPathss(1) = ""
    
    anOPCItemCollection. AddItems AddItemCount, anOPCItemIDs, _
             anOPCClientHandles, anOPCItemServerHandles, _
             anOPCItemServerErrors
             ', anOPCRequestedDataTypes, anOPCAccessPathss
    'Test for Error
    If anOPCItemServerErrors(1) >= 0 Then
        'Item was added
        anOPCItemCollection.GetOPCItem(anOPCItemServerHandles(1)).RequestedDataType 
        = DataType
    Else
        MsgBox anOPCItemServerErrors(1) & ": Error adding item " & ItemID
        Msg = AnOpcServer.GetErrorString(anOPCItemServerErrors(1))
        MsgBox Msg
    End If
        
    'Return the item's unique server handle
    AddItem = anOPCItemServerHandles(1)
    
    Exit Function
    
ErrorHandler:
    MsgBox "AddItem: Error!" & vbCrLf _
           & "Err.Number = " & Err.Number & vbCrLf _
           & "Err.Description = " & Err.Description & vbCrLf _
           & "Err.Source = " & Err.Source & vbCrLf
End Function

Add the following parameters to the RemoveItem function:

Public Function RemoveItem(GroupName As String, ItemID As String) As Integer

In the RemoveItem function, add the following code:

    Dim anOPCGroupHandle As Long
    Dim anOPCItemCollection As OPCItems
    
    Dim RemoveItemCount As Long
    Dim anOPCItemServerHandles(1) As Long
    Dim anOPCItemServerErrors(1) As Long
    
    Dim Msg As String
    
    RemoveItem = False
    On Error GoTo ErrorHandler
    
    anOPCGroupHandle = LookUpGroup(GroupName)
    If (anOPCGroupHandle < 0) Then
        MsgBox "Error - Group not found"
        Exit Function
    End If
    
    'Link item collection to group
    Set anOPCItemCollection = MyGroups(GroupName).OPCItems
    
    RemoveItemCount = 1
    anOPCItemServerHandles(1) = LookUpItem(anOPCGroupHandle, ItemID)
    If (anOPCItemServerHandles(1) < 0) Then
        MsgBox "Error - Item not found"
        Exit Function
    End If
    
    anOPCItemCollection.Remove RemoveItemCount, _
        anOPCItemServerHandles, anOPCItemServerErrors
    'Test for Error
    If anOPCItemServerErrors(1) >= 0 Then
        'Item was removed
    Else
        MsgBox anOPCItemServerErrors(1) & ": Error removing item " & ItemID
        Msg = AnOpcServer.GetErrorString(anOPCItemServerErrors(1))
        MsgBox Msg
    End If
    
    RemoveItem = True
    
    Exit Function
    
ErrorHandler:
    MsgBox "RemoveItem: Error!" & vbCrLf _
           & "Err.Number = " & Err.Number & vbCrLf _
           & "Err.Description = " & Err.Description & vbCrLf _
           & "Err.Source = " & Err.Source & vbCrLf
End Function

RemoveItem sets the ItemDisp interface to nothing. This releases the interface in the OpcServer.

Add a support function for looking up the group handle from the name. The function should be called "LookUpItem". It is a private function with the following declaration:
```
Private Function LookUpItem(GroupHandle As Integer, ItemID As String) 
As Integer
```

In the function LookUpItem, add the following code:

Dim done As Integer
Dim Count As Integer
LookUpItem = -1
On Error GoTo errHandler
done = False
Count = 0
While Not done
    If Count > Groups(GroupHandle).ItemCount Then
    done = True
    ElseIf ItemID = Items(GroupHandle, Count).ID Then
    LookUpItem = Count
        done = True
    Else
        Count = Count + 1
    End If
Wend
Exit Function
ErrHandler:
    MsgBox Err.Description
Exit Function

This function takes the Server Handle for the group and the Item ID. If the client handle is not known, the application calls LookUpGroup with the group name. LookUpGroup returns the client handle for the group.

Save the project and run with full compile to highlight any errors.

Related information

OPC constants for Visual Basic

Use the new class