Opc data Access Automation Specification

4.4.5.3

 

AsyncRead 

Description 

Read one or more items in a group. The results are returned via the AsyncReadComplete event 

associated with the OPCGroup object. 

Reads are from ‘DEVICE’ and are not affected by the ACTIVE state of the group or item. 

Syntax 

AsyncRead( NumItems As Long, ServerHandles() As Long, ByRef Errors() As Long, 

TransactionID As Long, ByRef CancelID As Long) 

 

 

Part Description 

NumItems 

The number of items to be read. 

ServerHandles 

Array of server item handles for the items to be read 

Errors 

Array of Long’s indicating the status of the individual items to be 

read.  

TransactionID 

The client specified transaction ID. This is included in the 

‘completion’ information provided in the Corresponding Event. 

CancelID 

A Server generated transaction ID. This is provided to enable the 

client to cancel the “transaction”. 

55 

 

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OPC Data Access Automation Specification2.02 

 

Remarks 

The AsyncRead requires the OPCGroup object to have been dimensioned with events (Dim 

WithEvents xxx As OPCGroup) in order for the results of the AsyncRead operation to be returned to 

the automation client application.  The AsyncReadComplete event associated with the OPCGroup 

object will be fired (called) by the automation server with the results of the AsyncRead operation.  

See Also 

IOPCAsyncIO2::Read from the OPC Data Access Custom Interface Specification 

Example Private 

Sub 

AsyncReadButton_Click() 

Dim NumItems As Long  

Dim ServerIndex As Long  

Dim ServerHandles(10) As Long  

Dim Values() As Variant  

Dim Errors() As Long  

Dim ClientTransactionID As Long  

Dim ServerTransactionID As Long 

Dim Qualities() As Variant  

Dim TimeStamps() As Variant 

NumItems = 10 

ClientTransactionID = 1975 

For ServerIndex = 1 to NumItems 

‘ set up which items to be read 

ServerHandles(ServerIndex) = AnOPCItemServerHandles(ServerIndex) 

Next ServerIndex 

OneGroup.AsyncRead NumItems, ServerHandles, Errors, ClientTransactionID , 

ServerTransactionID  

End Sub