Me.T3_Port.ReceivedBytesThreshold = 41

Sub T3_Port_DataReceived(ByVal sender As Object, ByVal e As _
  System.IO.Ports.SerialDataReceivedEventArgs) Handles T3_Port.DataReceived
 
    If T3_DatenPruefen() = True Then 
    .....
    endif      
End Sub
 
 Function T3_DatenPruefen() As Boolean
       T3_DatenPruefen = False
 
        With T3_Port
            Try
                Dim count As Integer = T3_Port.BytesToRead
                ReDim T3_ByteArray(count - 1)
                T3_Port.Read(T3_ByteArray, 0, 41)
                If count = 41 Then
                    If T3_ByteArray(0) = 255 And Checksumme(T3_ByteArray) = _
                      True Then
                        T3_DatenPruefen = True
                    End If
                End If
            Catch ex As Exception
                MessageBox.Show(ex.Message, "Fehler beim Empfangen" & _
                  "EingangsComPort...")
            End Try
        End With
    End Function