[IRtdServer interface method] Heartbeat [Description] 
If the RTD server is no longer able to process RefreshData method calls, the Heartbeat method enables Excel to pop up a dialog box that says "The real-time data server 'XYZ' is not responding. Would you like Microsoft Excel to attempt to restart the server?" This method returns a Long value; a value of 1 indicates that the real-time data server connection still exists, and a value of zero (0) or a negative value indicates that the real-time data server connection no longer exists.