RTD Server

Introduced in Excel 2002
Designed to replace DDE for updating spreadsheets in real-time

An RTD server is a COM Automation server that implement the IRTDServer interface
This allows asynchronous calls which means that the user is not interrupted while the function is calculating
Unlike XLLs, RTD automation servers can pull data from the server automatically when it changes.

The MSN MoneyCentral Stock Quotes add-in is an example of a RTD Server.

The RTD server is instantiated by Excel when a user enters a new function (RTD) into Excel and specifies the RTD server's programmatic ID (ProgID).
Once the server is instantiated, Excel gives it a reference to a callback object and communication occurs via a push-pull mechanism.
That is, the RTD server notifies Excel that data has changed (push) and, when appropriate, Excel requests the changed data from the RTD server (pull).
This push-pull architecture allows for RTD to function even if there is a modal dialog box displaying or if there is something else happening that would normally prevent an update.

support.microsoft.com/kb/285339
msdn.microsoft.com/en-us/library/aa140061(office.10).aspx
msdn.microsoft.com/en-us/library/aa140060(v=office.10).aspx
msdn.microsoft.com/en-us/library/aa140059(v=office.10).aspx

link to E - RTD and also link from RTD to Excel interop page
The RTD function delegates to the COM object that implements the interface IRTDServer

Application.RTD

Returns a real time data object

Application.RTD.RefreshData

Application.RTD.RestartServers

Application.RTD.ThrottleInterval
HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Excel\Options\RTDThrottleInterval

RTD("MyRTDServer.ProgID", "MyServer", "StockQuote", "MSFT", "NASDAQ", "LAST PRICE")