Subscribe to atom changes
The user can subscribe to changes of any atom in any object of the earlier listed types. You cannot subscribe to I/O mode or object attributes, since they are part of the configuration and will never change.
Example: Showing the current process value
In Microsoft Excel this is done from a cell in a worksheet. Assume you want to subscribe to the process value of object q1. You write:
=IGSS_DDE | q1 ! Value
This will make Microsoft Excel subscribe to the atom "Value" (the actual process value) for the object "Flow_1" in the server "IGSS_DDE" (our server). The Microsoft Excel value #N/A is shown immediately while the conversation is being established. When the conversation is established, a request is sent to get the current value which will be shown in the cell. In future, the contents of the cell will change each time the value in the database changes and always show the current value. If the subscription fails, maybe because the object does not exist, Microsoft Excel will show the error value #REF!
Notes
If a client is too busy to handle notifications so that it has not handled the previous notification when a new one arrives, the previous notification is lost. However, the newest value will always be available. This usually occurs in overloaded systems.
All advice loops involving system objects are stopped, if the active configuration is stopped.
Both request, poke and advice loops will be rejected, if data collection is not running.