PLC to Excel with DDE on events

Hi,

I use DDE link to get tag data from a PLC to Excel via RSLinx Classic.
Currently, I get all the data every seconds, but I would get them on trigger, when PLC values change.
For example, I want to get the values of some scales and I don't want to miss data (if 2 measurements are the same, get values regularly will be a problem).
So is there a way to doing this ?

Thanks for any help.

EDIT : I've seen that I could use "Unsolicited Messages" in RSLinx topic configuration, but I don't know how to configure it.

I've found some answers here (p.13) : http://literature.rockwellautomation.com/idc/groups/literature/documents/ap/1756-ap002_-en-e.pdf
The problem is that I don't know what to put in the "Destination Element" in the MSG configuration ...