How to carry out entry validation in SM30 with events

Hi people,

I'm creating a Z table with Maintenance view in SM30...i've modified the view so all fields are required as Rich H explained in some other post. But now i need to validate user input for a particular field against a standard table, and force to reenter if data is not valid. I've been looking up the available events...event 05 triggers when the line is new, but not when modifying. I've tried a couple of other events but don't serve my purpose. What i want is to validate data upon user's entered all the required fields (whether the line was preeexistant or not). If data is valid then go on, if not trigger a message and force correction prior continuing (disallowing saving).

Anyone knows how to do this?

Many thanks