Custom Error message in VF11

Hi All,

In the Tcode VF11 i would like to display an error message with some validations when user press enter after entered the value .

I have tried in some userexits and the log i got is from the standard errors. I think my custom error message interrupting the standard process.

Can you please tell me , if i need to display the error message , do i need to display by the LOG option and not directly in the screen?.

Please share your ideas!

Thanks,

Pradeep.