how can i automate the changing a value of variable

[syrine syrina]

Hi every body,
the application i want to do is explain bellow:
((if i write in the IOFILED and the value existe in my database so a msg box will be shown that the value existe,else a msgbox will show that the value didn't existe.))
my problem is in the data entry. i have to press enter , but in reallity i want when the barcode is scanned so automatically the code will be executed.
if the value change , I want that the msg will be automatically written without pressing enter. I m using wincc professionel. 
I have made a lot of research , and i didn't find any solution. if somenone knows how to do that please help me.
Iwill be thankfull , 
Best regards