catch key stroke before displaying in an entry

Hello List

I have an application with a main window an several Entry's (input 
boxes). I would like to check the pressed key before it is displaied in 
the selected entry or even if no entry is selected (main window 
selected). I need this to determine if the input is comming from the 
keyboard or from a barcode scanner which is connected together with the 
keyboard (and acts like a keyboard, I can add some post chars for the 
barcode reader).
I tried to "bind" the "KeyPress" event to the main window and hoped this 
 will be the first callback which is called after pressing a key, but 
it seems to be the last callback. Then I could check the char and decide 
if  the callback should  break or the next call back should be executed.
I hope somebody can tell me an easy way to do this.

Thanks in advance,

Thomas
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