Hi,
I gone a bit confused 
You said
one can scan number using RF Gun should displayed on the input field (i.e. for Material Number), but he can not enter the value in the input field manually through RF Key pad... i.e. the value should only come from the scanner and not from the keyboard..
So you were referring to a field in the screen that shows the scanned number if so how come making a screen field display only impact scanning.
Cheers,
Arindam