I have an Id field and I would like to prevent anyone from typing in letters just numbers. the field has been set as a text field and is linked to others.??
Then you need to create an input mask for that field. Check the help files for more info on using Input Masts. Something like this would force a two digit numeric entry...
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As Mile-O-Phile suggested, using an event will allow you more options like customizing user error messages. Search the forum for the keyword "KeyPress" and the user "ghudson" for I know I have posted how to use the keydown or keypress event to not allow numbers or letters.