>>I see nobody answered this, did you ever solve the problem?
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>>What about an alternate interface that uses a combobox instead. In your case the user tabs or clicks to the textbox, then presses ctrl-n. With a combo then could just press "n". Saves a keystroke. Or you cold use radio buttons or in the valid of the text box replace "n" with "negative". Just some ideas.
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>This could'n work. Because texboxes has to be flexible with values. I mean not only "Negative " or "Positive"

Or make a textbox class that has two small buttons beside it for "postive" or "negative" to be loaded into the textbox. Just a thought.