Err, sorry, I wasn't very clear was I? I meant keep the period input as text but have the duty cycle controlled by a slider.

But - I am easy either way. It is Copernicus' masterpiece, not mine!

However I do think that if there are any hard limits then the UI should let you know somehow. A slider is an obvious indication that you are at the end. Text inputs can be limited by a number of digits but then you get to a situation like you experienced a few posts back when the input only took 999 even though the actual limit was 4999 because it refused the fourth 9.