Simulink by default can tune a constant block to any value, it does not know that the input must be limited to 255 unless you specify that. However, as in the documentation, the input to this block is between 0 and 255 so you can calculate the duty cycle by n/255. So the 200 is a duty cycle of 0.78. If you pass a higher value, it probably defaults to a duty cycle of 1, or it might throw an error. It would be betst to use a saturation block or the appropriate data type to keep the range between 0 and 255 at all times.
Regards,
Joel
