Do you have DCS ?
Basically once you set up that particular area, you can enable it or disable it with a safety out. I'm home right now, i will give you more details later
So, you want to allow the robot to enter or to be inside the zone as long as the safety device is not broken
Oh !!! light screen = light curtain If your "safety device" is a light curtain the solution is not that simple, Do a search in the forum for "background program logic", there's a good explanation how to do it