Hi, I'm trying to use my late 2009 iMac as a display for my 2011 Macbook Pro via display port-display port cable. It works fine when I have the lid open on my Macbook Pro. However when I close the lid the iMac display goes off after a few seconds. It seems like the Macbook Pro is going to sleep. Any ideas on how to prevent this. I have a wired keyboard and mouse connected to the Macbook Pro but the external display still goes to sleep. Any ideas on why this is happening?