I have a ViewSonic Monitor, and on the back I can hook up two cables if I want. One VGA and one DVI. My PC has a video card that has both. I tryed them both and I know the Monitor it's self is fine. My VGA adapter works fine on the MAC. The DVI adapter is the second one I purchased. and does not work on my MAC. Is it posable that the DVI part in my MAC is not working?