@fredthank you Fred, I am still working on a solution with the Mac Pro, but a new PC with the GTX10180 will arrive tomorrow. This  project is only using three HD video projectors, so I can use the 4th output from the GTX1080 as a control screen. The Isadora patch currently uses a 3840x1080 stage preview stage with Syphon before splitting it into three stages for the projectors, so it appears using the built-in video output for a control screen is not an option. The three video projectors for development are now running at 1920x1080, but this will change to 1920x1200 when I connect to the large projectors. I have never used an MST hub, but I have now read about it. Is it correct that I can attach an MST hub to a displayport on the GTX1080 and get two independent 1920x1080 signals,  and then  I can drive 4 HD projectors and one control monitor?many thanks, Don