Why use an $800 device to do the work of a $100 device??? That's just plain stupid, don't you think? It would be a waste of an iPad's processor to use it as a plain display. And on top of that, laggy.
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You could convert those diagrams to PDF, then view them on the iPad while programming, plus you could be making annotations of all kinds to it.
I would recommend the GoodReader App for this purpose. It's the most complete and stable of them all.
Combine it with Dropbox for synchronization, and you've got yourself a complete solution.
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Here's a better solution:
http://www.startech.com/usb-video-adapters
For portability, I agree with you.
But as I said before I wouldn't be using an $800 device to do the job of a $100 device. It's like buying a monster truck to deliver pizza in Brooklyn.
I would rather use the iPad as a second computer in my configuration, and take advantage of it this way.
I currently do Email, Skype, and other tasks on the iPad while working on my main Mac in multi-monitor mode.