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I have a MBP early 2011 model and when running on the integrated GPU I'm unable to dual screen. The screen is a VGA monitor running at 1400 x 900 yet it won't display so do I always have to switch to my AMD GPU to run dual screens? I'm running gfxCardStatus to switch between them and i'm just trying to save power.

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Are you going to accept an answer? You have two good (and correct) ones, mine and Gio's. –  CajunLuke Nov 12 '11 at 0:55
    
Sorry thought I had. –  Dean Jan 5 '12 at 16:13
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3 Answers

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No, it does not. You need the discrete GPU to run an external display.

Funny though, because the integrated GPU's of the MacBook Air can power up to a 27" thunderbolt display. But that's how Apple rolls.

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No, it appears that using an external display requires using the discrete graphics card.

The reason for this is probably how the electrical connections are hooked up. Usually with a switchable-graphics setup like this, the internal display is connected to the integrated GPU and the external display is hooked up to the discrete GPU. In discrete graphics mode, the integrated GPU acts as little more than a framebuffer, routing the discrete GPU's signals to the internal screen. In integrated graphics mode, however, the discrete GPU is off, so nothing can talk to the external display.

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No, a dual-display setup should work. I think the problem may be that it is a VGA monitor. I cannot troubleshoot your problem based on the information at hand, but in my experience recent Apple computers (and the iPad) are unreliable in sending video to VGA or other analog video displays. Can you test this with a different external monitor with DVI, HDMI or DisplayPort input, using the appropriate connector dongle?

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No only have display/thunderbolt to VGA. –  Dean Oct 12 '11 at 15:32
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I can confirm that a brand new MacBook Pro 13" (late 2012) with integrated graphics only can not use an external VGA monitor, but it works fine with an external DVI monitor. Also, two different MacBook Pros (15" and 17") with discrete graphics can use the same VGA monitor using the same adapter and VGA cable. –  Simon Apr 17 '13 at 8:50
    
So that is it, then. It depends on whether your MacBook has the integrated Intel graphics processor on the motherboard, or whether it has a discrete nVidia or ATI graphics card. I have been telling people for years that the computer industry is trying to discard support for the ancient VGA specification, and everybody should avoid using it, but it has not died yet. –  Wheat Williams Apr 17 '13 at 13:52
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