Mac Pro (Early 2009), Mac Pro (Mid 2010), Mac Pro (Mid 2012): Issues with three displays and multiple DVI, HDMI connections
If you have three displays connected to each graphics card in one of the following configurations:
a Mac Pro (Mid 2010) or Mac Pro (Mid 2012) that has either the ATI Radeon HD 5770 or ATI Radeon HD 5870 graphics upgrade kit
two ATI Radeon HD 5770 graphics cards
a Mac Pro (Early 2009) that has a ATI Radeon HD 5870 graphics card
... one of the following issues may occur:
When using a DVI connection, a Mini DisplayPort, and an HDMI connection, the display connected via DVI may turn black.
When using a DVI connection and two HDMI displays, the display connected via DVI may turn black.
When using a DVI connection and a HDMI connection, adding an additional HDMI connection or a Mini DisplayPort connection may cause the initially-connected HDMI device to no longer appear in the Graphics tab of System Profiler.
If you use three displays with an ATI Radeon HD 5770 or ATI Radeon HD 5870 card, you should use only one DVI or HDMI device.
Note: The Mini DisplayPort to DVI Adapter (different from the Apple Mini DisplayPort to Dual-Link DVI adapter) is considered a single-link, passive DisplayPort device. However, you should use only dual-link adaptors (also known as "active DisplayPort" adaptors) with multiple display configurations. Single-link DVI to Mini DisplayPort (or VGA) adaptors may not work as expected in multiple display configurations.
If you use three displays, you should connect at least two of them with a Mini DisplayPort. You can also use the Apple Mini DisplayPort to Dual-Link DVI adapter or the Apple Mini DisplayPort to VGA adapter or just connect the devices to the graphics card using their native Mini DisplayPort connectors.
Note: The Apple Mini DisplayPort to Dual-Link DVI adaptor and Apple Mini DisplayPort to VGA adaptor are considered active adaptors.
Display adapters (whether they're made by Apple or a another manufacturer) should be connected directly to the display and the video card. You should not "daisy chain" or otherwise connect a secondary video adapter to them.
To learn more about active and passive adaptors and using multiple displays, see this article by AMD.
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