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iMac 5K: Adding a 4K External Display

Note that with the NEC display (and most all brands), there is no Thunderbolt daisy chaining; the Mini DisplayPort cable plugs into the iMac Thunderbolt/MDP port and “dead ends”. So this leaves one (1) Thunderbolt port on the iMac for other expansion. It is one of several reasons a Mac Pro might be preferred by some users. However, the display should work at the *end* of a Thunderbolt chain (plug it in last)—not tested but ought to work.

MPG attached the NEC EA244UHD 4K displayto the iMac 5K. Observations:

  • The iMac 5K was able to drive the display in landscape or portrait orientation.
  • The display worked at “Best” as well as at scaled resolution 2560 X 1440.
  • Playing a 4K video clip on both displays simultaneously was smooth on both displays.
  • No apparent issues in Photoshop with the 2nd display attached.
  • While playing Unigine Valley (GPU intensive), mouse movement and image display in a web browser becomes herky-jerky and images draw a bit sluggishly (not that the 'Valley' demo is running at 1/2 res 1/4 pixels on the iMac). On the 2013 Mac Pro, the same activity results in relatively smooth mouse movement and image drawing—definitely superior to the iMac, though not completely unaffected by the activity.
  • Rebooting the iMac 5K, the 2nd display stayed black (“no signal”, odd since the iMac clearly saw it as there and a 2nd display). Rebooting again it appeared. This behavior is sometimes seen with the Macbook Pro also.
  • Drawing the desktop showed a visibly sluggish redraw with both displays upon first logging in.

In-depth testing was not performed, but a 2nd display at 4K resolution appears to work well enough for many uses. However as per above, the iMac 5K falls short as compared to a Mac pro when the GPU is under load (Unigine Valley).

iMac 5K with NEC EA244UHD 4K display attached
(OS X resolution on EA244UHD is erroneous, it is 3840 X 2160)
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