Friday, June 18, 2010

The 10.6.4 update breaks iMac external display function?

An iMac 27 can serve as an external monitor. Connect a DisplayPort input and it should switch to that.

It used to work. Until now ...
Apple - Support - Discussions - After 10.6.4 update I cannot use iMac ...
I've routinely connected my work laptop to my iMac via the DisplayPort to use it as an external display. This morning this is no longer working. I updated to 10.6.4 last night.
When I plug the DisplayPort cable into my laptop the iMac screen goes black as it should. I can then select dual monitors. For a moment it switches to the laptop setting. When I then click 'Apply' on the laptop to configure dual monitors it gracefully switches back to the Mac desktop (which is wrong).
There may be other causes of course. I strongly suspect a 10.6.4 bug however.
Updates to come if I learn more.

Update: I found a workaround
  1. Set iMac resolution to 800x600
  2. Plug DisplayPort cable into Laptop
  3. In laptop, click on secondary display and set external display resolution to 800x600
It then switched the iMac into display mode. It also jumped to the correct resolution.

Multi-monitor support on Windows has emergent properties, so I can't be sure this is a 10.6.4 bug. We'll have to see how many have trouble.

Update 7/25/2010: The workaround doesn't always work. I've submitted a bug report. I've also noticed that if you unplug a remote display from an iMac, the main display switches to the resolution of the removed remote display. So there are several things broken.

My current workaround is (Windows XP on Dell laptop) as below. I think behavior is different if you've never successfully connected to the external Mac monitor. The key is to switch the Mac to 800x600
  1. Laptop: set NVIDIA control panel to multiple monitor configuration screen. It will show only one monitor.
  2. Switch iMac user account to 800x600 resolution
  3. Plug in DisplayPort cable to laptop
  4. NVIDIA control panel display will switch to dual monitor view. Configure as needed.
Under some conditions you will have a very short period of time to configure the NVIDIA control panel before the iMac switches back to showing the OS X time period. My hunch is that this is the 10.6.4 bug -- a change made to the timing. In earlier releases the OS X desktop remained "black" (waiting for external input signal) for a much longer time, perhaps indefinitely. I suspect the 10.6.4 bug was introduced as a hasty hack for problems where the OS X desktop remained stuck in an inappropriate "black" mode, waiting for a signal that never came.

Update 7/28/10:

Another tip. I have the habit of turning off my iMac's OS X secondary monitor before disconnecting it and plugging in the laptop that will put the iMac in monitor mode. I think this causes problem. The iMac still thinks it has an external display and it won't switch over to being in display mode itself. If I simply unplug the external LCD, without first turning it off, the iMac goes into single display mode. I think this may make a difference.

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