Ask: Sharing Monitors between Computers

Got an Apple, Mac, or iOS tech question? Here's the answer. In this edition of Ask, we'll explain what to do when you're having trouble sharing monitors between your Mac and another computer.

Question

I have two computers and two monitors: a Mac Pro (running Mavericks) with a 27-inch Dell monitor connected using DisplayPort, and a Windows 8 PC with another Dell monitor that’s shared between the PC and the Mac Pro over DVI on the PC. Sometimes I want to use both monitors on the Mac, but if I unplug the monitor from the PC and connect it to the Mac Pro, the Mac recognises the new display, but the screen stays black. After a few seconds the monitor goes into standby. How do I fix this?

Answer

OS X sends a query to the monitor through the video cable when the Mac first boots or wakes from sleep, when you plug in the Mac end of the video cable, and when you hold å in Display Preferences and click the “Detect Displays” button. 

If the monitor doesn’t answer with its display ID at any of these points, the Mac will conclude that there isn’t anything at the other end of the cable and will stop sending video data through it. The monitor, in turn, will drop to standby as soon as it goes a few seconds without receiving a video signal.

The Detect Displays button can force a stubborn monitor to notice you.

It may be that your monitor isn’t responding because it’s only listening to the PC through the DVI port at the time and doesn’t switch over immediately. You could try using the Detect Displays button on the Mac to send a few extra ID queries down the cable and see if the monitor catches any of them. If that doesn’t work, try cycling the power on the monitor first, before you plug it into the Mac. Even if this works, however, it’s a little inconvenient to have to turn the monitor off and on again every time you switch computers. 

However there is a glimmer of hope. There are a few monitor detection bugs in Mavericks that Apple is aware of. If you upgrade to Yosemite, there is a chance that Apple has already fixed this issue in the display driver. The only way to be sure is to actually try it with your specific Dell monitors and your PC.

Got an Apple tech question? Email ask@maclife.com.