Family troubles: Macs won't talk - 21.5' iMac/MacBook Pro
August 10, 2019 8:17 AM   Subscribe

Help needed getting 21.5' iMac tow as external display for Macbook Pro ...

.. there must be a way. After a week of computer conniptions, with deadlines looming, I bought a new MacBook Pro. I didn't realize that it only has a USB C port, and so quickly added a hub to my purchases. It was only after unpacking everything that I found only 27.5' imacs work in target display mode and that the 21.5 won't.

Has anybody successfully worked out this configuration? I can get cords/adapters etc. Both are running Mojave, and the iMac while sluggish, has no problems.
posted by bwonder2 to Computers & Internet (3 answers total) 1 user marked this as a favorite
This Apple Support page has the details.

You need the following to use Target Display Mode:
* A Mac notebook or desktop with a Mini DisplayPort or Thunderbolt port. If your Mac has Thunderbolt 3 (USB-C) ports, you can connect it using the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.
* A Thunderbolt cable or a Mini DisplayPort to Mini DisplayPort cable
* A supported iMac with OS X v10.6.1 or later

note that it also says iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can't be used as Target Display Mode displays.
You don't mention what version of 21.5" iMac you are using, but if it's fairly recent, it can't be done. The underlying support doesn't exist.
posted by blob at 8:42 AM on August 10 [2 favorites]

Target Display Mode does not exist on the 21" iMacs, as has been pointed out. The one sort of option I can think of to use the 21.5" screen that doesn't involve taking the old iMac apart and literally disconnecting the screen and figuring out how to adapt whatever connection is in it to HDMI/DispalyPort for use with the MacBook Pro would be something like Synergy, but that would only allow you to control the iMac via the MacBook Pro (i.e. the iMac runs normally, and you control it with the MBP's mouse and keyboard).

The cheaper/easier option would be to just get another monitor and figure out what to do with the iMac later, especially if your hub thing has an HDMI port on it (such as this from Anker or this official Apple one) or if you can source the appropriate cable locally. Brand and size don't matter too much; as long as you can get a cable between the machine and the monitor, you'll be good. The 21.5" iMac has a 1080p (1920x1080) resolution, so you might want to aim for at least that resolution, and maybe avoid anything 4K or higher as running those at their true resolution may require more care as far as cables go. Trying to reuse the 21.5" screen is pretty much a lost cause, though.
posted by mrg at 9:59 AM on August 10

Seconding a new monitor. You can get a 21” 1080p monitor for $90, and could easily spend that on various adapters trying to make the iMac work.

And if you want something really nice, the newest set of entry level LG UHD displays are very nice and in the $350 range.
posted by b1tr0t at 10:27 AM on August 10 [3 favorites]

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