iMac 27” mid 2011 target display mode not working

emmajunli wrote:

Thanks for the tip. Yup the target disk mode works. So we can rule out the possibility of a bad thunderbolt 2 cable connection. Too bad I’ve already ordered a new thunderbolt 2 cable… Plus the issue persists - iMac still won’t display MacBook Pro

I’m no hardware expert but I gather that a Thunderbolt cable carries 2 signals (besides power): data and video. There is still a chance that your current cable fails on the video signal (and not on the data signal) and that the replacement cable might work.

So now we know why Apple dropped Target Display Mode, even when it is supposed to work, it doesn't work.

Video on the Thunderbolt cable is also digital data, ones and zeros, unlike the old analog cables that carried current.

It's a mystery to me why it ever works. 😉

There is still a possibility that the Thunderbolt cable you have is deficient in some way.

I'd try again with the new cable when it arrives and see if you get a different result.

I'm inclined to agree with @Keith Barkley. There's a reason Apple didn't stick with the feature beyond the 2014 hardware and macOS Catalina. It's so much easier to buy a monitor than to rig up an iMac computer to be a monitor - something it really wasn't designed to be.

D.I. Johnson wrote:

There's a reason Apple didn't stick with the feature beyond the 2014 hardware and macOS Catalina.

Apple dropped Target Display Mode when the first 27" 5K Retina iMac came out in Late 2014.

5K video has four times the bandwidth requirements for 2.5K video. That iMac had Thunderbolt 2 ports that had only half the bandwidth of Thunderbolt 3 ones, so there is a distinct possibility that it would have been necessary to split 5K video over two physical cables (rather than over two DisplayPort sessions within one cable).

Handling high-bandwidth video input in real time might have been too much to expect of the CPU – and the GPU might not have had provisions for offloading that work from the CPU.

Try using Target Disk Mode to see if the cable or connection is responsible.

Nothing happened when you pressed Command-F2 on the keyboard of the iMac?

Are you sure that this is a Thunderbolt 1 or 2 cable – not a Mini DisplayPort one? Thunderbolt 1 and 2 use the same connector as Mini DisplayPort, but I believe, require a higher-quality cable.

"I was thinking maybe my thunderbolt 2 cable is too old or something? "

Or, just bad.

Thanks, D.I. Johnson!

You're welcome. 👍🏽

Not disagreeing with you. 👍🏽