Why is target display mode not possible on the 5K iMac?

[rainydays]

I understand that the limitations of thunderbolt and other macs doesn't make it possible to drive the display via TDM at 5K. But why don't they simply lower the resolution of the display when put in TDM?

I can't think of a reason that they couldn't have worked that feature out. But I understand that it's not top priority and that they wanted to get it out there.
I wonder if it's something that can be made possible through software updates however?
But knowing Apple they would most likely release a new iMac with that feature instead.

 

[patrickdunn]

Current Thunderbolt can't support 5K, the retina's have an extra chip-set inside to help drive it.

 

[jaxhunter]

I suspect that the reason is one or more of the following:

-An agreement with the LCD manufacturer to prevent Apple from effectively selling a $2500 5K display (which is well below retail price for a similar display)
-It may not be possible for the custom TCON and other controllers in the LCD panel to be used via Target Display Mode (TDM)
-It may not be possible for the hardware to operate at full 5k in TDM and Apple has historically completely disabled functions that dont work 100% rather than implement half measures (ie, it may be possible for it to do TDM at a lower resolution but this is the Retina iMac and so what would be the point?).
-The pass through functionality may not have been included in the new GPU due to lack of physical space, available resources, something.

 

[5iMacs]

Even Apple has limits in its feats of engineering.

 

[nrubenstein]

I suspect that the reason is one or more of the following:

-An agreement with the LCD manufacturer to prevent Apple from effectively selling a $2500 5K display (which is well below retail price for a similar display)
-It may not be possible for the custom TCON and other controllers in the LCD panel to be used via Target Display Mode (TDM)
-It may not be possible for the hardware to operate at full 5k in TDM and Apple has historically completely disabled functions that dont work 100% rather than implement half measures (ie, it may be possible for it to do TDM at a lower resolution but this is the Retina iMac and so what would be the point?).
-The pass through functionality may not have been included in the new GPU due to lack of physical space, available resources, something.

You're almost completely wrong here.

It's very simply because Thunderbolt has insufficient bandwidth. You need two current thunderbolt buses to run a 5K display, and the iMac only has one shared between two ports.

Additionally, running two thunderbolt cables is a gigantic kludge, which is the sort of thing that Apple is not highly likely to want to stand behind. That's how the Dell monitor will work (if it actually works).

 

[kgapp]

does TDM work the other way

So we all know that the iMac 27 Retina 5k does not support TDM for what is most likely technical reasons (if they could have made it work they probably would have).

My question is are there any issues going the other way? In other words, can I hook up a previous generation iMac 27" (that does support TDM) to the new Retina 5k model and utilize the old machine as a second display with TDM? My assumption would be yes but I have not seen any definitive information on that yet. Can anyone confirm this for me?

 

[rainydays]

It's very simply because Thunderbolt has insufficient bandwidth. You need two current thunderbolt buses to run a 5K display, and the iMac only has one shared between two ports.

As I said, they could have lowered the display resolution to 2560x1440 when it's in TDM. I don't think that would have been a half baked solution.
But perhaps it's more complicated to do that than I think.

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My question is are there any issues going the other way?

That works just fine! I even saw a video of someone demonstrating it. Don't remember which one however.

 

[kgapp]

As I said, they could have lowered the display resolution to 2560x1440 when it's in TDM. I don't think that would have been a half baked solution.
But perhaps it's more complicated to do that than I think.

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That works just fine! I even saw a video of someone demonstrating it. Don't remember which one however.

Even if it was technically possible as you suggested for Apple to lower the display resolution to support TDM what would be the point?

I can't really envision too many real world scenarios where someone would want to use their 5k monitor in 2560X1600. The only possibility I can think of is if there was a component failure that rendered it unusable as a computer. However, if this were the case it would be unlikely you could even boot the machine up properly to put it in TDM in the first place.

Even if you were in a situation where the machine was working fine but the internals were outdated to the point where you no longer wanted to utilize the computer functionality and just turn it into a display could you really see yourself going back to using 2560X1600 after running 5k for any period of time? I could not see myself doing that under any circumstance (by that time 4k and 5k monitors will likely be the norm and will be much cheaper than they are today).

 

[nrubenstein]

As I said, they could have lowered the display resolution to 2560x1440 when it's in TDM. I don't think that would have been a half baked solution.
But perhaps it's more complicated to do that than I think.

A solution that lets you "use" it, but in which it looks awful is not half baked? Okaaaaayyyy...

 

[rainydays]

I can't really envision too many real world scenarios where someone would want to use their 5k monitor in 2560X1600.

Really? How about pluggin in your work MacBook when you are at home? Quite a lot of people do that, including myself. And I know at least one developer who keep a Mac Mini plugged in to his iMac and switches to it with TDM, don't know how common that is though.
And there are plenty of people who use their gaming consoles with the iMac through the Kanex adapter.

Switching to the riMac would require them to get a second screen to plug their devices into. Don't you think they would rather keep that functionality?

 

[WilliamG]

Really? How about pluggin in your work MacBook when you are at home? Quite a lot of people do that, including myself. And I know at least one developer who keep a Mac Mini plugged in to his iMac and switches to it with TDM, don't know how common that is though.
And there are plenty of people who use their gaming consoles with the iMac through the Kanex adapter.

Switching to the riMac would require them to get a second screen to plug their devices into. Don't you think they would rather keep that functionality?

The iMac hasn't been able to be used with a Kanex adapter/game consoles since the 2011 iMac's introduction.

 

[kgapp]

Really? How about pluggin in your work MacBook when you are at home? Quite a lot of people do that, including myself. And I know at least one developer who keep a Mac Mini plugged in to his iMac and switches to it with TDM, don't know how common that is though.
And there are plenty of people who use their gaming consoles with the iMac through the Kanex adapter.

Switching to the riMac would require them to get a second screen to plug their devices into. Don't you think they would rather keep that functionality?

Something tells me you have not actually seen the 5k display in person yet?

Very few people who use a 5k display for more than 10 minutes would likely ever be compelled to plug their macbook into it so they can run it at 2560X1600. 2560X1600 looks so terribly bad after spending just a few minutes at 5k that it will spoil you forever. It did for me.

 

[rainydays]

Very few people who use a 5k display for more than 10 minutes would likely ever be compelled to plug their macbook into it so they can run it at 2560X1600. 2560X1600 looks so terribly bad after spending just a few minutes at 5k that it will spoil you forever. It did for me.

So what's worse, having to run the 5k display at 2560x1600 or buying a separate screen with that resolution? I completely understand the "being spoiled" thing but it would still be a perfectly useful feature. You wouldn't expect a HDTV to display DVDs at 1080p would you? Would you rather not watch those old movies because you've been spoiled by BlueRay?

I doubt that many people use their iMacs in TDM as a default setup. It's more a convenient way of being able to plug in other devices to it when needed.

Sure, for the MacBook scenario there is the possibility to boot in target disk mode. Not as convenient, but you would gain the power of the iMac.

 

[nrubenstein]

So what's worse, having to run the 5k display at 2560x1600 or buying a separate screen with that resolution? I completely understand the "being spoiled" thing but it would still be a perfectly useful feature. You wouldn't expect a HDTV to display DVDs at 1080p would you? Would you rather not watch those old movies because you've been spoiled by BlueRay?

I doubt that many people use their iMacs in TDM as a default setup. It's more a convenient way of being able to plug in other devices to it when needed.

Sure, for the MacBook scenario there is the possibility to boot in target disk mode. Not as convenient, but you would gain the power of the iMac.

Apple doesn't do kludged edge case solutions. Almost no one uses TDM in the first place. The negative press from ****** TDM would vastly outweigh the three people who it would make happy.

And they would logically ask why the hell you would want to plug your laptop in?

 

[redheeler]

Really? How about pluggin in your work MacBook when you are at home? Quite a lot of people do that, including myself. And I know at least one developer who keep a Mac Mini plugged in to his iMac and switches to it with TDM, don't know how common that is though.
And there are plenty of people who use their gaming consoles with the iMac through the Kanex adapter.

Switching to the riMac would require them to get a second screen to plug their devices into. Don't you think they would rather keep that functionality?

Yup, my 2012 Mac mini is now headless and I have to put up with a screen sharing connection full of lag. Oh well, at least the HDMI display emulator I got for it allows for HiDPI display of the Mac mini's screen over screen sharing. It's artifacting a bit in higher HiDPI resolutions though, must be the integrated graphics.

 

[xav8tor]

Hope the mods don't consider this a cross-post, but can anyone here explain TB bridge and screen sharing to get around no TDM on the 5K iMac and answer my questions buried in the news sub forum? I doubt it gets seen there much, if at all. Thanks.


Originally Posted by ChrizTheWiz:
I've got the iMac Retina 5k and can confirm that it doesn't support Target Display Mode. However, there's no problem with using thunderbolt bridge and screen sharing instead, i.e. "same same but different"

My Question:
So, are you saying that if I set up a TB bridge between my new Mac Pro with D500 GPU's (now connected to an Apple TB display) and a new 5K iMac, and then enable screen sharing, I can output UHD material, whether video or real time app running a simulation, and see it displayed as 3840x2160 upscaled to 5K on the Retina iMac with no choppiness? How will the nMP GPU know to send a 3840x2160 signal via TB to the iMac since it isn't connected as a display, but rather, just screen sharing? I don't get it. And again, just how smooth will the iMac display look if I'm kicking out complex, detailed, fast moving graphics in UHD at 60 FPS?

 

[nrubenstein]

Hope the mods don't consider this a cross-post, but can anyone here explain TB bridge and screen sharing to get around no TDM on the 5K iMac and answer my questions buried in the news sub forum? I doubt it gets seen there much, if at all. Thanks.


Originally Posted by ChrizTheWiz:
I've got the iMac Retina 5k and can confirm that it doesn't support Target Display Mode. However, there's no problem with using thunderbolt bridge and screen sharing instead, i.e. "same same but different"

My Question:
So, are you saying that if I set up a TB bridge between my new Mac Pro with D500 GPU's (now connected to an Apple TB display) and a new 5K iMac, and then enable screen sharing, I can output UHD material, whether video or real time app running a simulation, and see it displayed as 3840x2160 upscaled to 5K on the Retina iMac with no choppiness? How will the nMP GPU know to send a 3840x2160 signal via TB to the iMac since it isn't connected as a display, but rather, just screen sharing? I don't get it. And again, just how smooth will the iMac display look if I'm kicking out complex, detailed, fast moving graphics in UHD at 60 FPS?

Without having tried it, I would assume that you will *not* be able to get things to look perfect (and possibly not even good). There are just too many steps involved and I doubt that anyone has put the time in necessary to optimize the software. That's a LOT of data to process non-natively.

 

[kgapp]

My Question:
So, are you saying that if I set up a TB bridge between my new Mac Pro with D500 GPU's (now connected to an Apple TB display) and a new 5K iMac, and then enable screen sharing, I can output UHD material, whether video or real time app running a simulation, and see it displayed as 3840x2160 upscaled to 5K on the Retina iMac with no choppiness? How will the nMP GPU know to send a 3840x2160 signal via TB to the iMac since it isn't connected as a display, but rather, just screen sharing? I don't get it. And again, just how smooth will the iMac display look if I'm kicking out complex, detailed, fast moving graphics in UHD at 60 FPS?

I don't think that is at all what they are implying. I believe they are suggesting that you can network the two machines with a Thunderbolt Bridge (i.e. networking via thunderbolt) and then you can use screen sharing to control the other machine.

You will likely not be able to achieve a 4k resolution and it most certainly will not be with out the choppiness that goes along with any type of screen sharing. VNC (the screen sharing protocol built into OS X) is not the best and it will most certainly have a degree of lag regardless of how fast your network connection is to the other machine. Windows RDP protocol is a bit better than VNC but it still has its share of related issues.

 

[xav8tor]

Thanks for the answers. I figured as much. Apple has really dropped the ball on 4K, marketing it so heavily on the nMP, then not offering a display or TDM at UHD on the 5K iMac. UHD drivers have been pretty bad too. Meanwhile, Windows users seem to have few issues. Hopefully the new Dell IPS UHD monitors that went online yesterday will work at 60 Hz. via DP 1.2 single stream and hassle free. The price on both sure is nice.

 

[TheBearman]

Without some option this could be a deal breaker for me. I really want a 5k iMac but being able to use TDM is very important for me. I connect my Macbook Pro when I work from home one or two days a week. I know I could keep the old iMac but really don't want the extra display (no desk space).

Maybe what I need to do is test the TB Bridge setup and see the results. Does anyone have a link on setting this up? Lag may not be an issue as work is mostly dealing with office type documents (spreadsheets, docs, presentations).

 

[redheeler]

Without some option this could be a deal breaker for me. I really want a 5k iMac but being able to use TDM is very important for me. I connect my Macbook Pro when I work from home one or two days a week. I know I could keep the old iMac but really don't want the extra display (no desk space).

Maybe what I need to do is test the TB Bridge setup and see the results. Does anyone have a link on setting this up? Lag may not be an issue as work is mostly dealing with office type documents (spreadsheets, docs, presentations).

Since I posted my comment about the Mac mini I did just that (Thunderbolt Bridge) and it hugely improved the performance in Screen Sharing. It's simple to set up, just go into System Preferences > Network and assign both computers an IP address under Thunderbolt Bridge. Then use OS X's Screen Sharing.app utility to connect to the MBP's IP address.

 

[rainydays]

Without some option this could be a deal breaker for me. I really want a 5k iMac but being able to use TDM is very important for me. I connect my Macbook Pro when I work from home one or two days a week. I know I could keep the old iMac but really don't want the extra display (no desk space).

Didn't you hear the other guys? No one is using TDM and especially not the way you're using it.

 

[redheeler]

Didn't you hear the other guys? No one is using TDM and especially not the way you're using it.

Well he could just copy the documents to his iMac

 

[tillsbury]

Well he could just copy the documents to his iMac

Indeed. Or if he really didn't want to do that he could screen share the MBP, or run it with Parallels Access or similar. Or, just edit the documents on the iMac accessing the MBP files through the network.

 

[TheBearman]

Well he could just copy the documents to his iMac

Not if he wants to stay employed! Thanks for the info I'll give screen share a shot!


Update: Damn! Just realized I can't test this with my current iMac as it doesn't have a TB port, just a display port. So redheeler, how is the quality of the image during the share? Do you feel you could work with spreadsheets without issue? Compared to TDM, does it have as good image quality? Sorry if these sound like basic questions but I've had problems with networked windows (AIX/Solaris) where quality of the shared image was so bad I could hardly read the text/numbers. Looked more like a poorly rendered picture.

Update 2uh! Just realized that all the TB port would be doing is a faster/direct network connection. The quality is more than acceptable and surprisingly fast from my test over wireless. My only concern now is finding a way to scale the image to a bit larger size(filling the screen). Understanding this scaling could result in possible quality issues but this is a workable option (and fairly simple).