Using 27" 5k iMac as extended display for MacBook Pro

snyp1193

macrumors member
Original poster
Nov 2, 2019
50
12
Canada
Hi,

I am aware Apple removed Target Display Mode with the 5k iMac, however I am receiving a work MacBook Pro soon and I was wondering if there are alternative hardware based option to extend a MacBook display onto a 2019 iMac 5k? It would be great to use my home computer as a monitor when Im working from home. I was thinking of trying Luna Display but from the reviews I've read it does not display at 5k and it's quite choppy since it's using a network connection instead of a hardwired connection like target display mode.
 

casperes1996

macrumors 601
Jan 26, 2014
4,953
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Horsens, Denmark
There’s no way of doing this, No. Sorry. Target Display mode should just come back already. The removal for the first 5k was understandable with the lack of connectors that could carry that much bandwidth, but it should’ve returned
 

snyp1193

macrumors member
Original poster
Nov 2, 2019
50
12
Canada
That sucks. Hopefully some device or cable comes out that allows you to mimic target display mode again.
 

casperes1996

macrumors 601
Jan 26, 2014
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Horsens, Denmark
That sucks. Hopefully some device or cable comes out that allows you to mimic target display mode again.
You can't just make that though. Target Display mode requires wiring from the port to the display and a switch to toggle whether the monitor takes input from the iMac's GPU's frame buffer or the connected device. All the ports on the 5Ks are physically only wired to the logic board as output. At best you could transfer it as a data stream of video to then be displayed by the iMac GPU's frame buffer, but that'd be similar to Lunar Display (they basically use this technique)
 

snyp1193

macrumors member
Original poster
Nov 2, 2019
50
12
Canada
You can't just make that though. Target Display mode requires wiring from the port to the display and a switch to toggle whether the monitor takes input from the iMac's GPU's frame buffer or the connected device. All the ports on the 5Ks are physically only wired to the logic board as output. At best you could transfer it as a data stream of video to then be displayed by the iMac GPU's frame buffer, but that'd be similar to Lunar Display (they basically use this technique)
Couldn’t this data stream be transferred over Thunderbolt 3 instead of Wifi, so that it could eliminate the latency and choppiness of the data?

EDIT: Just properly read about the ports being wired to the logic board, so i’m assuming you can’t transfer a video stream from a thunderbolt 3 port of a macbook to the iMac...
 

casperes1996

macrumors 601
Jan 26, 2014
4,953
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Horsens, Denmark
Couldn’t this data stream be transferred over Thunderbolt 3 instead of Wifi, so that it could eliminate the latency and choppiness of the data?

EDIT: Just properly read about the ports being wired to the logic board, so i’m assuming you can’t transfer a video stream from a thunderbolt 3 port of a macbook to the iMac...
You can't transfer a video stream that way, no. In terms of wiring, they're both only made to send video out that way. You could however encode it as a video stream (performance cost), send it as data packages to the iMac's system memory (performance cost), send it to the iMac's GPU buffer (performance cost) and have that GPU display it to the monitor again. It's faster than Wi-Fi, but it doesn't eliminate any of the issues. It merely lowers their severity. You'll burn more CPU/GPU power on both systems, have latency and lower image quality than a native display solution.
 

professor peach

macrumors newbie
Mar 12, 2007
25
3
So, is the general consensus of opinion that it's not technically possible to bring back Target Display mode to the 2019 iMac for example?

Like a lot of others in these terrible times, I am now working from home using my work PC and a rubbish old Samsung monitor, when I have a lovely 5K 27 inch iMac screen sat right next to it.

If it is technically possible, it would be a good PR move by Apple to enable this with a software upgrade at this time!
 
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curnalpanic

macrumors newbie
Mar 26, 2008
17
3
It would be very handy, indeed. We still use a few 2009-era iMacs at work just for the display alone, connected to newer Macs.
 

snyp1193

macrumors member
Original poster
Nov 2, 2019
50
12
Canada
I’ve been using Luna display with my work supplied 16” MacBook Pro and my personal 27” iMac. It’s not the best but it gets the job done for now. I really wish they would bring back Target display mode, even if it’s at a lower resolution.
 

casperes1996

macrumors 601
Jan 26, 2014
4,953
2,816
Horsens, Denmark
So, is the general consensus of opinion that it's not technically possible to bring back Target Display mode to the 2019 iMac for example?

Like a lot of others in these terrible times, I am now working from home using my work PC and a rubbish old Samsung monitor, when I have a lovely 5K 27 inch iMac screen sat right next to it.

If it is technically possible, it would be a good PR move by Apple to enable this with a software upgrade at this time!

They would definitely need to change hardware to do it. I mean, it could be done similar to how iPad does it through just software; With an encoded video stream, but to actually have it act like an external display would require hardware modification
 

Moonjumper

macrumors 68020
Jun 20, 2009
2,267
1,954
Lincoln, UK
I hope Sidecar is extended to be able to use a Mac as a second display. If games can be streamed over the internet (Google Stadia for example), then a display should be able to stream locally, especially if connected via the Ethernet port.
 
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