Target display mode

No, that setup is not supported by Apple. The iMac cannot be used as a monitor for any of the newest Macs and Target Display mode is no longer a feature of macOS, having been deprecated several years ago. TDM was never supported for use with any other PC.

Please see: Use your iMac as a display with target display mode - Apple Support

Sam Seprish wrote:

The 2009-2010 iMacs did support connectivity to PCs

Yes, but not intentionally, and not all non-Apple devices would work. Apple never intended, documented, nor supported the use of TDM with non-Apple devices.

You asked about a 2012 iMac. Very different animal. When Thunderbolt replaced Mini-display port in 2012, any hope of cross-platform friendliness went the way of civility at a political convention.

Apple started the deprecation of TDMin Late 2014 and threw the last shovel of dirt in its face in Early 2020.

Hahah! Yeah, I wouldn’t expect you to!

I’m also not expecting Apple to care too much about the issue, given that they’ve already been clear on their position on TDM. Unfortunately, that doesn’t help me out, as I do need the multiple displays and there’s no other way (that I’m aware of) of getting multiple displays without over pressuring the GPU. Moreover, there’s nothing that looks as cohesive (the cat adds extra class).

Overnight my main computer (2019 iMac) updated itself (which it is set to do, but never normally does 🙄) to Sonoma.

With my configuration, the TDM still operates with the two 2012 TDs running Catalina. So in this case (at least) all is right with the world. Though it is not on Apple's radar, the TDM is one of the most appreciated functions offered by Apple, and is invaluable to both my research and aesthetic preferences; more screen realty (a bigger screen) is not better when you use computers for academic work (research, writing, teaching, online instruction, and academic publication), in which you need multiple screens to separate search browsers, .pdfs, e-mails, and Pages).

If the next OS fails to support TDM, I am not at all sure what I will have to do -- presumably, one would be forced to buy a Mac Mini and three Apple monitors (which are too big for my desk). I am not even sure if a Mac Mini would drive three Apple 4k monitors. In this respect, Apple's direction is less good.

Apple's technical specifications for the 27" Late 2009 iMac, on a late November 2009 archive snapshot of their site, say:

Built-in iSight camera; Mini DisplayPort output with support for DVI, dual-link DVI, and VGA video (requires adapters, sold separately); supports input from external DisplayPort sources (requires cables and adapters, sold separately)

http://web.archive.org/web/20091203155316/http://www.apple.com/imac/specs.html

There is no qualification that the external DisplayPort sources have to be Macs released in 2019 or earlier and running Catalina or earlier. Apple added those restrictions after people like me had already bought the iMacs.

A 27" Late 2009 iMac, with a 2560x1440 pixel screen, would still have some relevance as a TDM mDP display today, if not for restrictions added after the fact. Granted, comparable standalone 27" monitors have dropped from $1,200 (back then) to maybe $315 –$350 (now).

RE: "When Thunderbolt replaced Mini-display port in 2012, any hope of cross-platform friendliness went the way of civility at a political convention."

There wasn't much cross-platform friendliness with Thunderbolt 1 & 2 because, while PC vendors could have widely adopted the technology, most of them didn't.

Once Intel moved Thunderbolt 3 to USB-C, and it was out a while, we did start seeing cross-platform adoption of Thunderbolt 3 (& 4). But even with Thunderbolt 3 & 4, there are products such as the 27" Apple 5K Studio Display that are not cross-platform friendly. (AFAIK, there are no menu controls on the monitor tiself to change things like brightness, and it doesn't come with Windows or Linux software for setting those controls, either.)

Hmmm, I’m running a 2019 21.5” iMac with two 2012 iMac as target displays. I run Ventura currently without any issue (though some updates are more stable than others for TDM).

I want to know whether or not Sonoma supports TDM as a vestigial system operation (knowing it’s more a BIOS thing than a software thing). TDM is critical to my setup, and I won’t update if there’s no TDM support.

I disagree, it critical to my set up. I run two 2012 21/5” iMacs as target displays, and Apple has not offered any alternative that works as well, given that any non-Apple screens have scaling issues and tax the GPU excessively making the computer sluggish. Not everyone can afford two subordinate Apple monitors anyway — I’m lucky to have inherited mine, which is how I can run them as slave displays.

2019 iMac running Ventura.

shedlock2000 wrote:

Hmmm, I’m running a 2019 21.5” iMac with two 2012 iMac as target displays. I run Ventura currently without any issue (though some updates are more stable than others for TDM).

I want to know whether or not Sonoma supports TDM as a vestigial system operation (knowing it’s more a BIOS thing than a software thing). TDM is critical to my setup, and I won’t update if there’s no TDM support.

If you are running Ventura on the 2019 iMac ("the other Mac"), you are already operating using a configuration that Apple implies won't work, based on the restrictions they added on "the other Mac" after the fact. If there was a security update to Ventura that just happened to break your TDM setup, they wouldn't treat that as a bug.

You have no TDM support now, and will have no TDM support if you upgrade.

As to whether there is vestigial code that might let TDM work in an "unsupported, could break at any moment, with any change" sort of way, I don't know. And I'm not going out to buy a used 2019 iMac, and a pair of used 2012 iMacs, and Thunderbolt connecting hardware, just to find out.

shedlock2000 wrote:

I disagree, it critical to my set up. I run two 2012 21/5” iMacs as target displays, and Apple has not offered any alternative that works as well, given that any non-Apple screens have scaling issues and tax the GPU excessively making the computer sluggish.

Those 21.5" 2012 iMacs don't have Retina 4K displays. Their displays have a resolution of 1920x1080 pixels, the same resolution as many 24" 1080p (not 4K) monitors. They might offer decent quality for the resolution that they have, but you're certainly not getting Retina sharpness.

Furthermore, since you are driving these iMacs without any scaling, and would drive actual 24" 1080p monitors without any scaling, I'm not sure what scaling issues you think you're avoiding. On the flip side, you say you're running Ventura on the 2019 iMac. My understanding is that Ventura does not bend over backwards as much to accommodate low-DPI displays (like those 21.5" 1080p iMacs, or a 24" 1080p monitor) as High Sierra did. There were changes somewhere along the line to remove sub pixel anti-aliasing

Not everyone can afford two subordinate Apple monitors anyway — I’m lucky to have inherited mine, which is how I can run them as slave displays.

These days, most people who use standalone monitors with Macs use third-party monitors. Apple only offers two monitors under their own name: a 27" 5K one, and a 32" 6K one (which isn't compatible with your 2019 iMac). If you want a monitor with one of the other common resolutions – ranging from 1920x1080 to 3840x2160 – you buy it from a third-party.

shedlock2000 wrote:

Hahah! Yeah, I wouldn’t expect you to!

I’m also not expecting Apple to care too much about the issue, given that they’ve already been clear on their position on TDM. Unfortunately, that doesn’t help me out, as I do need the multiple displays and there’s no other way (that I’m aware of) of getting multiple displays without over pressuring the GPU.

Two real 24" 1920x1080 or 1920x1200 monitors presumably would not "over pressure the GPU" much (if any) more than the two 21.5" 1920x1080 TDM iMacs that you have now. You'd probably have to use up both of your Thunderbolt 3 ports to connect the real monitors, though.

Moreover, there’s nothing that looks as cohesive (the cat adds extra class).

That's true. Also, with multi-monitor setups that are not precisely matched, the mouse pointer will "jump" up or down at most of the crossing places between screens.

Oh, I know they’re not 4K (my intention was to slowly upgrade the TD display iMacs to 4K ones). 4K monitors have a slightly different resolution to the native 21.5” iMac (from what I’m told) 4096x2304 is different than the standard resolution of something like the HP (which is 3840x2160). Upgrading the two 2012 iMacs (which would be the next logical step) would push the scaling on the GPU in ways it’s not good at handling (by all accounts).

Of course, I could replace the iMacs with 1080 resolution monitors, but I’d lose the glossy screen look of the iMacs (thus causing eye strain as you move from one type of screen to another and back), the loss of cohesion in the set up, and I’d spend more money on the monitors because the iMacs are relatively worthless now. So there’s really no benefit with that idea save running the new OS. I’m now worried that if I update the OS, I won’t even be able to run what I already have.

If that’s the case, then I’ll don’t know what to do, as I certainly don’t want 3 different looking monitors on the desk with 3 different screen types and appearances — it’d look dreadful and be really hard to work with for 18 hours a day! I’ve downloaded the update, but I’m hesitant to install it.

Hmm, well my TDs are currently using both the USBC ports on my 2019 (into a thunderbolt converter). Are you saying that the OS on the TD is the limiting factor or on the primary computer? (Or is it both).

shedlock2000 wrote:

Hmm, well my TDs are currently using both the USBC ports on my 2019 (into a thunderbolt converter). Are you saying that the OS on the TD is the limiting factor or on the primary computer? (Or is it both).

The version of macOS on the 2019 iMac ("the other Mac") is a problem. The support article says that computer must be running Catalina or earlier – but you're running Ventura, and hoping to run Sonoma.

You didn't say what version of macOS you are running on the 2012 iMacs. Apple's support document says that you have to be running High Sierra or earlier on them to use them as TDM displays. If you have upgraded those iMacs to Mojave or Catalina, that would pose a second problem.

The two DMs are running Catalina, and the driving machine is running Ventura.

The setup is largely faultless, but there have been more stable versions of TDM on the other iMac when running both Catalina and Ventura. The target displays have always run Catalina.