Using a 2008 iMac 27 as an external monitor for a 2019 iMac 27?

[pshufd]

You can — enable Screen Sharing on the iMac and connect to it using that. The iMac keeps running normally while TDM is active.


It would be cool if you checked and compared the network traffic caused in both cases (via Activity Monitor).

I just added a number of tests to my TDM and Screen Sharing todo list. Checking network bandwidth is a very good idea. I can do that in a spreadsheet to test out various systems and resolutions.

One other test that I'm curious about is what happens if you have an attached monitor and start up TDM. My guess is that it disables the external display but it would be most cool if the second display just kept on working. One thing about this testing is that it's a good workout. Moving 27 inch iMacs around takes a bit of effort.

 

[pshufd]

I ran my screen sharing experiment on the Late 2009 iMac 27 and it uses a maximum of 10.2 MBPS. The test was to run 4k video at QHD. Bandwidth was quite variable running a small screen video at about 1.5 to 4.0 MBPS.

I couldn't run the test to see what happens to the external screen if you plug in for Target Display Mode because the Late 2009 and 2010 iMacs only have one MiniDisplayPort port. It's the Thunderbolt models that have two ports and I don't have any of those though there are some for sale nearby really cheap.

I'm going to try the rest of the scaled resolutions of the 2009 iMac to see what I get. I thought that supporting QHD would require more bandwidth but maybe 4k will work with this approach.

Playing 4k video at HD resolution results in a maxium of 15.8 MBPS. I guess this makes sense. I'm not going to bother with the lower resolutions as it looks like it scales linearly with the number of pixels.

 

[Amethyst1]

I ran my screen sharing experiment on the Late 2009 iMac 27 and it uses a maximum of 10.2 MBPS. [...] I couldn't run the test to see what happens to the external screen if you plug in for Target Display Mode because the Late 2009 and 2010 iMacs only have one MiniDisplayPort port.

So you have the 2009 27" iMac hooked up to your 5K iMac via MiniDisplayPort and it's in Target Display Mode? In that case you don't need an additional network connection between the two Macs. Or am I missing something?

 

[pshufd]

So you have the 2009 27" iMac hooked up to your 5K iMac via MiniDisplayPort and it's in Target Display Mode? In that case you don't need an additional network connection between the two Macs. Or am I missing something?

The bandwidth tests are for screen sharing.

The TDM tests were to see if the CPU is used in TDM and also to see what happens if you have an external monitor and a TDM input.

The CPU is used for TDM for Audio. There are two audio process that run on the target iMac all the time, whether or not you are sending audio. The percentage of CPU is under 1% though. I did not see any video processes.

 

[unixfool]

I've a Late 2009 iMac 27" that I'm using as an external monitor for my 2020 M1 Mini. It's easy to get TDM working...you just have the correct cabling and correct iMac. I'm using a mini-DP to USB-C cable to connect them.

 

[Amethyst1]

I've a Late 2009 iMac 27" that I'm using as an external monitor for my 2020 M1 Mini. It's easy to get TDM working...you just have the correct cabling and correct iMac. [...]

It's worth mentioning that Target Display Mode via Thunderbolt (used by 2011 and later non-Retina iMacs) does not work with an Apple Silicon Mac as the host. So you're limited to 2009/2010 27" iMacs that do TDM via DisplayPort in this case.

 

[unixfool]

It's worth mentioning that Target Display Mode via Thunderbolt (used by 2011 and later non-Retina iMacs) does not work with an Apple Silicon Mac as the host. So you're limited to 2009/2010 27" iMacs that do TDM via DisplayPort in this case.

That is correct. I didn't mention that because that's not what the OP was asking, and my comment wasn't implying that the opposite of what I did would work.

 

[jman995x]

I ran my screen sharing experiment on the Late 2009 iMac 27 and it uses a maximum of 10.2 MBPS. The test was to run 4k video at QHD. Bandwidth was quite variable running a small screen video at about 1.5 to 4.0 MBPS.

I couldn't run the test to see what happens to the external screen if you plug in for Target Display Mode because the Late 2009 and 2010 iMacs only have one MiniDisplayPort port. It's the Thunderbolt models that have two ports and I don't have any of those though there are some for sale nearby really cheap.

I'm going to try the rest of the scaled resolutions of the 2009 iMac to see what I get. I thought that supporting QHD would require more bandwidth but maybe 4k will work with this approach.

Playing 4k video at HD resolution results in a maxium of 15.8 MBPS. I guess this makes sense. I'm not going to bother with the lower resolutions as it looks like it scales linearly with the number of pixels.

Hello,

Just to reconfirm what I've read (and currently understand):

2019 Retina 5K iMac --> (USB-C - to - MiniDisplay Port Cable) --> 2009/10 27" iMac (for TDM purposes)

Then, once connected,....on the 2019 iMac, go to
Settings > Displays > (Option) Detect Displays....and it should see the 2009/10 27" iMac connected, and willing to be a TD?

Just wanted reconfirmation because if this will work, I'll have to buy the 2009/10 iMac to act as a TD,...BUT don't want to buy one until I get confirmation, because if it doesn't work, then I've just spent money on an old iMac I'll have NO other use for, and would like to prevent that.

Thanks,

- J

 

[R2K2]

If you have no other use for the 2009/2010 iMac, than you're better off buying a new 4K 27" monitor. The 2009/2010 iMac will be constantly running an unused macOS in the background while acting as a target display. It runs really hot in this mode (been there&done that), consuming over 150W all the time, and it only has a 2560x1440 resolution. Which will be very noticable because that's only 109ppi as opposed to your 5K iMac that has double the ppi. A modern 27" 4K monitor isn't that sharp, but comes much closer at around 163ppi and uses considerably less power at probably around 40-50W on average.

 

[Nguyen Duc Hieu]

If you have no other use for the 2009/2010 iMac, than you're better off buying a new 4K 27" monitor. The 2009/2010 iMac will be constantly running an unused macOS in the background while acting as a target display. It runs really hot in this mode (been there&done that), consuming over 150W all the time, and it only has a 2560x1440 resolution. Which will be very noticable because that's only 109ppi as opposed to your 5K iMac that has double the ppi. A modern 27" 4K monitor isn't that sharp, but comes much closer at around 163ppi and uses considerably less power at probably around 40-50W on average.

A modern 27" 4k display doesn't come as cheap as an old 2009/2010 27" iMac.
And a cheap 27" 4k display also can't offer the brightness of the old 2009/2010 27" iMacs.

Just some comments from another aspect, I myself prefer the modern 27" 4k display, if I can afford to buy one.

Around my place, I've seen offer of a HP 27" 2k display (64$) which use the same LG LCD panel as iMac 27" in that era.
I would just buy this HP display if I ever need a display, rather than the iMac.

 

[FreakinEurekan]

Just wanted reconfirmation because if this will work, I'll have to buy the 2009/10 iMac to act as a TD,...BUT don't want to buy one until I get confirmation, because if it doesn't work, then I've just spent money on an old iMac I'll have NO other use for, and would like to prevent that.

Absolutely don’t “buy” a Mac for this purpose. TDM is for old Macs you already own and are too cheap (frankly) to replace. If you have to buy something anyway, just buy a display.

• iMacs generate a lot more heat than a display. They’re heavier too, and require a keyboard to enable TDM that you have to keep track of.
• An iMac will use (depending on your electricity rate) anywhere from $20 to $60 a year in power. That’s assuming 8 hours a day, 270 days a year… but you can calculate the cost for your own rate & usage; figure 150 (wattage of Mac) X hours used per day X days used per year, divide by 1000 to get kWh, multiply by your electricity rate. Whatever the result, divide that by 5 to get the cost for an actual external display of 30w. A display will “eventually” pay for itself; how long that takes depends on your usage & power cost.
• iMacs will eventually break - we’re talking about 14-15 year old machines right now, and they’re not gonna get any younger.
• iMacs have a decent brightness level (if working properly - see previous bullet). But the resolution is easy to beat in a 27” display.

 

[jman995x]

If you have no other use for the 2009/2010 iMac, than you're better off buying a new 4K 27" monitor. The 2009/2010 iMac will be constantly running an unused macOS in the background while acting as a target display. It runs really hot in this mode (been there&done that), consuming over 150W all the time, and it only has a 2560x1440 resolution. Which will be very noticable because that's only 109ppi as opposed to your 5K iMac that has double the ppi. A modern 27" 4K monitor isn't that sharp, but comes much closer at around 163ppi and uses considerably less power at probably around 40-50W on average.

Very insightful (never really thought about the OS running in the background, since it wouldn't be "doing anything", so to speak, except TDM).

Once you brought up this issue, I started looking up 27" 4K monitors, and found one that I like,...BUT then ran across this guy who brings up an interesting point about how Mac Scaling to an extended monitor, and how I'd be going from a 5K monitor to a 4K monitor, and the scaling would be off, because Mac likes to run at 110ppi, or 220ppi (Retina), and 4K falls right in the 163ppi range, which means the GPU is having to calculate / render that at like 60xsecond, and thus, he recommends a 1440p monitor (which, 5K, cut in half by the Mac, would fall exactly at 1440p, thus no scaling), instead of 4K.

Worth watching from the beginning, and he gets into a interesting chart at about the 4m05s mark:
VIDEO: Mac Scaling Explained

Thoughts?....

- J.

 

[R2K2]

The 27" 2009/2010 iMac actually are 1440p (which is 2560*1440), and that's the relatively low ppi of 109ppi.
If you want your iMac and external monitor to have the exact same UI scaling/size, you would need a 5K 27" screen, and these are rare and expensive (Apple Studio Display is one of few the options)

 

[R2K2]

[deleted]

 

[zarmanto]

... 4K falls right in the 163ppi range, which means the GPU is having to calculate / render that at like 60xsecond, and thus, he recommends a 1440p monitor (which, 5K, cut in half by the Mac, would fall exactly at 1440p, thus no scaling), instead of 4K.

Worth watching from the beginning, and he gets into a interesting chart at about the 4m05s mark:
VIDEO: Mac Scaling Explained

One thing that Hunter mentions early on in his video is that he found varied information in online forums and such that were all just "people's opinions..." but what he doesn't mention is that in reality, his video falls much into the same vein; he's just offering yet another opinion -- his just happens to be in video format instead of text. And while he makes a point of backing up his opinion with technical statistics, I would argue that those statistics may or may not be relevant to your specific use case.

Take me for example: I paired up my 5K 2019 iMac with a 4K Dell monitor which I purchased in April 2021, and I've been quite satisfied with it -- but I'm admittedly not the same kind of "graphics specs geek" as Hunter. As a software engineer and casual gamer, I use a very different mix of software on my Mac than Hunter uses for his professional graphics workload, so unsurprisingly, I've never observed any of the rendering delays that he bemoans. Beyond that, my own configuration is different than his in other ways: my 4K monitor has always performed double duty, connected to a Windows based gaming laptop on one side and the iMac on the other side, switching the 4K secondary monitor between the two computers depending upon what I'm doing. This "split-use case" is quite different than Hunter's use case and tends to make me want to lean into the higher resolution as more of a preferred feature, rather than color accuracy and such.

So really, you need to decide what's important for your specific use case; if you have a professional graphics heavy workload like Hunter, maybe his analysis has relevance to you.