Is there a switch or toggle for a Mini-Displayport cable between iMac 2009 and Mac mini?

This is Apple's guidance for using target display mode of old iMacs with other older Macs:

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

Per the link above:

Only the following older iMac models could be used:

• iMac models introduced in 2011, 2012, 2013, and mid 2014 using Thunderbolt or Thunderbolt 2 cable.
• 24-inch and 27-inch iMac models introduced in 2009 and 2010 using Mini DisplayPort cable.
• The intended target iMac must be running macOS 10.13 High Sierra or earlier; it does not work with macOS 10.14 Mojave or later, or with Boot Camp and Windows.
• Retina display iMacs won’t work as target displays.
• In all cases the source Mac that you're connecting FROM must have been introduced in 2019 or earlier and have macOS 10.15 Catalina or earlier installed.

Please note that the technology is officially dead to Apple.

They killed the feature in hardware when the first retina iMacs were released in 2014, and they finally buried it in software when they released macOS Big Sur in late 2020.

The only "switch or toggle" is the enabling keystroke on the target iMac; Command-F2 (⌘F2).

What year Mac mini did you buy?

And which version of macOS is that Mac mini running?

natedrew23 wrote:

I think I have to boot up the iMac first then connect the Mini-DP to trigger the Mac Mini to display on the screen. Perhaps there is a way to switch it on from the iMac, but I don't know the command. I'm not to sophisticated on the Mac, so I might be missing something obvious.

These are the instructions for using Target Display Mode:

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

Note that Apple added restrictions on the video source that mean that your M4 Mac mini does not support using any iMac as a Target Display.

D.I. Johnson wrote:

So you actually have the 2009 27" iMac running High Sierra working as a monitor for a 2024 Mac mini M4 running macOS 15 Sequoia?! The connection is by USB-C to DisplayPort cable?

Color me impressed. By all accounts given by Apple for the past few years this shouldn't be possible.

It seems that maybe something in the specs of the newest computers has changed that allows this interconnection.

Apple did claim in Technical Specifications that the 27" Late 2009 iMac could accept DisplayPort input.

I believe that for a lot of other Macs that supported Target Display Mode, Technical Specifications did not mention the feature.

I understand you have TDM running but you are concerned about wear resulting from repeatedly plugging and unplugging the Mini-DP cable. Is that correct?

If so I would just get two Mini-DP "extension cables" and disconnect the cheap parts instead. Leave the expensive parts alone. Either way it will probably endure thousands of plugging / unplugging cycles, but if something wears out you just have to purchase another inexpensive pair of cables.

You didn't mention which Mini model you purchased but it obviously must be "old" as well. Pre-"M" series, with a commensurately older macOS version.

John Galt wrote:

... I would just get two Mini-DP "extension cables" ...

Actually you just need one "extension cable" and your existing Mini-DP cable. I have never heard of a Mini-DP switch but if one exists it's going to be a lot more expensive than an eight dollar cable, and switches wear out too.

natedrew23 wrote:

[...] I purchased the Mac Mini with M4 chip 16GB RAM. The iMac is running High Sierra. [...]

So you actually have the 2009 27" iMac running High Sierra working as a monitor for a 2024 Mac mini M4 running macOS 15 Sequoia?! The connection is by USB-C to DisplayPort cable?

Color me impressed. By all accounts given by Apple for the past few years this shouldn't be possible.

It seems that maybe something in the specs of the newest computers has changed that allows this interconnection.

I wonder if we're going to hear from others who can make this work. 🤷🏽‍♂️

I'll be looking out for the posts.

You should be able to leave the cable plugged into both computers and use Command-F2 on the keyboard of the iMac to turn Target Display Mode on or off. With Target Display Mode off, the iMac should just act as a stand-alone computer.

I know of at least one circumstance in which TDM worked with an "unsupported" Mac, but it stopped working subsequent to a macOS update.

It was a long time ago and it wasn't an M series Mac.

Its OP was short on details also, leading me to believe the Mac may have been modified in some way.

I did not ask why you need to disconnect the Mini-DP cable. I just assumed you needed to do that. It's not necessary to disconnect it just to turn off or otherwise disable the other Mac's display.

Wow! That's a very interesting find and good to know.

And obviously it's something Apple really didn't wish to support afterward.

Thanks! 🙏🏽