- You can repurpose your old iMac into a second monitor for your new one using Target Display Mode.
- The number of external displays you can connect depends on the chipset and available ports of your host system.
- Older iMacs have physical and software limitations that make it harder to use them as a second monitor.
If you’ve recently purchased a new iMac model, you might wonder what you can do with your old iMac. These machines are relatively expensive. So, throwing it out might feel wasteful. The good news is that you can repurpose your old iMac into a second monitor for your new one. You need to make a few considerations depending on the make and model of the machines, but most iMacs can function as an external display for another macOS device using Target Display Mode. Let’s look at how you can get that set up.
Make and Model Requirements: Host System
First, you’ll want to ensure your host system can support multiple displays. The computer’s chipset and available ports determine how many external displays you can connect. Both iMac and MacBook models can use external displays with a compatible chipset and available ports.
Machines with M1 Chipsets
M1 iMacs and MacBooks can only install one additional monitor. Adding a dock to the system will not increase the number of monitors you can connect to the machine.
Machines with a Thunderbolt or Thunderbolt 2 Connector
macOS machines with Thunderbolt or Thunderbolt 2 connectors can connect two additional displays. If the external displays also have a Thunderbolt connector, you can daisy chain monitors together and connect two interconnected external displays to a single Thunderbolt connector on the host PC.
Machines with a Mini DisplayPort Connector
Machines with Mini DisplayPort connectors can use up to two external monitors through these connectors. If using a chain of Thunderbolt-connected devices, the display must be the last device in the chain.
Machines with a Thunderbolt 3 Connector
Macs with at least one Thunderbolt 3 connector can connect one display to each Thunderbolt 3 connector. If you interconnect multiple Thunderbolt devices, the display must be the last one in the chain. If you use a Thunderbolt 3 display that contains additional USB ports, you can use those for data transfer and charging devices.
Machines with a Thunderbolt 4 Connector
macOS machines with Thunderbolt 4 connectors can connect up to eight additional displays, depending on the model’s port availability.
Make and Model Requirements: Display Models
The older the iMac you want to use as a second monitor, the more difficult it is to use it as an additional display. Older iMacs have physical and software limitations that make it harder to use them as a second monitor. Let’s examine the system requirements for the different iMac models.
- macOS High Sierra or an earlier operating system installed
- Mini DisplayPort connection between machines
- The host system must be a 2019 or earlier macOS machine with High Sierra or an earlier operating system installed
- macOS High Sierra or an earlier operating system installed
- Thunderbolt or Thunderbolt 2 connector
- The host system must be a 2019 or earlier macOS machine with Catalina or an earlier operating system installed
Step 1: Shut Down Both the Host and Display System
First, you’ll need to completely power down both the system you intend to use as a display and the one you intend to connect to. Select the Apple menu, and choose the “Shut Down” option on both systems. Wait for them to power off.
Step 2: Plug the Display Machine into the Host Machine
Plug the display and any devices you daisy chained into the host machine. Ensure that the connection is secure and firm. Remember that chained Thunderbolt devices before Thunderbolt 4 require the display to be the last device in the chain.
Step 3: Activate Target Display Mode on Your iMac
With a keyboard plugged into the iMac you intend to use as a second display, press Command + F2 or Command + Fn + F2 to trigger Target Display Mode, which will mirror your host computer’s display onto the iMac.
Once you do this, the iMac will begin mirroring the host machine’s desktop display. Apps that were open on the host machine will not close or pause when Target Display Mode is activated. So, for instance, if you have music playing on your host machine, your music won’t pause or stop when you activate Target Display Mode.
When using Target Display Mode, the iMac can’t use additional features like the camera or the built-in USB ports unless the host computer has Thunderbolt connectors that are generation 3 or 4. As per Apple’s support documentation, host computers with Thunderbolt 3 and 4 connectors can connect to another display with built-in USB or Thunderbolt connectors active. However, as with any feature that relies on a port’s generation, your mileage may vary.
If you need to connect a device to your computer to charge or transfer data, we recommend connecting it directly to the host computer.
Controlling the Brightness and Built-In Speakers on an iMac Being Used as a Second Display
You can output sound from your host computer through your iMac’s speakers. Using your iMac as the speaker device might be helpful if you use the iMac as a second display for a MacBook device, which doesn’t have the same speaker quality as the iMac all-in-one computer.
To change which device speakers you use to output the sound from the host computer, start by going to the Apple menu and navigating to the System Preferences dialog. Select the “Sound” submenu and go to “Output.” Change the output device to the second display.
If your second display is set as the output device, you must plug a keyboard into it to control the volume and sound settings. Additionally, if a keyboard is plugged into the second display, you can use the media keys on the keyboard to control the second display’s brightness.
What to Do If Target Display Mode Doesn’t Work
You can take a few steps to resolve the issue if Target Display Mode isn’t working on your second monitor. Let’s look at what you can do if Target Display Mode isn’t working as expected.
Step 1: Ensure Both Devices Are Target Display Mode Compatible
Go to the Apple Menu and select “About this Mac.” Doing this will bring up a window with all the system information you need to ensure that both devices are compatible with Target Display Mode, like the device manufacturing year.
As stated above, iMacs up to mid-2014 can be used as second monitors, and the host computer cannot be newer than a 2019 model. If either device is outside these requirements, you cannot use Target Display Mode, regardless of what you do.
Step 2: Restart Both Devices
We know it’s frustrating to hear “turn it off and turn it back on,” but this step is always the first step you should take when experiencing technical difficulty. First, turn off the iMac you’re using as a second display and try to activate Target Display Mode again when the device powers back on. If Target Display Mode still doesn’t activate, restart the host computer.
Step 3: Unplug the Thunderbolt or Mini-DisplayPort Cable
Another issue can be with the Thunderbolt or Mini-DisplayPort cable connecting the second display to the host computer. Start by unplugging the cable and plugging it back in. Ensure that the connection is secure and physically stable. Check the port for any damage. If you have a can of compressed air, you can blow some air into the port to clean it out. Finally, try using a different cable altogether. Not all cables are made equal, especially Thunderbolt cables. Some Thunderbolt cables may lack the specifications necessary to be used as a connection between an iMac and another macOS device.
Step 4: Log Out of the iMac You’re Trying to Use as a Second Display
Sometimes, you can run into trouble trying to use an iMac with an account logged into it as a second display. Go to the Apple menu and select “Log Out” to return to the login screen. Then, try to activate Target Display Mode while on the login screen.
Step 5: Check to See If the Function Keys Are Active
Another issue that can arise is with the function keys. Go to the Apple menu and select “System Preferences.” Navigate to “Keyboard” and see if “Use F1, F2, etc. keys as standard function keys” is selected. If it is, you can activate Target Display Mode using Command + Fn + F2 rather than the standard Command + F2.
Step 6: Try a Different Keyboard
The best keyboard to use to activate Target Display Mode is the one that came with your iMac. Not all keyboards support the command for Target Display Mode. So, if you’re using an alternative keyboard or even an older macOS keyboard made before Target Display Mode, your keyboard might not send the correct signal when you try to activate it.
Step 7: Check Your Operating System
It’s also crucial that you check your operating system. If you regularly upgrade your operating system, you may have a version of macOS that does not support Target Display Mode. The second display and the host machine must both have macOS High Sierra 10.13.6 or earlier. Any macOS version after High Sierra 10.13.6 will not support Target Display Mode.
If one or both machines have an incompatible operating system, you cannot use Target Display Mode unless you roll the operating system back to an earlier version.
How Can I Use a Newer Mac Computer as a Second Display?
Apple Silicon Macs can use an older iMac as a display, but not with Target Display Mode. Newer macOS operating systems do not support the feature. Additionally, newer macOS devices cannot roll the operating system back to High Sierra or Catalina. Here are your options if your host device is too new to use Target Display Mode:
- AirPlay (macOS Monterey or later)
- Luna Display (supports 5K)
- Cloud Duet Display (subscription service only)
If you have an iMac that was manufactured from 2010 until mid-2014 running High Sierra or earlier, you can use Target Display Mode. Any 2019 or earlier macOS device that runs Catalina or earlier (2011-2014 iMacs) or High Sierra or earlier (2009/2010 iMacs), can serve as your host machine. Follow the process we’ve outlined above, and you should be up and running in no time.
The image featured at the top of this post is ©Krisda/Shutterstock.com.