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last month I asked about how many monitors an iMac 21.5 4k can handle.

So as suggested I went and checked in apple support page, turns out this iMac only supports up to two additional displays and I currently have 3 displays so 1 of them is totally inactive.

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I have the adapters but when I connect the 3 displays at the same time it randomly selects 2 of them.

Is there a way to enable 3 display config? Thanks

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Yes, you can enable a 3 display config in roughly three ways:

1) Using an eGPU. This is a real, full powered external GPU connected to the iMac over the relatively fast Thunderbolt 3 bus. You can buy different eGPUs at various speeds, but will generally enable you to do everything on your third monitor that you can do on the first two.

This is how an eGPU is used: https://support.apple.com/en-gb/HT208544

An example of an eGPU where you bring your own GPU (299$): https://www.razer.com/gaming-laptops/razer-core-x

Another example with pre-installed GPU (399$): https://www.sonnettech.com/product/egfx-breakaway-puck.html

2) Using a DisplayLink adapter. This is essentially a kind of "light weight" external GPU that is connected to the iMac over a relatively slow USB device - enabling an extra display suitable for "light weight" jobs. I.e. it is going to be just fine when working with Excel, but if you're going to be playing 3D games, it's probably going to be overworked. Note that there seem to be some complaints about driver instability or upgrade problems, so it might not be a perfect solution.

You can find a list of DisplayLink products here: https://www.displaylink.com/products

An example of a product: https://www.startech.com/uk/Cards-Adapters/Laptop-docking-stations/usb-3-docking-station~DK30A2DH

Another example product: https://store.hp.com/us/en/pdp/hp-usb-c-a-universal-dock-g2

Note that not all of these are necessarily compatible with your Mac.

3) Using an external computer to control a third display communicating with it over either WiFi or wired ethernet. This can be everything from an iPad connected with Sidecar to a complete computer running various remoting software.

The three options are listed in order according to performance. However, note also that the eGPU is usually the most expensive option (if you consider option 3 as using spare hardware that you have lying around anyways).

  • Thanks for the very detailed explanation. I was aware of option number 3 but as you mention is kind of expensive. – Karlo A. López Nov 25 '19 at 1:34
  • The thing is I currently own the following adapter: startech.com/AV/display-and-video-adapters/… Does that count like a "DisplayLink"? I currently have 2 USB-C ports and one is attached to this device, but if I attach an original apple HDMI adapter one of the displays remains with no content. Do you know exactly what device could do the trick? – Karlo A. López Nov 25 '19 at 1:36
  • @KarloA.López No, that adapter is not DisplayLink. DisplayLink is the name of a company that developed a specific technology that is licensed and marketed as DisplayLink - it is not just any piece of cable/adapter that links a computer and a display together. So you should look for the trademarked name "DisplayLink" to be in the description of the adapter for it to actually feature DisplayLink. The device you have linked cannot be used to in itself get 3 seperate displays working on your Mac. – jksoegaard Nov 25 '19 at 1:42
  • I have added a link to the list of DisplayLink products, as well as a link to two example products. – jksoegaard Nov 25 '19 at 1:48
  • Really helpful links. Thank you – Karlo A. López Nov 25 '19 at 3:50

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