iMac as a monitor.
Which cable actually works.
For a Mac from 2020 or later, the cable carries a network connection, not a video signal, so it depends on the app. Encore needs at least 400 Mbps between the Macs: Gigabit Ethernet on any iMac from Late 2009, or Thunderbolt on 2011 and later. An HDMI cable alone won't work, because iMacs don't have an HDMI input. The exact parts for your iMac are on the cable page.
The first question: what is the cable for?
There are two very different things a cable can do between a Mac and an iMac.
- A video signal. Only Target Display Mode does this, and only with a Mac from 2019 or earlier on Catalina or older, on an iMac from 2009 to Mid 2014. Apple's rules.
- A network link. Encore and the other apps turn the cable into a fast connection and send the picture over it as compressed data. This is what works with a Mac with Apple silicon.
Most confusion about cables comes from mixing these up. For a video signal, the cable must match Apple's list exactly. For an app, the cable needs to be fast enough.
Cables that don't work
- HDMI. iMacs don't have an HDMI input. Apple's Target Display Mode page only lists Mini DisplayPort and Thunderbolt, and on later iMacs the Thunderbolt ports are outputs: a poster in a TidBITS Talk thread notes those TB2 ports "support video out only, not video in". To put HDMI on an iMac you need a capture card and a viewer app.
- A plain USB-C charging cable. It doesn't carry Thunderbolt. For Thunderbolt 3 or 4 you want the lightning-bolt logo. Encore needs Thunderbolt or Gigabit Ethernet.
- A 10/100 Ethernet adapter. It tops out at 100 Mbps, well below the 400 Mbps Encore needs. On a label, look for Gigabit or 10/100/1000.
- Docks and hubs, for Target Display Mode. Plugable says its docks and adapters don't pass Target Display Mode. Software routes are more forgiving, but every extra part slows things down.
Mini DisplayPort and Thunderbolt look identical
This catches people out. The Mini DisplayPort port on a 2009 or 2010 iMac and the Thunderbolt port on a 2011 to 2015 iMac share a connector shape. Macworld warns that the connectors look the same but are wired differently, and to check for the lightning-bolt icon on a Thunderbolt cable. If you are buying used cables, check before you plug in.
Macworld's advice for Target Display Mode with a Thunderbolt 3 Mac (Intel, not Apple silicon): a USB-C to Mini DisplayPort adapter for 2009 and 2010 iMacs, and Apple's Thunderbolt 3 to Thunderbolt 2 adapter with a Thunderbolt 2 cable for 2011 to 2014 iMacs.
What to buy for each iMac (with Encore)
| iMac | Port on the iMac | Connect with |
|---|---|---|
| Late 2009 and 2010 | Gigabit Ethernet; Mini DisplayPort is for TDM only | USB-C to Gigabit Ethernet adapter and a Cat6 cable |
| 2011 to 2013 | Thunderbolt and Gigabit Ethernet | Apple's Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter and a Thunderbolt cable, or Gigabit Ethernet |
| Mid 2014 and 2015 | Thunderbolt 2 and Gigabit Ethernet | The same adapter and a Thunderbolt 2 cable, or Gigabit Ethernet |
| 2017 to 2020 | Thunderbolt 3 and Gigabit (10 Gigabit on some 2020 models) | A Thunderbolt 3 or 4 cable with USB-C on both ends, or Gigabit Ethernet |
Every part in the chain counts: a Thunderbolt 3 adapter, a Thunderbolt 2 cable, and a Mac port that supports Thunderbolt. The full list, with typical prices and traps, is on the cable page.
Ethernet or Thunderbolt?
Gigabit Ethernet is cheap, works on every iMac from Late 2009, and gives Encore a full 1 Gbit/s link: well above the 400 Mbps minimum. Thunderbolt costs more but carries several Gbit/s, which Encore uses for Ultra (ProRes 4444) at up to 1440p, and gives other apps the bandwidth for very high resolutions. For a 1080p iMac, Gigabit is plenty. For a 2560 x 1440 or Retina iMac, Thunderbolt has more headroom.
What other apps want
- RetinaRelay: Thunderbolt, USB SuperSpeed or a Gigabit Ethernet adapter.
- Second Display: Thunderbolt for its lossless mode; USB-C, Wi-Fi or Ethernet are compressed.
- MacDisplay: Thunderbolt 3 or later, or Gigabit Ethernet. Its page says Thunderbolt 2 Macs should use Ethernet or Wi-Fi.
- Luna Display: the dongle goes directly into your Mac with no adapters, and a separate cable connects to the iMac.
How to check your cable worked
In Encore on your Mac, the iMac's card offers Cable and marks it Recommended as soon as it finds the cable. If an adapter is too slow, the card says so, for example that it only does 100 Mbit/s. If Cable doesn't appear, try another port, and see the help center. Sharp, lossless text needs 400 Mbps or more, so a 10/100 adapter will still leave you short.
Questions
Can I use HDMI to use my iMac as a monitor?
Not directly. iMacs don't have an HDMI input. A capture card plus a viewer app can show HDMI on the iMac, but that is not the same as a monitor connection.
Is a USB-C charging cable enough?
No. Use a Thunderbolt 3 or 4 cable with the lightning-bolt logo, or Gigabit Ethernet.
Do I need a Thunderbolt cable for Encore?
No. Gigabit Ethernet works on every iMac from Late 2009. Thunderbolt adds headroom, especially for Ultra and larger screens.
Why does my cable work but the picture is still soft?
Check the link speed in Encore: a 10/100 adapter or a slow port in the path caps you below 400 Mbps. Then see the help center.