Using Target Display Mode on iMac with USB-C MacBook

[nshinka]

Hi everyone,

If I have an iMac compatible with Target Display Mode, would I be able to connect it to a newer USB-C MacBook using a USB-C to Thunderbolt 2 adaptor + Thunderbolt 2 cable? Or in the instance of the 2009/2010 27" iMac, would a USB-C to Mini-DP adaptor + Mini-DP cable work? If no, why not?

Thanks!

 

[joevt]

It looks like you know everything about Target Display Mode.

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For a Mini DisplayPort iMac, use a USB-C to Mini DisplayPort cable or USB-C to DisplayPort cable with DisplayPort to Mini DisplayPort adapter. The display should perform as a display connected directly to the GPU of the MacBook.

For a Thunderbolt iMac, use the Apple Thunderbolt 3 to Thunderbolt 2 adapter with a Thunderbolt 2 cable. But a USB-C MacBook does not have Thunderbolt so this will not work. You cannot make this work by including an externally powered Thunderbolt 3 add-in card such as a GC-TITAN RIDGE or GC-ALPINE RIDGE because the Thunderbolt Target Display Mode signal is not just DisplayPort over Thunderbolt (I'm guessing). You would need a PCIe connection to the add-in card and the add-in card would need Apple Thunderbolt firmware.

I wonder how a Thunderbolt Target Display Mode display looks in ioreg -flw0 and /System/Library/Extensions/AppleGraphicsControl.kext/Contents/MacOS/AGDCDiagnose -a > AGDCDiagnose_a.txt 2>&1? I guess it would not behave or perform like a display connected to a GPU. It would behave like a DisplayLink display except Apple could make a better Thunderbolt display driver than the DisplayLink people can make a USB display driver. Well, the output from AGDCDiagnose should be able to prove that Thunderbolt Target Display Mode is not using DisplayPort over Thunderbolt.

 

[nshinka]

Thanks for your response.

I was under the impression that USB-C *is* Thunderbolt 3? In that case, the TB-2 cable with the TB-2 to USB-C adaptor from Apple should work, right? If I check the adaptor that Apple sells, it has a USB-C connection at one end and at the other end a port to plug a TB-2 cable into.

 

[joevt]

USB-C is not Thunderbolt 3.

USB-C is a connector. Thunderbolt is a signal type. The USB-C connector can be used for many signal types (USB 2.0, USB 3.x, USB4, DisplayPort, Thunderbolt, power delivery ,VirtualLink, HDMI, MHL).

USB-C - Wikipedia

en.wikipedia.org

A Thunderbolt 3 port can support Power Delivery, USB 2.0, USB 3.x, DisplayPort, and Thunderbolt.

The USB-C port of the MacBook (not MacBook Pro) does not support Thunderbolt. It can support Power Delivery, USB 2.0, USB 3.x, DisplayPort.

Adapters can be used to convert DisplayPort to VGA, HDMI, DVI, Dual Link DVI.

The Apple Thunderbolt 3 to Thunderbolt 2 adapter only passes Thunderbolt signals. The MacBook can't output a Thunderbolt signal. Therefore the Thunderbolt adapter is not usable with the MacBook.

 

[chrfr]

Thanks for your response.

I was under the impression that USB-C *is* Thunderbolt 3? In that case, the TB-2 cable with the TB-2 to USB-C adaptor from Apple should work, right? If I check the adaptor that Apple sells, it has a USB-C connection at one end and at the other end a port to plug a TB-2 cable into.

If you’re trying to use a Thunderbolt equipped iMac (at least a 2011 model) in target display mode, yes, you’d need to use the TB2 to TB3 adapter as well as a Thunderbolt cable. If you have the 12” MacBook with only one USB-C port then you cannot use Thunderbolt iMacs as displays.
Older models would require a USB-C to Mini DisplayPort cable and/or adapter as appropriate.

 

[diddl14]

Hi everyone,

If I have an iMac compatible with Target Display Mode, would I be able to connect it to a newer USB-C MacBook using a USB-C to Thunderbolt 2 adaptor + Thunderbolt 2 cable? Or in the instance of the 2009/2010 27" iMac, would a USB-C to Mini-DP adaptor + Mini-DP cable work? If no, why not?

Thanks!

Did you ever get this working? I'm looking into buying an adapter and cable and also wonder what is the best option..