Big Sur 11.3.1 No Detection of Extended display

[Nat501]

New iMac with Big Sur 11.3.1 - OSX will not detect Extended display (4K Samsung HDMI) through Apple USBC to Thunderbolt 2 adapter.

 

[joevt]

Update to 11.5.2?

Why are you using a Thunderbolt adapter for an HDMI display? The Thunderbolt adapter only works with Thunderbolt signal. You cannot connect Mini DisplayPort to it directly - you need a Thunderbolt dock to convert Thunderbolt signal to DisplayPort. Read the product page and support page:
https://www.apple.com/ca/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter
https://support.apple.com/en-gb/HT207266 (Thunderbolt 2 cable does not mean DisplayPort cable)

Instead, you should just use a USB-C to Mini DisplayPort adapter or better yet, a USB-C to HDMI adapter (since the Mini DisplayPort to HDMI adapter you're trying to use probably doesn't support HDR? - what adapter are you using?)

Consider one of the following:
- Apple USB-C Digital AV Multiport Adapter A2119
- Club 3D CAC-1085 (DisplayPort 1.4 to HDMI 4K120Hz)
- Cable Matters 201388 USB-C to HDMI 2.1 Adapter

https://forums.macrumors.com/thread...ransfers-recommendation.2278473/post-29466205

 

[Nat501]

Great. Thanks for the heads-up. I will get a usbc to hdmi.
My reasoning was quite simple. My original iMac had TH2, and I used a TH2 - HDMI cable. This worked fine.
So bought a USBC - TH2 for the new iMac assuming theory would work the same way.
All good. Thanks again.

 

[joevt]

My reasoning was quite simple. My original iMac had TH2, and I used a TH2 - HDMI cable. This worked fine.
So bought a USBC - TH2 for the new iMac assuming theory would work the same way.

I knew that was probably the reason. It was probably rude of me to ask. It's a strange thing. In the case of the iMac's Thunderbolt 2 port, the Thunderbolt controller chip that the port is connected to can output a Thunderbolt signal or a DisplayPort signal through the port. A Thunderbolt 2 dock has a similar Thunderbolt controller that can do the same thing.

The Apple Thunderbolt 3 to Thunderbolt 2 Adapter doesn't have a Thunderbolt controller chip though. Whatever chip it has (if it has one) does not have the ability to convert Thunderbolt to DisplayPort. Whatever chip it has (if it has one) does not attempt to detect that a DisplayPort device is attached and does not therefore indicate to the host computer that it should switch to USB-C DisplayPort Alt Mode. There's a teardown at ifixit.com. Looks like most of the chips are for power.
https://www.ifixit.com/Teardown/Apple+Thunderbolt+3+(USB-C)+to+Thunderbolt+2+Adapter+Teardown/124055

The dual nature (Thunderbolt / DisplayPort) of the Thunderbolt 1/2 port is kind of like a precursor to the USB-C Alt Modes of USB-C. There is a USB-C Alt Mode for Thunderbolt and a USB-C Alt Mode for DisplayPort. There are other USB-C Alt Modes that are less common (and are not supported by Macs). I guess USB4 is a USB-C Alt Mode supported by M1 Macs and is compatible with Thunderbolt (I'm not sure if they're exactly separate or just different versions of the same Alt Mode - the USB4 spec will have that info).