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External monitor support for the M1 iMac

I have an M1 iMac with 2 usb c with thunderbolt 4 support. I am trying to use an adapter usb c to hdmi to try to connect my monitor with the iMac, but it is simply not detecting the monitor. can someone help debug the problem? the monitor i have is LG ultragear monitor and i am trying to use the given HDMI cable with the adapter i got on amazon. I have checked that the adapter is working with my other laptop and the same hdmi cable and it is working fine.

iMac 24″, macOS 14.1

Posted on Dec 31, 2023 2:32 AM

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10 replies

Jan 1, 2024 7:53 AM in response to barkha33

barkha33 wrote:

The LG monitor that i have is no longer in production. However i can provide you with the specs if that makes it easier. It is with 27 inch G-Sync compatible 240 Hz monitor, FHD, 16:9 aspect ratio: https://www.amazon.in/gp/aw/d/B07MKT2BNB?psc=1&ref=ppx_pop_mob_b_asin_title
Also the monitor is not fixed at 240 Hz, i can set it to even 30Hz in the monitor settings, so that should not be an issue.

Also here is the link to the adapter: https://www.amazon.in/gp/product/B0CHQJLMRL/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&th=1


The description of the adapter says that it has three ports:

  • A HDMI port that supports 4K resolution at 30 Hz.
  • A USB-A (USB 3.0) port.
  • A USB-C port that supports up to 100 watts of USB-C Power Delivery from a charger to a notebook computer


The description of the monitor says that it is a LG Ultragear 27Gn750 – a 27" monitor with 1920x1080 resolution and a 240 Hz refresh rate.


This would appear to be its description on the LG site.

LG – 27" UltraGear Gaming Monitor (27GN750-B)


Someone answered a question about its refresh rate over HDMI saying that

"It … supports 1920 x 1080 @ 60/120/144Hz over HDMI."


In the user manual

  • The Factory Support Mode table (page 36) shows no support for 30 Hz refresh rates.
  • The HDMI Timing (Video) table (page 37) only shows 30 Hz as an option if you are running in the 2160P Preset Mode with HDMI Compatibility Mode turned off. (That means that whatever device was providing the monitor with a HDMI signal would have to support HDMI version 2.0 or better.)


Since the monitor does not have 4K (2160P) resolution, I'm guessing that in 2160P Preset Mode, it accepts a low-refresh-rate 4K signal and proceeds to throw 3/4th of the resolution away.


If the AmazonBasics adapter can drive a 4K monitor at 30 Hz, I would hope it could drive a 1080p one at 60 Hz or better … but I wouldn't absolutely count on it


The specifications in the manual say that the monitor has both HDMI and DisplayPort inputs. This suggests that another way you could connect it to your M1 iMac would be with a USB-C to DisplayPort adapter cable.

Dec 31, 2023 2:45 AM in response to barkha33

What is the specific model of LG UltraGear monitor? (I see a bunch of UltraGear monitors on the LG site, and they seem to have refresh rates that the Mac and/or adapter might not support. Many also have resolutions above the 1920x1080 that some HDMI adapters or cables might support.)


Do you have a link to the description of the AmazonBasics adapter on the Amazon site?

Jan 1, 2024 1:28 AM in response to Servant of Cats

The LG monitor that i have is no longer in production. However i can provide you with the specs if that makes it easier. It is with 27 inch G-Sync compatible 240 Hz monitor, FHD, 16:9 aspect ratio: https://www.amazon.in/gp/aw/d/B07MKT2BNB?psc=1&ref=ppx_pop_mob_b_asin_title

Also the monitor is not fixed at 240 Hz, i can set it to even 30Hz in the monitor settings, so that should not be an issue.


Also here is the link to the adapter: https://www.amazon.in/gp/product/B0CHQJLMRL/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&th=1

Jan 3, 2024 3:36 AM in response to Servant of Cats

Thank you very much for taking the time and effort to reply. I will follow the solution you pointed out.


One thing that you mentioned that the adapter supports 4k 30hz, and it should go higher for 1080p 60hz, is absolutely true because like i mentioned, it works perfectly fine when i connect my monitor to my lenovo laptop using the adapter. So this issue i supposed is somehow related to mac silicon processing the signals rather than the adapter/monitor/cable.

External monitor support for the M1 iMac

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