External monitor support for the M1 iMac

First I would try moving the adapter to the iMac's other port.

Next I would test a 1.5m or shorter premium certified HDMI cable.

Then if neither of those work, I wound test the official Apple or Belkin adapter.

USB-C Digital AV Multiport Adapter - Apple

Belkin USB-C to HDMI Adapter - Apple

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.

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?

iMac connection looks good (assuming the HDMI cable is plugged in.. hard to tell if that is fully seated in photo).

Have you tried the traditional IT trick of power cycling the LG monitor off/on?

A short USB-C to DisplayPort 1.4 8K cable will offer greater resolutions and refresh rates.

for example > https://www.amazon.com/dp/B09M3BFHPP/ref=sspa_dk_detail_3?

and/or > https://www.amazon.com/dp/B0B8GP2H17/ref=sspa_dk_detail_6?

Plus it will cost less than upgrading to a less reliable USB-C to HDMI adapter and HDMI cable setup.