Connecting 2019 iMac to Type C monitor

Hi,

Following article could help your issue.

Connect an external display to your iMac - Apple Support (CA)

Please refer to the Tech Specs for your 2019 27" iMac:

Video Support and Camera

• FaceTime HD camera
• Simultaneously supports full native resolution on the built-in display at 1 billion colors and:
  • One 5120‑by‑2880 (5K) external display at 60Hz with support for 1 billion colors, or
  • Two 3840-by-2160 (4K UHD) external displays at 60Hz with support for 1 billion colors, or
  • Two 4096‑by‑2304 (4K) external displays at 60Hz with support for millions of colors
• Thunderbolt 3 digital video output
• Native DisplayPort output over USB‑C
• Thunderbolt 2, HDMI, DVI, and VGA output supported using adapters (sold separately)

You need a Thunderbolt 3 to HDMI adapter!!!!

That's a 4K UHD monitor. It has HDMI 2.1, DisplayPort 1.4a, and USB-C inputs.

https://www.msi.com/Monitor/G321**

https://download.msi.com/archive/mnu_exe/monitor/G321CUv1.0_English.pdf (user manual)

The specifications page and manual claim that all of the inputs support 3840 x 2160 at up to 144 Hz. (To get 144 Hz refresh rate over HDMI, you must be using HDMI 2.1.)

That doesn't mean the the Mac does. I don't know whether you have a 21.5" iMac or a 27" one, but the Technical Specifications for both only refer to a 60 Hz refresh rate. There's no promise of anything higher.

iMac (Retina 5K, 27-inch, 2019) - Technical Specifications

iMac (Retina 4K, 21.5-inch, 2019) - Technical Specifications

It does appear that the monitor should be able to fall back to 3840 x 2160 @ 60 Hz (monitor manual, page 22). So the answer to "Why can't I get 120 Hz or 144 Hz?" might be "It's not supported.", but I'm not sure why you are not able to get the monitor to work at all.

ebolaboy wrote:

I'm confused if the 2019 iMac does not work with a type C monitor because it does not have DP alt mode capability

It supports that ALT mode. That's how you'd attach most monitors – short of an 27" Apple 5K Studio Display.

rkaufmann87 wrote:

You need a Thunderbolt 3 to HDMI adapter!!!!

Most USB-C to HDMI adapters go from USB-C (DisplayPort) to HDMI, not from USB-C (Thunderbolt 3) to HDMI. Even when vendors mention Thunderbolt, they're usually just trying (in a clumsy way) to reassure customers that their USB-C (DP) adapter will, in fact, work with a Mac that has Thunderbolt ports.

There are plenty of monitors that take USB-C (DisplayPort) input that a 2019 iMac should be able to drive with a quality USB-C to USB-C cable. The thing that stands out here is not the USB-C connector, per se, but the high (120 Hz and 144 Hz) refresh rates.