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tobzzzz Author

Level 1

9 points

LG 28UK850 External 4k monitor to iMac 2015 5K - What cable type/settings?

I have a USB-C to USB-3 cable from the monitor to iMac.

The external screen doesn't come on, and in display settings, no external monitors are found.

When I plug it in, I get a warning on the iMac that reads:

Accessory Needs More Power - Disconnecting other accessories may allow this to work.

The external monitor is plugged into the wall / mains electricity. I am not trying to power it, just trying to use it as a second monitor.

Can I use the cable I have somehow by altering settings?
If not, what kind of cable do I need for the 2015 iMac (see port options below)?

LG monitor has USB-C, HDMI, DisplayPort (large/standard size)

iMac 2015 has USB-3 and ThunderBolt 2.

iMac 27", macOS 10.14

Posted on Apr 1, 2020 5:15 AM

Top-ranking reply

Malcolm J. Rayfield

Level 7

23,059 points

Posted on Apr 1, 2020 7:48 PM

You need a Mini DisplayPort to DisplayPort Cable, such as:

https://www.startech.com/Cables/Audio-Video/DisplayPort/2m-White-Mini-DisplayPort-to-DisplayPort-Adapter-Cable-Male-to-Male~MDP2DPMM2MW

Plug it into one of the iMacs Thunderbolt ports.

Thunderbolt cables or adapters won't work.

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3 replies

Top-ranking reply

Malcolm J. Rayfield

Level 7

23,059 points

Apr 1, 2020 7:48 PM in response to tobzzzz

You need a Mini DisplayPort to DisplayPort Cable, such as:

https://www.startech.com/Cables/Audio-Video/DisplayPort/2m-White-Mini-DisplayPort-to-DisplayPort-Adapter-Cable-Male-to-Male~MDP2DPMM2MW

Plug it into one of the iMacs Thunderbolt ports.

Thunderbolt cables or adapters won't work.

Apr 1, 2020 6:00 AM in response to tobzzzz

tobzzzz wrote:

....

LG monitor has USB-C, HDMI, DisplayPort (large/standard size)

iMac 2015 has USB-3 and ThunderBolt 2.

First displays are not supported on the USB version on

your iMac.

The first and cheapest solution may be Mini Display Port to Display Port cable.

The other and more expensive option, is a Thunderbolt (1 or 2) to Thunderbolt 3

adapter.

tobzzzz Author

Level 1

9 points

Apr 6, 2020 6:11 AM in response to woodmeister50

Thanks @woodmeister50 and Malcolm R...

I went for this which supports up to 4k in the spec and isn't an adaptor but a cable:

https://www.amazon.co.uk/gp/product/B00YOQYQWY/

It works, but intermittently flickers - every minute or two. I've tried default scaling (big) and scaled. Refresh rate is greyed-out so cannot be altered, at 60 - which seems default. From what I've read, a lot of people with external monitors have this issue with Macs, (both MBPs and iMacs). It seems Apple just aren't very good at supporting external monitors. I'll probably just live with it - it's only during this covid-19 WFH time. Once I'm back in the office I'll use my old display.

LG 28UK850 External 4k monitor to iMac 2015 5K - What cable type/settings?