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I have a mid-2015 MacBook Pro running High Sierra. I normally use it as a desktop computer in clamshell mode with an external monitor.

I just replaced my 24-inch external monitor with a 32-inch monitor. The new monitor is an Acer EB321HQU-B, which is supposed to support up to 2560x1440 resolution.

According to the technical specifications my Mac is supposed to support

full native resolution on the built-in display and up to 3840 by 2160 pixels on up to two external displays, both at millions of colors.

and

Support for 3840-by-2160 resolution at 30Hz

I have the new monitor connected via a Mini DisplayPort to HDMI adapter.

But the best I can get is 1920x1080, which is the same as my old monitor. In Display preferences, the only options I see in Scaled are 1600x900, 1080p, 1080i, and 720p, whether I select 60Hz or 30Hz. So I can't show any more on the screen, it's just all scaled up 33%.

Is there some way I can take advantage of the new monitor?

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The problem is the cable you’re using to connect the monitor to the computer.

The easiest way to ensure that you have the full resolution is to use a standard mini-DisplayPort to DisplayPort cable and you should be able to use the full resolution without problems.

You can also use a mini-DisplayPort to DVI cable, but these are usually harder to come by in a standard home electronics store.

Last but not least, it might be possible to get the full resolution by using a better HDMI cable. You'll need one that supports HDMI 1.3 or 1.4 to support the 2560x1440 resolution.

  • Thanks. I went to Staples, they didn't have any Mini-DP to DP in stock, but they ordered one for me. Assuming it solves the problem I'll accept this answer. – Barmar Mar 17 at 23:59
  • I probably won't be able to accept the answer for a couple of weeks; the cable may not arrive until Friday, and I'll be away on vacation. I'm just wondering if you think it will also address another issue I'm seeing: when I do screen sleep, the monitor backlight stays on. Will it be able to go completely black with DP? – Barmar Mar 19 at 0:42
  • The cable came today, the result is beautiful. – Barmar Mar 20 at 15:03
  • The issue with screen sleep is actually something else. It's ignoring the "Put the screen to sleep" setting in Energy Saver, and showing the screen saver, which is set to a black screen. I didn't have this problem with my previous VGA monitor. I'll open a new question. – Barmar Mar 20 at 15:05
  • Sounds good with the cable :-) – jksoegaard Mar 20 at 15:24
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Why did you not use a Mini DisplayPort to DVI adapter? For example, this item.

A mini-DisplayPort actually has the DVI signals. Only the jack is smaller. The converter does not need to convert the signals. There are just wires connected between the smaller mini-DisplayPort pins and the larger DVI jack pins.

  • Because that's what the MicroCenter salesman sold me. – Barmar Mar 17 at 22:06
  • The monitor came with an HDMI cable, so he sold me the adapter for it. – Barmar Mar 17 at 22:07
  • Can I assume since you have a HDMI port, that there is also a DVI port, but no Display port (DP)? – David Anderson Mar 17 at 22:20
  • @DavidAnderson I know the monitor - it has HDMI, DVI and DisplayPort as I wrote in my answer. – jksoegaard Mar 17 at 22:23
  • Right, it has all those ports, but the only cable I have is HDMI. I've ordered a new cable. – Barmar Mar 18 at 0:01

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