External display not detected

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Level 1

13 points

External display not detected

Hi,

I have a MacBook Pro (16-inch, 2019), running Catalina 10.15.7. I also have two external monitors:

LG 27UL850
AOC G2590FX

Both monitors are connected using DisplayPort to USB-C connectors. Problem: Only the LG 27UL850 is detected. Here is everything I have tested to resolve the issue:

Switched connectors. Still, only the LG Monitor is detected which means it is not a problem with the connectors
Connected both monitors to a Windows 10 PC. Both monitors are detected, which means it is not a problem with the monitors.
Connected only one monitor at a time. Again, only the LG is detected.
Reset my NVRAM.
Reset my SMC

I'm out of ideas. Any help would be appreciated. Thanks.

MacBook Pro 16″, macOS 10.15

Posted on Oct 22, 2020 5:18 AM

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Answer 1

jfgauron Author

Level 1

13 points

Oct 22, 2020 7:04 AM in response to Sharhari

I've ordered a USB-C to VGA adapter, I should receive it tomorrow. In the mean time, could you explain why this could fix the issue? On my Mac the resolution for the LG Monitor is set to 1920x1080, so I assume it should be able to use the same resolution on the AOC Monitor, no?

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Answer 2

Sharhari

Level 5

6,293 points

Oct 22, 2020 7:41 AM in response to jfgauron

I'm not sure this could fix, but it would help to isolate the issue.

I'm aware when you set resolution other than native resolution most of the time they are scaled and it will work.

Also an important detail to look into is the refresh rate, where the LG has is 60Hz. which is in sync with the supported resolutions spec in your MacBook pro.

I'm leaning towards this spec not matching with your other monitor hence checking with alternate input method to monitor.

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Answer 3

Oct 22, 2020 9:05 AM in response to jfgauron

<<Connected only one monitor at a time. Again, only the LG is detected.>>

Based on your debugging the AOC monitor is defective.

--------

to get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display.

This query is only sent at certain times:

• at startup

• at wake from sleep

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• on invoking Option-(Detect Display) button in Displays preferences (from another display)

so try doing some of those things and see if the display comes alive.

Modern Displays with multiple ports are sometimes busy scanning the other ports, looking for an input, and miss the query from the Mac. They need to pay attention to the port you are actually using, or they will miss the query.

Some displays have On-Screen Display settings that can be used to tell the display a computer is attached on a certain port, or a certain port should be highest priority. Changing those may make your display more responsive.

Some displays include their own private "sleep" settings for the display alone. This can allow the display to enter its own sleep mode, on top of the Mac's not sending it data. A display that is sleeping on its own cannot respond to the Mac's query, and will stay dark.

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Answer 4

jfgauron Author

Level 1

13 points

Oct 22, 2020 9:20 AM in response to Grant Bennet-Alder

<<Connected only one monitor at a time. Again, only the LG is detected.>>

Based on your debugging the AOC monitor is defective.

How so? The AOC monitor works on Windows 10 without any problem.

to get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display.

This query is only sent at certain times:

• at startup

• at wake from sleep

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• on invoking Option-(Detect Display) button in Displays preferences (from another display)

so try doing some of those things and see if the display comes alive.

When I do any of these operations, the AOC monitor shows "no signal" on a black screen.

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Answer 5

Oct 22, 2020 10:19 AM in response to jfgauron

on Windows, you manually set the resolution. Then the computer sends out a data stream, regardless of whether a display is actually connected or not.

To work on a Mac, the display MUST respond to the Mac's Query, or the screen stays dark.

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Answer 6

Sharhari

Level 5

6,293 points

Oct 22, 2020 6:06 AM in response to jfgauron

Check if you can get display output with USB-C to VGA adapter (Since the resolution of monitor is 1920x1080).

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