Dual display problem

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PhilTheHat Author

Level 1

18 points

Dual display problem

I have a problem with my LATE 2013 iMac running 2 external displays.

- OS: Catalina 10.15.7
- Processor: 3.5 Ghz Quad-Core Intel Core i7
- Graphics: NVIDIA GeForce GT 755M 1 GB

HISTORY:

For around 5 years I have had 2 external Acer S271HL Dbid 27" Full HD Monitors connected to my Mac using Apple-supplied (Belkin) Thunderbolt-to-HDMI cables. Ocassionally , one or the other monitor would fail to start. Unplugging the TB-HDMI cable and/or a restart of the iMac would bring both monitors back to life. But not anymore.
In March this year I had the motherboard replaced after the Mac would not start.
1. Dual displays worked as normal after its return.
In October this year, the HDD failed and was replaced.
1. Dual displays worked as normal after its return.

PROBLEM:

Two monitors - A and B.

B will always come on.

A will NEVER come on.

- I have reset SMC numerous times
- I have reset PRAM numerous times
- I have reinstated macOS
- I have tried and failed to update the Thunderbolt firmware. Cannot find it.
- I have swapped TB-HDMI cables
- I have swapped power supplies
- I have tried using only Monitor A (disconnecting Monitor B completely
- I have plugged Monitor A into my TV, my Apple TV and my PS4 using their respective HDMI cables and IT WORKS FINE!

System Information shows this for "Graphics/Displays":

NVIDIA GeForce GT 755M:

Chipset Model: NVIDIA GeForce GT 755M

Type: GPU

Bus: PCIe

PCIe Lane Width: x16

VRAM (Total): 1 GB

Vendor: NVIDIA (0x10de)

Device ID: 0x0fea

Revision ID: 0x00a2

ROM Revision: 3782

Metal: Supported, feature set macOS GPUFamily1 v4

Displays:

iMac:

Display Type: LCD

Resolution: 2560x1440 (QHD/WQHD - Wide Quad High Definition)

UI Looks like: 2560 x 1440

Framebuffer Depth: 24-Bit Colour (ARGB8888)

Main Display: Yes

Mirror: Off

Online: Yes

Automatically Adjust Brightness: No

Connection Type: Internal

S271HL:

Resolution: 1920x1080 (1080p FHD - Full High Definition)

UI Looks like: 1920 x 1080 @ 60 Hz

Framebuffer Depth: 24-Bit Colour (ARGB8888)

Display Serial Number: LUWEE0258538

Mirror: Off

Online: Yes

Rotation: Supported

Automatically Adjust Brightness: No

"Thunderbolt" shows this:

Thunderbolt Bus:

Vendor Name: Apple Inc.

Device Name: iMac

UID: 0x0001000D1B6E0000

Route String: 0

Firmware Version: 23.10

Domain UUID: 46EC23B7-3D50-A055-BCBC-0002FFB5C47F

Port:

Status: No device connected

Link Status: 0x101

Speed: Up to 10Gb/s x2

Receptacle: 1

Link Controller Firmware Version: 0.12.3

Port:

Status: No device connected

Link Status: 0x101

Speed: Up to 10Gb/s x2

Receptacle: 2

Link Controller Firmware Version: 0.12.3

I am now at a complete loss. Can anyone help?

iMac Line (2012 and Later)

Posted on Nov 7, 2020 1:31 AM

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Answer 1

PhilTheHat Author

Level 1

18 points

Nov 11, 2020 10:21 PM in response to Alwayswantingtohelp1

Since I posted the original, the situation has deteriorated to the point that neither monitor would come on. The message on the screen was "No signal detected" which means there was no HDMI signal from either port (remember - these monitors work fine when plugged into another HDMI source. Apple diagnostics could find no errors so on a hunch I ordered a cheap Mini DP - VGA cable and the monitor camera to life immediately! This is both monitors and from either TB port. So this either points to both HDMI cables being broken OR no HDMI output at all from the Mac. I have ordered another cheap cable (this time Mini DP - DVI) and will see if this gets me back to a dual external monitor situation once more.

Is there any other way of testing the Mac for HDMI output? I can't test the cable without some other source with a Mini DP for TB port (which I don't have) so if I could somehow eliminate the issue being with the Mac, then I can go and order new HDMI cables.

Thanks

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Answer 2

Nov 12, 2020 7:40 AM in response to PhilTheHat

PhilTheHat,

Testing the HDMI cables with other devices that use HDMI connections could determine if there is an issue with the cables. If they work with other devices like a TV or game console, but do not with your Mac, then that could be an indication that the issue is with the ports on the Mac.

Mac Repair - Official Apple Support

Take Care.

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Answer 3

Nov 11, 2020 1:16 PM in response to PhilTheHat

Hello PhilTheHat,

Thank you for using Apple Support Communities!

We understand from your post that you have previously used two external displays with your iMac which continued to work after having both the motherboard and hard drive replaced. Did you have the motherboard and hard drive replaced by an Apple Authorized Service Provider? If you have completed the steps from this article and the issue persists, you may need to contact Apple Support for additional assistance.

Get help with video issues on external displays connected to your Mac - Apple Support

Get Support

Best Regards.

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Answer 4

PhilTheHat Author

Level 1

18 points

Nov 14, 2020 3:05 AM in response to Alwayswantingtohelp1

This is now defying all logic!

I moved my Mac to my TV room and hooked up my Sony Bravia TV - with both cables, both ports and even switched them around.

BOTH cables and BOTH ports sent HDMI perfectly to my TV.....

While I was there, I brought both monitors from the office and connected them to the PS4 via HDMI. BOTH worked flawlessly again.

I then returned my Mac to the office and hooked up both monitors again. This time monitor B came back to life with HDMI. Monitor A still lifeless - until I switched it back to VGA.

So now I have one monitor which simply will not work with Mac using HDMI, but will via VGA; and another monitor which now appears to be working with Mac via HDMI....until the next time it doesn't.

NONE of this makes sense!

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Dual display problem

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