Mac only detects one display, mirrors to two screens

kernel_45 wrote:

I have seen a similar post regarding this matter but the general conclusion was that the user had a cheap docking station and that was the issue, and even though my dock is relatively cheap too (40$) im absolutely certain that's not the issue, since I have 2 other laptops (one with Windows and another one with Linux) and they work perfectly fine with 4 other monitors i have tried connecting (in pairs of 2). I'm pretty sure it's an OS programming bug.

If you were trying to connect both screes through your cheap $40 dock, I can guarantee that despite your protests, your cheap docking station absolutely was the issue.

At that price point, you do not get Thunderbolt docks. Any dock at that price point that has multiple places to plug in monitors is using

• DisplayPort MST – which the Mac does not support, or
• Some second-class workaround like DisplayLink (instead of first-class, hardware-accelerated video output), or
• Both

The typical symptom of trying to use DisplayPort MST on a Mac is that the Mac only detects one monitor, and that monitors appear to "mirror" each other (since the dock is feeding them copies of the same signal). These are the same symptoms that you reported.

To connect two monitors to an external dock, hub, or adapter, and be able to run them in extended desktop mode, that dock, hub, or adapter must be a genuine Thunderbolt one. Thunderbolt docks typically can support up to two (non-Thunderbolt) monitors with resolutions of up to 4K each, or a single 5K/6K Thunderbolt monitor such as the 27" Apple 5K Studio Display or the 32" Apple 6K Pro Display XDR.

If you do not want to buy a Thunderbolt dock, hub, or adapter, then you will need to connect your two monitors via different Mac host ports.

kernel_45 wrote:

Hello, I'm not going to thank you for your condescending message. Luckily, I've managed to temporarily solve the problem, as I mentioned here, so the dock definitely wasn't the issue. I repeat, it worked on Windows and Linux, so despite your protests, the dock is not the problem. It is a thunderbolt dock and it's selling for 37€ right now. https://www.amazon.es/NOVOO-Docking-Station-Pantalla-Adaptador/dp/B0D2NVJ5PC?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&psc=1&smid=A1AT7YVPFBWXBL&language=es_ES

That is not a Thunderbolt dock. Using Google Translate on the URL indicates that it is a "NOVOO Triple Display Docking Station, 14-in-1 USB-C to Dual HDMI 4K 60Hz Adapter with DP 8K 30Hz, 6 USB-C/A Ports, 100W PD, RJ45, SD/TF, 3.5mm Audio for MacBook/Dell/HP."

Thunderbolt 3, 4, and 5 run over the USB-C connector, but that does not mean that every hub or dock that plugs into a USB-C port (or whose manufacturer claims that it is Thunderbolt-compatible) is a Thunderbolt hub or dock. Most inexpensive docks run off a combination of DisplayPort Alt Mode and USB signals. They know nothing about Thunderbolt and cannot take advantage of it. As far as I can tell, the Amazon description of this dock never once claims that it is a Thunderbolt device, or even a Thunderbolt-compatible one. You are making a bad assertion that "it is a thunderbolt dock", unsupported by the manufacturer or by the evidence.

Furthermore, as Mr. Bennet-Alder discovered, in the description, it says "Note: macOS only supports mirroring or single-display (A-AAA or A-BBB), does not support multi-monitor MST." Just because Windows and Linux might support MST does not mean that macOS does.

As long as you continue to plug both monitors into this dock, you will continue to have the problem you described.

kernel_45 wrote:

I'ts a plain M3 from 2023. I managed to fix it by unplugging and plugging a couple times through the dock and through the built in hdmi port on the laptop but I still think it would be nice if this got a OS fix sometime in the future.

It might be of value to read the technical specifications for your Mac:

MacBook Pro (14-inch, M3, Nov 2023) - Tech Specs - Apple Support

"Display Support

Simultaneously supports full native resolution on the built-in display at 1 billion colors and:

• One external display with up to 6K resolution at 60Hz
• Close the MacBook Pro lid to use a second external display with up to 5K resolution at 60Hz

HDMI digital video output

• Support for one display with up to 4K resolution at 120Hz (M3)"

if this is your MacBook Pro model:

MacBook Pro (14-inch, M3, Nov 2023) - Tech Specs - Apple Support

says it supports ONE external display, unless/until you close the lid:

Display Support 

Simultaneously supports full native resolution on the built-in display at 1 billion colors and: 

• One external display with up to 6K resolution at 60Hz

• Close the MacBook Pro lid to use a second external display with up to 5K resolution at 60Hz

HDMI digital video output 

• Support for one display with up to 4K resolution at 120Hz (M3)

You say that this is a 14" MacBook Pro with a plain M3 chip.

The plain M3 MacBook Airs and Pros are a bit of an anomaly in that they can switch one internal video output, so that it drives either the built-in screen or an external screen. Without that arrangement, the plain M3 notebooks would have been limited to one external screen, period, like the plain M1 and plain M2 notebooks. (With plain M4 and M5 chips, Apple increased the number of display generators so that these chips could drive a built-in screen, and up to two external displays, at the same time.)

Use dual monitors with your MacBook Air and MacBook Pro with M3 chip - Apple Support

The Apple Support article for MacBook Air and MacBook Pro notebooks talks about connecting the second display after you have connected the first display, and closed the laptop lid.

If you have both displays plugged into a dock full-time, you might need to keep the second one turned off any time the lid is open – only turning it on after the lid is closed and the Mac has recognized the first monitor.

Hopefully you would not need to unplug and replug the second display.

from the US English version of that ad on Amazon:

• 📈【Triple Display Made Easy】Perform multitasking work with MST(A-BCD) support. In Windows system, this USB-Hub allows you to display different content on triple 4K@30Hz monitors and easily manage complex projects with spreadsheets, documents and presentations across screens.

🔺Note: macOS only supports mirroring or single extension (A-AAA or A-BBB), not MST multi-monitor support.

exactly what model-year and what processor { M12345 (plain) PRO MAX ULTRA } ??

to quickly toggle the sense of "mirror displays" use:

Command-BrightnessDown

If this a MacBook Pro, as indicated in your profile, then it’s certainly capable of supporting two external displays.

I am puzzled: you say the two external displays are mirroring each other, but also it appears that the mac only shows one external display. Is this really the case? Weird.

If you click the “Arrange” button, what does it show you?

kernel_45 wrote:

it's Monitor - PEAQ PMO S245-VFC, 24"

that display appears to be a 1920 by 1080 display, 8 bits/color at up to 120Hz refresh rate:

interfaces include:

HDMI 1.4

DisplayPort 1.2

you can turn the capabilities of the Dock into a non-issue by connecting one display using a PREMIUM or ULTRA HDMI cable, directly to the Mac's HDMI port.

--------

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet"   (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive. 

<< I repeat, it worked on Windows and Linux. >>

The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video with NO dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or Transmit errors due to low-spec or overly long cables. 

These acceptable settings are obtained by a query to the display itself. In most recent versions of MacOS, no transmit errors are tolerated, or the display will drop out. 

Under Windows, you manually choose the settings you prefer, or download a popular "Driver" (which is actually a package of resolutions and setting thought to be good ones).  Whatever happens is your problem.

What exact model MNT display?