Dual 4k OLED Setup not working with Macbook Pro M3 Pro

To determine display names and capabilities, your Mac uses something resembling "plug-and-play". It sends a query to the display, and the display responds with its name, its capabilities, including default and preferred resolutions, and its SERIAL NUMBER. The serial number is how the Mac uniquely tells TWIN displays apart.

What we have seen here with TWIN displays, sometimes the manufacturers have been sloppy about what field contains the serial number. The Mac uses VESA standard, and if the display-maker does not supply the serial number in the expected field, the Mac can not tell the TWIN displays apart, and they 'swap around'.

Your displays have a design defect. Only the display maker can fix this defect, and would have to issue a display firmware update to fix it. You should attempt to pressure them to fix the defect.

As a work around, two different models or different makers will not exhibit this symptom.

Another work around found by users is buying a little stunt box that intercepts the EDID information and lies about what display is connected:

https://www.amazon.com/s?k=edid+emulator+4k

Additional background information:

The details can be seen using a Utility like SwitchResX (and probably BetterDisplay as well) which reads out the EDID information the Mac gets back from a query of the device for its name, capabilities, and serial number. In an earlier investigation a User found that the serial number was there, but NOT in the correct field, so the Mac could not tell TWIN displays apart.

Apple has not responded favorably to suggestions that they "accept whatever junk the display provides, whether it meets the accepted standard or not", but you can certainly ask again.

Grant Bennet-Alder wrote:

What we have seen here with TWIN displays, sometimes the manufacturers have been sloppy about what field contains the serial number. The Mac uses VESA standard, and if the display-maker does not supply the serial number in the expected field, the Mac can not tell the TWIN displays apart, and they 'swap around'.

Your displays have a design defect. Only the display maker can fix this defect, and would have to issue a display firmware update to fix it. You should attempt to pressure them to fix the defect.

A similar defect is when a manufacturer puts a serial number in the expected field – but reuses the same serial number for several displays of the same type manufactured in the same batch.

This happens when some manufacturers get lazy and decide that they don't want to take the time to program a slightly different firmware image – one that differs in the serial number field – for each monitor on a production line. They just take an image with one serial number, and program it into a whole batch of monitors. (They may change the serial number before programming the next batch, but it is a violation of standards for them to have reused the same serial number at all.)

So if you get two monitors of the same type – and they happen to have been manufactured in the same batch – they incorrectly report the same serial number, which potentially could confuse your computer.

Dell Alienware AW2725Q appears to the a 4K display with HDR (10 bits/color) and

2x HDMI 2.1

1x Displayport 1.4

no direct USB-C feed is available, since it can barely support a display this wide at 60 Hz.

Ordinarily, you should be able to connect the first display to the HDMI port with an ULTRA Cable, and the second display using some means that takes advantage of ThunderBolt speeds, such as the Caldigit Dock.

this line concerns me:

<< Using BetterDisplay to rename the monitors -when the second monitor is connected, it will take the name of the first monitor that I renamed >>

That suggest your two display have the same string in the official serial number field, and are therefore effectively ONE display.

(continued)

even if you do one of the items on this list?

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. "No signal detected" is generated by the DISPLAY, not by the Mac.

This query is only sent at certain times:

• at startup

• at wake from sleep — so momentarily sleeping and waking your Mac may work

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

• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)

¿Does reducing the direct HDMI connected display's refresh rate to 120 Hz or below allow the second display to connect?

We used to say that the resolution on any display had no effect on any other display. However, Servant of Cats observed that the initial direct-connect HDMI display may be a special case.

A careful re-reading of the specs for your Mac suggests that you MUST lower the resolution on the HDMI display to use ANY other displays at high resolutions.

Display Support

M3 Pro

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

• Up to two external displays with up to 6K resolution at 60Hz over Thunderbolt, or one external display with up to 6K resolution at 60Hz over Thunderbolt and one external display with up to 4K resolution at 144Hz over HDMI
• One external display supported at 8K resolution at 60Hz or one external display at 4K resolution at 240Hz over HDMI

NB the last bullet point.

PTHG wrote:

It makes me wonder, what change happened between Sonoma and Sequoia in which caused this issue to start appearing? As mentioned above, rolling back to Sonoma resolves the issue completely and no other issues are seen with the displays on that MacOS version.

There have been some reports on these forums of problems with dual-monitor setups that started with Sequoia 15.4 – and that might not have been cleared up by Sequoia 15.4.1. I'm not sure whether the reports are thorough enough to definitely conclude that there is a bug in Sequoia 15.4 / 15.4.1, but I'm open to the possibility.

What's odd in your case is that you say that things worked fine on 15.4.1 but not on 15.5.

It’s disappointing as well as there’s no documentation on Apple’s end to list this as an issue and note monitors/situations to avoid (unless I missed something somewhere) and something I tried to take into consideration before I purchased the monitors I did.

It would be good for Apple to document this issue – but it is something that is very technical – so not the type of thing they seem to like to write about in user-level documentation.

As far as a list of monitors to avoid, maintaining that list could be a nightmare. When manufacturers cheat – and use duplicate serial numbers – they don't go out of their way to advertise that fact. And someone who buys two copies of the same monitor might get lucky and get monitors from different batches. Monitors which don't have "globally unique" serial numbers, but do have serial numbers that are distinct from each other, as far as the Mac that is using them knows. It would thus be impossible for Apple to maintain a "guaranteed complete" list even if they were in the habit of buying multiple copies of every model of third-party monitor to hit the market.

"no success" is more than a little bit vague.

what happens when you try?