M5 Pro MacBook Pro: Third display not detected

nullvoid324 wrote:

• ASUS XG32UCG

Asus – ROG Strix XG32UCG ROG Strix XG32UCG Gaming Monitor – 32-inch (31.5 inch viewable) 3840x2160, dual mode (4K 160Hz/ FHD 320Hz)

This monitor has DisplayPort v1.4, USB-C (DIsplayPort) and HDMI v2.1 inputs.

• ASUS VG27AQ

• ASUS VG27AQ3A

Asus – TUF Gaming VG27AQ HDR Gaming Monitor – 27 inch QHD (2560x1440), IPS, 165Hz

Asus – TUF Gaming VG27AQ3A Gaming Monitor – 27-inch, QHD(2560x1440), 180Hz,

These monitors each have a DisplayPort v1.2 input, and two HDMI v2.0 inputs.

I can get 2 monitors working, but the 3rd is never detected, even when I try connecting it directly to the MacBook Pro using either HDMI or USB-C to DisplayPort.

All three of your displays have maximum refresh rates in excess of 144 Hz. The specifications for the M5 Pro MBP indicate that when you use displays with very high resolutions (e.g., 8K) or refresh rates, you reduce the maximum number of external displays that you can have from three to two.

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M5 Pro

Supports up to three external displays over any combination of Thunderbolt and HDMI ports:

• Three displays up to a native resolution of 6K at 60Hz or 4K at 144Hz or
• One display up to a native resolution of 8K at 60Hz or 5K at 120Hz or 4K at 240Hz plus a second display up to a native resolution of 5K at 120Hz or 4K at 200Hz

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If I disconnect the dock and connect only a single display directly to the Mac with an HDMI cable or USB-C to DisplayPort cable, that display works fine.

If I connect the dock and with two displays connected to the dock, both displays work fine.

Either of those configurations would appear to fall within the limits mentioned in Technical Specifications.

Any attempt to have 3 displays connected simultaneously in any configuration results in only 2 displays being detected.

Plug in that third display, now you are outside the limits. Quite possibly, the MBP, having learned the capabilities of the other displays, has set up resources for the high-refresh-rate, two-display-maximum case.

I also had the same issue with the same model of MacBook (m5 pro). I was able to resolve this by editing the UI Settings on the monitors themselves. In order to make the Thunderbolt 5 dock recognize all three displays when plugging it into the MacBook, I had to turn off DSC on each of the monitors I have. Once I did this, all of the displays were recognized every time I plugged them in. Hopefully this helps you as much as it did me.

I’m seeing a consistent display-detection issue with both my M5 Max and M5 pro MacBook Pro when using two Studio Display XDRs plus a third 4K monitor.

My setup:

• MacBook Pro with M5 Max and M5 pro
• Two Apple Studio Display XDRs, connected via Thunderbolt 5
• Third 4K display: tested with both Dell U2725QE and an EIZO 4K monitor
• Third display tested via both HDMI and USB-C/Thunderbolt

The issue appears to depend heavily on connection order.

What works:

If I connect the 4K display first, then connect the two Studio Display XDRs in a Thunderbolt daisy chain, all three displays can work.

What does not work:

If I connect the two Studio Display XDRs first, then connect the third 4K display, the 4K display gets no signal.

This happens whether the third 4K display is connected via HDMI or USB-C/Thunderbolt.

Additional observations:

• If all three displays are connected and the 4K display has no signal, unplugging either one of the Studio Display XDRs immediately restores signal to the 4K display.
• Lowering the Studio Display XDRs to 60 Hz does not fix the issue.
• Using three separate ports instead of daisy chaining shows similar connection-order behavior.
• The same behavior occurs with both the Dell U2725QE and an EIZO 4K display, so it does not seem specific to one monitor model.
• Since the same hardware and cables can work when connected in a different order, this does not seem like a simple GPU capability issue or a single bad cable.

My impression is that this may be a macOS / Apple Silicon / Thunderbolt display-allocation or hot-plug negotiation issue. It seems that when the two Studio Display XDRs are connected first, macOS allocates display resources in a way that prevents the third 4K display from being activated. However, when the 4K display is connected first, macOS appears to choose a different allocation that allows all three displays to work.

Has anyone else seen similar behavior with multiple high-resolution Thunderbolt displays on Apple Silicon, especially with daisy-chained Studio Display XDRs or connection-order-dependent behavior?

All three of your displays have maximum refresh rates higher than 144 Hz. One is a 160 Hz monitor, another is a 165 Hz monitor, and the third is a 180 Hz monitor.

I do not know for certain whether it is a display's maximum refresh rate, or its current rate, that counts against the limits. That said, it seems reasonable that the maximum rate might count. Especially since that is consistent with the behavior that you observed.

You should connect that ASUS XG32UCG using DIRECT HDMI port out of the Mac, because that is the highest speed interface the display supports.

Use extreme caution when connecting displays using USB-C to DisplayPort. If the supported port of the computer or Dock really is a USB port, it may be limited to 10 G bits/sec, and some adapters are also limited. Connecting that to a DisplayPort 2.1 input can NOT get you close to 40 G bits/sec operation, because the pinch point is USB-C at 10 G bits/sec.

I would mention in passing that there is a fairly new Thunderbolt5 to DisplayPort 2.1 adapter from CableMatters available. HOWEVER, your ASUS XG32UCG does not support that speed on its DisplayPort 1.4 input, which is limited to HBR3 at 25.92 G bits/sec PROVIDED you are driving it with a Thunderbolt output, not merely USB.

HBR3 is limited to 4K at 10 bits/color at 75 Hz without Display Stream Compression.

What we suspect is happening in cases like this is that the Mac sees it can not attain the highest speeds directly. This is often due to protocols limited to only certain speeds, and not using the maximum possible speeds the interface could theoretically support.

So it uses up TWO display generators and divides the display into Left and Right halves. In some cases, this composite data stream is faster. That reduces the Total number of displays that can be supported by one.

When it crashes, there is a very telling artifact, where you can see that the left and right halves are being treated separately: