Thunderbolt 5 backward compatibility issue (DP alt-mode)

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Level 1

4 points

Thunderbolt 5 backward compatibility issue (DP alt-mode)

Hi there!

I have a 14" M4 Pro MBP and 2 external monitors (Acer Nitro XV272KLV) that are not working as expected. They're connected directly to my macbook via usb-c to usb-c cables. Both 4K 160Hz monitors use Display Port alt-mode. The issue is that I got it to work only in 4K60Hz mode, but previously I had an M1 Pro machine (Thunderbolt 4) and my setup was working great. I use the original cables (USB 3.2 gen 1) that shipped with my monitors.

I've also tried a Thunderbolt 4 cable, but it didn't work out, since the monitors are not Thunderbolt-capable (exactly same result).

I suspect that there is an ongoing issue with the USB 3.2 gen 2x2 compatibility, which I've already seen on the Apple Support Community.

The issue seems to happen only with Thunderbolt 5 devices, so it broke backward compatibility.

Is there any hope it will be mitigated?

Does anybody have the same problem?

Thanks in advance!

MacBook Pro 14″, macOS 15.4

Posted on Apr 17, 2025 8:56 PM

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Answer 1

Top-ranking reply

Grant Bennet-Alder

Level 10

140,291 points

Apr 19, 2025 6:40 AM in response to vanchesky

that Acer display appears to be a 4K display with 10 bits/color:

to be used for this very high data rate, USB-C cables used to connect the display must be ONE meter or shorter. Longer cables, or non-certified cables, may fail of performs adequately due to signal losses. Later versions of MacOS are increasingly fussy about data errors reported by the Display, and will either refuse to connect or disconnect frequently if error occur.

Leroy Dougals' suggestion of using a longer ACTIVE cable is not off base, because these ACTIVE cables re-drive the signals under closely-controlled conditions are are far less error-prone. But yes, they are FAR more expensive.

Cables 'shipped in the box' often turn out to be 'lowest bidder' cables, just good enough to get a picture on the display on day one so that you do not return the device as defective.

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Answer 2

Apr 18, 2025 8:47 AM in response to vanchesky

vanchesky wrote:

Is there any hope it will be mitigated?

Does anybody have the same problem?

Thanks in advance!

see if there is any relief here

https://www.apple.com/shop/product/MC9C4AM/A/thunderbolt-5-usb%E2%80%91c-pro-cable-1-m

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Answer 3

Apr 19, 2025 6:59 AM in response to Grant Bennet-Alder

For USB-C outputs, Standard timing tables suggest 60 Hz is attainable at 10 bits/color (HDR ON) with a USB-C cable. Higher refresh rates may require an adapter or Dock that is capable of converting a full ThunderBolt signal to MUCH HIGHER data rates using DisplayPort.

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Answer 4

Apr 19, 2025 6:15 PM in response to vanchesky

<< But I'm 100% sure it's not a cable issue. >>

DisplayPort 4K at 10 bits/color higher at 60 Hz requires HBR2 transmission mode, which runs at 17.28 G bits/sec.

That is as fast as a USB-C cable carrying display data can run. Higher speeds would require faster data rates, which CAN be achieved over Thunderbolt, (or adapters from ThunderBolt) up to possibly 144 Hz refresh rates.

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Answer 5

vanchesky Author

Level 1

4 points

Apr 19, 2025 6:31 PM in response to Grant Bennet-Alder

Depending on the number of DP lanes it utilizes, there are typically two modes of DisplayPort Alt-mode, 2-lane, and 4-lane mode. 4-lane mode is enough for 4k120hz even without DSC.

P.S. I'll say it again: the same exact setup worked great on the M1 Pro machine.

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Answer 6

Apr 20, 2025 6:31 AM in response to vanchesky

<< 4-lane mode is enough for 4k120hz even without DSC.>>

its not just a matter of lanes, it also depends on the SPEED of those lanes. Thunderbolt 3 and 4 operate as high as 40 G bits/sec across all four lanes turned outbound. USB speeds are more modest.

The speed limit for USB-3, if every lane is turned outbound is 20 M bits/sec.

a 4K display at 10 bits/color over USB would use HBR2 transmission mode, which consumes 17.28 GB of that 20 G bits/sec.

At that speed, it can support 60 Hz refresh, not more.

if you turn off HDR and use 8 bits/color, it can support up to 75 Hz refresh

Sstandard timing tables from a DisplayPort article on Wikipedia. These are for 10 bits/color

https://en.wikipedia.org/wiki/DisplayPort

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Answer 7

vanchesky Author

Level 1

4 points

Apr 19, 2025 5:30 PM in response to Grant Bennet-Alder

Thanks for trying to help me!

Replying to both of your messages: I totally understand what you're talking about. But I'm 100% sure it's not a cable issue. I also tested the monitor with a quality Thunderbolt 4 cable - same thing.

The small amount of time while I had both of my M1 Pro and M4 Pro machines I tested them with my setup and get 100% consistent results: 4k160hz mode working with the M1 Pro and only 4k60hz mode working with the M4 Pro.

I couldn't find any way to debug the DisplayPort alt-mode, so I can see the actual data rate. I want to see that, for example, usb-c works on 3.1 mode rather that 3.2 (which is expected), how many lanes are used for the Display data purposes, is DSC enabled, and so on. By the way, the M4 Pro should be capable of DP 2.1 which is capable of 4k240hz over usb-c. Even DP 1.4 is enough with DSC for 4k144hz.

I'm pretty sure I'm not alone with the issue and I'd love to test it further to find the solution. I dig through all the "System Information" and there is not enough information in there. Is there any tools to dig deeper?

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