Precision 5690 TB4 port, two U2724DE daisy chained, stuck at 75Hz?

Question

Hi,

I'm using a Precision 5690.

Left Type-C ports are TB4

Right Type-C port is USB4.

Both U2724DE are TB4 and daisy chain with TB4 Downstream (monitor 1) to TB4 IN (monitor 2).

It's funny because :

- When connecting 5690 with TB4 port : I'm stuck at 75hz

- When connecting 5690 with USB4, the ultrasharp ask If i'm okay to reduce Ethernet and data speed, I say "yes" then I'm able to reach 120hz on both.

How do you explain that ?
I did try playing with MST setting when using TB4, it does not change anything.

My understanding is that the TB4 port of the Precision 5690 is limited ?

Or is it the TB4 of the ultrasharp ?

I don't understand.

Also take note that, this is the second time TB4 is limiting while USB4 works fine as I explained another issue here for which I have ZERO answers : Host PC TB4 to U2724DE cannot daisy chain over USB-C TB4 Downstream to U2424HE

DELL-Daniel V · Accepted Answer

The 5690 would be using the full Thunderbolt 4 spec, wouldn't be able to call it Thunderbolt without it. Also the right hand C port is USB 3.2 Gen 2. Thunderbolt 4 supports two 4K displays and with the U2724DE being 1440p, we should have more than enough head room to run these at 120hz too. Per the monitor user guide, there really shouldn't be any issue with daisy chaining the two together as well. Are you using the thunderbolt 4 USB C cables that came with the monitors? This is really the only limiting factor I can think of. Do you get the full 120hz with just 1 monitor attached through the TB4 port?

Peterzfr · Answer

Thanks for your reply.

For the daisy‑chain setup I normally use a 1‑meter Dell USB‑C Gen2 5A PD3 cable.

Following your recommendation, I also tested with an official Dell Thunderbolt 4 cable (between both monitor, I also use a Dell USB‑C Gen2 5A PD3 cable for my PC to primary monitor) and the issue remains exactly the same.

Additional details:

When I connect through the right USB 3.2 Gen 2 port, Windows shows a warning: “Use a different USB port.”
Then both monitors display a warning: “Follow these steps for the best image performance. The data and network speeds through the monitor will be reduced. Select Yes to continue.”
After pressing Yes, I’m able to run dual QHD at 120 Hz.

Display information at that point:

Monitor 1
MST: ON
USB‑C Prioritization: High Resolution
USB: 2.0
USB Upstream: Thunderbolt
Stream Info: 8.1 Gbps 4‑lane

Monitor 2
MST: OFF (????)
USB‑C Prioritization: High Resolution
USB Upstream: Thunderbolt
Stream Info: 5.4 Gbps 4‑lane

If I enable MST on Monitor 2, it also reports 8.1 Gbps 4‑lane, same as Monitor 1.
Interestingly, both monitors report “USB Upstream: Thunderbolt,” which suggests the 5690 motherboard may be routing Thunderbolt 4 over the right USB 3.2 Gen 2 port ?

Now, with MST enabled on both monitors, if I unplug the right port and switch to a TB4 port on the left side, only the primary monitor comes up at QHD@120Hz.There is definitely a bug:

• Power‑cycling the monitors: no change.
• Unplug/plug cycle: eventually the second monitor appears, but only at 2048×1080 60 Hz (??? It seems that it try to utilize all the remaning bandwidth and the result is this odd resolution --while the primary is 2560×1440 120 Hz).

Something is clearly wrong.

The only way to get both monitors working is to lower the primary monitor from 120 Hz to 59.95 Hz. Then the second monitor can also run at 2560×1440 59.95 Hz, and I can even push both to 74.97 Hz.

At that point, both monitors show:
USB Upstream: Thunderbolt
Stream Info: 8.1 Gbps 4‑lane
USB: 3.2 Gen2
MST: ON
USB‑C Prioritization: greyed out (makes sense, since TB4 should provide full bandwidth for video and USB).

So maybe the issue is in the cable I use to connect the 5690 to the primary monitor, it's a Dell USB‑C Gen2 5A PD3 cable. I need to find out a second TB4 Certified cable to test that part.

Regards,

Peter

Peterzfr · Answer

Also what is the meaning of this ?

On my previous post, I'm not convinced about the cable as it works with right side port using Dell USB‑C Gen2 5A PD3 cable both for laptop and daisy chain (even though the monitor falls back to USB 2.0)

Peterzfr · Answer

Guess what - It Works using the two Original TB4 Cable that comes with the monitors.

This is Office setup, people who installed the monitor did not take care of the cabling.

So from my understanding the USB‑C Gen2 5A PD3 cable is not capable as this setup requires a cable that support HBR3 (8.1 Gbps × 4 = 32.4 Gbps) as stated in the display info OSD : 8.1Gbps 4-lane.

In the end I guess any USB4 or TB4 is required as a minimum for this setup to works with TB4. Seems logic lol, my bad for tryning with USB‑C Gen2 5A PD3 which is not a 40gbps cable.

BUT : What about DSC ? I guess DSC has to be ON but I have no information about it.

DELL-Daniel V · Answer

@Peterzfr​ Glad to hear it worked with the correct cabling! Unfortunately, not all USB C cables are built the same. DSC is a handshake that the laptop and monitor will work out together automatically, it does not require any user enablement nowadays so it should already be on. Im also a fan of this video from LTT and they go into depth of creating true spec cables.

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Precision 5690 TB4 port, two U2724DE daisy chained, stuck at 75Hz?

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