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XPS 15 9500: Intermittent signal on 4K DisplayPort 1.2 monitor with left-side USB-C/Thunderbolt ports

I recently bought an XPS 15 9500 (with UHD+ display), and I noticed that when I connect my external 4K 60Hz display to either of the left-side USB-C/Thunderbolt 3 ports (via a USB-C to DisplayPort cable), the monitor flickers on and off, intermittently displaying my desktop, showing "No Signal", or trying (but usually failing) to re-establish the connection. I would describe the behavior as similar to what I'd expect if the cable were faulty/loose. However:

  • The monitor works correctly (and maintains a stable connection) when connected to the USB-C port on the right side of the laptop (the one that does not support Thunderbolt 3)
  • The monitor works correctly when connected with the same cable to two other laptops (one Mac, one Windows; both Thunderbolt 3), and it works correctly on all ports on those laptops.
  • I tried two different USB-C to DisplayPort cables (different brands), and observed identical behavior with both.
  • I tried two 4K DisplayPort 1.2 monitors (different brands/sizes), and observed identical behavior with both.
  • If I force the monitor into DisplayPort 1.1 mode, the monitor works correctly on the left-side ports. However, DisplayPort 1.1 is limited to 30Hz at 4K, which is not ideal.
  • If I leave the monitor in DisplayPort 1.2 mode, decreasing the refresh rate to 30Hz (or decreasing the resolution) does not fix the issue.
  • USB devices and the power adapter both work correctly (and stably) when plugged into the left-side USB ports. I do not have any Thunderbolt 3 devices to test the Thunderbolt functionality.
  • If I boot into Ubuntu from a USB stick, the monitor seems to work correctly in DisplayPort 1.2 mode at 4K 60Hz, even when connected to the ports on the left side.

Note that in all cases, even though the monitor doesn't have a stable signal, Windows does correctly recognize all of the monitor's available resolutions and refresh rates.

On my XPS 15, I've tried the following:

  • Updated the BIOS to the latest version
  • Installed all available Windows updates, including manually updating to Windows 10 Version 2004 (the May 2020 update)
  • Updated the Intel graphics drivers:
    • To the latest version offered by Dell
    • To the latest stable version offered by Intel
    • To the latest beta version offered by Intel
  • Updated the Nvidia graphics drivers to the latest version
  • Verified that the Thunderbolt 3 drivers are up to date
  • Disabled Thunderbolt 3 in the BIOS
  • Attempted to rule out possible sources of electromagnetic interference:
    • Tested with and without the power adapter plugged in
    • Tested with and without a WiFi connection
    • Tested with the WiFI networking device disabled in Device Manager
  • Installed the driver offered by the monitor manufacturer to replace the default monitor driver that Windows offers

Does anyone else have the same issue, or does anyone have any other ideas for things to try before I give up and send the laptop back?

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Re: XPS 15 9500: Intermittent signal on 4K DisplayPort 1.2 monitor with left-side USB-C/Thunderbolt ports

@ingjald  This might be related to the issues that are being reported with the also-new XPS 13 9300.  That system is generating all sorts of reports of issues using Thunderbolt peripherals and docks, and a recent BIOS update seems to have completely broken the ability to use certain USB-C to DP and USB-C to HDMI adapters/cables that worked perfectly fine on older BIOS releases.  Based on your test results and what you've tried thus far, I'm guessing this is going to be a BIOS or TB3 controller firmware issue that hasn't been fixed yet.  So if keeping the display connected to the non-Thunderbolt port on the right isn't an acceptable workaround for you (or you don't want to risk ending up past the return period and putting up with this issue for an indeterminate time), then unfortunately I would consider sending the system back.  Sadly there aren't exactly a lot of XPS 15 equivalent systems out there.  Its only real competitor in the PC world is the X1 Extreme Gen 2.  I did a small comparison of those two systems here if you're interested.

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Re: XPS 15 9500: Intermittent signal on 4K DisplayPort 1.2 monitor with left-side USB-C/Thunderbolt ports

@jphughan, thanks for your response!

I suspect you may be right. After reading your response, I booted into Ubuntu again, and the monitor did the same thing as it does in Windows—I guess I just got lucky on my previous boot into Ubuntu. I also tried downgrading the BIOS to the only other available version, but the behavior is the same.

That said, I'm curious to see if anyone else has the same issue (or conversely, if anyone doesn't have the same issue). While I don't mind the workaround for now, I can't rule out a hardware issue, and I certainly wouldn't want to find out that it is a hardware problem after the return period has run out.

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Re: XPS 15 9500: Intermittent signal on 4K DisplayPort 1.2 monitor with left-side USB-C/Thunderbolt ports

I just found another mention of the same issue with the 9500 (on Reddit), so at least I know it's not just me.

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Re: XPS 15 9500: Intermittent signal on 4K DisplayPort 1.2 monitor with left-side USB-C/Thunderbolt ports

As another update to this, it looks like the problem is much less severe when the charger is plugged into the other left-side USB-C port. Even then, though, the connection to the monitor is not stable.

Again, this all seems consistent with the issue that folks have reported on the XPS 13 9300.

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