DP Alt Mode not working

[mariachi76]

Hi,

I have a ProArt X870E and a Astral RTX 5090 OC. My setup was to use DP-Alt mode, i.e. route the DP-Signal from the RTX5090 into the DP-In of the motherboard, and then use USB4 (40G) to connect my monitor.

The setup worked fine for some weeks until around early August this year. Since then, when booting in this configuration, the white POST led is on, indicating a graphics card problem. The screen stays black, but boots into windows. If I connect the DP-Out of the RTX5090 to my monitor directly, it works fine and I see windows booted.

I have not done any updates/changes directly directly before the problem appeared. So no obvious trigger.

I did about a month of troubleshooting, including swapping the motherboard with an identical one, different BIOS versions etc. What I have found out so far:

- DP-out from RTX5090 to monitor works

- In DP-Alt mode, when I connect the USB4 cable to EC2 (the lower of the two USB40 ports), it connects perfectly to the monitor, but EC2 routes the AMD iGPU signal to the monitor, not the RTX 5090. (Thats per design/according to the manual). Obviously I want to use my RTX5090...

When connecting the cable to EC1 (the upper USB4 40GB port), the RTX5090 signal is not passed through, no image. The  white POST led stays on.

I swapped cables, Motherboard, CPU, used different BIOS versions and older Nvidia drivers. All drivers are up to date. Windows is up to date.I can't get it to work anymore.

It might be somewhat of a handshake problem of the DP-Alt Mode between DP-In and USB4 controller. 

I guess my question is if anyone with a similar setup can reproduce that problem. Asus support is not very helpful, they tell me to RMA the motherboard and or the RTX5090. Which I am sure won't really help since I swapped the MB already, and the GPU/DP out is working perfectly when connecting it to a monitor directly.

Any clues?

CPU is a Ryzen 9 9950 X3D.

thanks

mariachi76

[mariachi76]

Nobody has any similar experience - or a setup like this working?

[nemen]

most people plug cable directly from gpu to monitor.

you can turn off igpu in bios settings and maybe that change will force the 5090

but why bother when you can connect directly to the gpu without any issues ?

[mariachi76]

I have a whole setup based on Thunderbolt - a TB Monitor, a TB KVM switch with a TB Laptop connected to it. So it makes sense to use the TB/USB4 with DP-Alt mode to connect it via the TB KVM switch. That switch then switches the monitor and all other periperhals.

As mentioned, I had this setup working for months flawlessly, until the mentioned symptom appeared. I did buy a DP-KVM switch in the meanwhile, but I hate all the cluttered cables etc.

If Asus advertises the board with DP-Alt and describes it in the manual, I expect it to work. So I'd just want to know if others have similar problems.

[n0pslide]

I wouldn't expect a gang of users simply because using DP alt mode on desktop is akin to taking a Lambo to a dirt road. Bruh, u castrating the signal - why..? DP GPU-to-monitor is superior to dp alt mode in just about any meaningful aspect. And get that DP2.1 cable for some 4K@240Hz woooooohooooooooo:)

Edit: But for real tho I got $20 on the cause being Thunderbolt. That mothaf#cka just can't stay stable on Windows. I have a goddamn ptsd dealing with TB on W10/11 😞

[mariachi76]

now that helped really a lot to solve the problem... 

[n0pslide]

Hold on a second - I'm trippin lol what Thunderbolt if u on AMD lol my bad. ******, I'd start with disabling iGPU - that'll be in the north bridge settings. Kill the ******. Plug in EC1 (EC2 is bound to iGPU) and see what happens. In monitor OSD set DisplayPort version to 1.4 if the choice exists and it didn't default to that value.

If u're still out of luck -> back to square one: clean install, disable all automatic driver updates (group policy searcher if u prefer this method) before u connect to internet. Install the old-ass drivers that came with your mobo, then update em with asus updater and clean up the old ****** with Driver Store Explorer . Consider RMA if this fails, not before.