Precision 5540 dual monitors at 60hz

Question

Hi everyone I'm trying to run two external monitors at 60hz but it will only let me run one at 4k and the other at a max of 1024x768. If I switch to 30hz I can run both at 3840x2160. I am using an UpTab usb-c to dual displayport adapter, its specs say it can run dual 4k at 60hz. I have also tried a WD19TB dell dock and had the same issues. The monitors I am trying to connect are Asus ROG Swift PG27u monitors.  I would be happy even with dual 1920x1080 or 1280x720 at 60hz but that doesn't appear as an option on the second monitor, only 800x600 and 1024x768. Is there anything I can do? Thanks!

jphughan · Accepted Answer

@Eric235  Dual 4K 60 Hz through a regular USB-C MST hub requires a source system that supports DisplayPort 1.4/HBR3, and the Precision 5540 only supports DisplayPort 1.2/HBR2.  Thunderbolt 3 will however allow dual 4K 60 Hz from a DP 1.2 system, including through the WD19TB, but the WD19TB allocates video bandwidth across its outputs differently based on the DisplayPort revision supported by the attached system.  If you want to run dual 4K 60 Hz through the WD19TB from an HBR2 system like the Precision 5540, one of the displays has to be connected to the downstream TB3 port at the edge of the dock -- not to be confused with the USB-C port near the HDMI output.  Use a USB-C to DisplayPort adapter/cable if needed.  The other display can be connected to any other output.  If you're using any display output combination on the WD19TB that does NOT involve the downstream TB3 port, then your setup is resulting in both displays having to share only half of the total available video bandwidth coming from the source system, and that's only enough for a single 4K 60 Hz display plus some trivial amount left over, which is why you only have low resolution left for the second display.  Putting one of the displays on the downstream TB3 port, which has half of the total video bandwidth available, will result in each display getting the amount of bandwidth it needs for 4K 60 Hz.  This is laid out in the WD19TB's manual available on the Dell Support site, fyi.

jphughan · Answer

@Eric235  If you want to understand this better, regular USB-C to Dual DP adapters can only tap into a single source DisplayPort interface worth of bandwidth, since that’s all regular USB-C allows. If you have a system and adapter that support DP 1.4/HBR3, then that single interface is enough for dual 4K 60 Hz. But your system only supports DP 1.2/HBR2, and in that case a single interface is only enough bandwidth for dual 4K 30 Hz (or single 4K 60 Hz). But if you have an adapter or dock that can actually tap into Thunderbolt 3, then that device can tap into two full DP 1.2/HBR2 interfaces. And at that point you have enough bandwidth to run dual 4K 60 Hz even from an HBR2 system.

jphughan · Answer

@Eric235  Is it a USB-C to Dual DisplayPort adapter, or a Thunderbolt 3 to Dual DisplayPort adapter? They look very similar, but on your system the latter will be able to tap into twice as much video bandwidth as the former — and that is the difference between being able to run dual 4K 60 Hz vs dual 4K 30 Hz. The UpTab products I found appear to be regular USB-C products.

Eric235 · Answer

ok thanks that explains a lot. I will look for a thunderbolt dual adapter then. In the meantime, using the dock like you suggested with one monitor in a displayport and my other monitor plugged into the downstream thunderbolt using my adapter has gotten it working! Thanks a lot for your help.

Eric235 · Answer

at the moment I am not using the dock, just a thunderbolt usb-c to dual displayport adapter. I could try with the dock again I guess

jphughan · Answer

@Eric235  Happy to help! A variety of vendors make TB3 to Dual DP adapters, including Plugable, StarTech, and Sabrent, to name a few solid vendors. Good luck!

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Precision 5540 dual monitors at 60hz