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WD19TB - 7490 - Dual 4k at 60hz - Impossible?

I picked up one of the new WD19TB Dell docks and hooked it up to a nearly-new 7490 with TB3. I tried every combination of HDMI and Diplayport (WD19TS has dual DP and HDMI, my monitors have both DP and HDMI) and cannot get this dock to talk to my 4k monitors at 60hz. They only go as high as 30hz.

I have a Caldigit TS3+ dock which hooks up to this same 7490 via TB3, and runs dual 4k at 60hz without a problem. I just can't get the new Dell WD19TB to do it. What am I doing wrong?

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@Mik68 

I'm actually having that same problem with a brand new WD19TBS, a brand new 7420, and dual 1440p monitors. I can get one monitor to 144/120/100hz at 1440p and 8bit color, but the 2nd one only goes as high as 100hz at 6 bit color.

I think this is an HBR3 problem as @jphughan has mentioned before. I was only on site for a few minutes to try to figure it out so I need to dive a bit deeper, but I could have sworn I had dual 1440p's working on HBR3 laptops and WD19TB (not S) docks in the past.

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@Mik68  The reason that cabling setup was necessary isn't because the downstream TB3 port supports DP 1.4.  The DP outputs and the USB-C output near the HDMI output can all support DP 1.4 as well, but actually NONE of them will support that when you're using an XPS 13 9380 anyway, because that system is only sending a DP 1.2 signal to the dock.  The reason that cabling setup was necessary is because the 9380 is a DP 1.2 system, and as such, when it's connected to a Thunderbolt 3 dock, it sends two DP 1.2 interfaces to the dock.  When the WD19TB(S) is paired with a DP 1.2 system, it allocates one of those interfaces to the downstream TB3 port, and the other interface is shared across all other ports.  A 4K 60 Hz display requires almost an entire DP 1.2 interface worth of bandwidth, so in your original setup, you were trying to have two 4K 60 Hz displays share a single DP 1.2 interface of bandwidth, leaving the entire second DP 1.2 interface completely unused.  That's why your second display didn't work properly.  By moving the second display to the downstream TB3 port, you've now got each display using its own DP 1.2 interface, and therefore each display having access to enough bandwidth to run properly.  This requirement is explained in the WD19TB(S) User Guide, for what it's worth.

@decker12  My memory from our discussions over PMs was that you were able to get dual 1440p 120 Hz running from an HBR3 system, but not dual 1440p 144 Hz.


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@jphughan Yes, that is now my recollection as well. The trick was for me when I messed with it in March was to use both the DP ports on the WD19TB and ignore the USB-C and TB ports on the dock.

Out of the various cable combinations, that was the one I forgot to try during my limited time with the user on Monday. I'll give it another try next time I'm in the office, and should be able to manage 1440p @120hz for two monitors with a WD19TBS and a HBR3 7420.

 

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Quick edit to my post above, the user was able to connect dual 1440p @ 120hz @ 8bit color, using two DP 1.4 cables and his HB3 7420 and his WD19TBS dock. Quick and easy! No need for DP to USB-C and no need for any changes in the monitor's OSD. Thanks for the reminder @jphughan 

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