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pkolaczk
1 Nickel

Dell WD15 Dock - problem connecting VGA monitor

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I have Dell Precision 5520 laptop connected to Dell WD15 dock. 

There is a Dell U3011 2560x1600 display connected to MiniDP of the dock. 

The laptop display works in 1920x1080 mode, in order to get DPI similar to the one on U3011 (that's on purpose I'm fine with that). 

Today I tried adding the third display with 1280x1024 resolution connected to VGA of the dock.

It doesn't work. The third display gets detected by the Ubuntu display settings, but it can't be turned on. It works only if I unplug U3011. 

It is also possible to run all 3 displays in mirror mode, but not in extended desktop mode.

Is it a hardware limitation (Intel graphics in Precision, WD15 dock) or just problem with Intel Linux graphic drivers? 


BTW: Nvidia card is turned off.

From  the WD15 and laptop specs it looks like the config I'm trying to do should be supported:

- 1920x1080 builtin laptop screen
- 2560x1600 through WD15 and miniDP 
- 1280x1024 through WD15 and VGA

Any ideas, what I can try?

Thanks,

Piotr

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4 Replies
jphughan
5 Rhenium

RE: Dell WD15 Dock - problem connecting VGA monitor

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You can't use a DP monitor with a resolution that high and a VGA display all at the same time.  USB-C docks only have HALF the bandwidth of a full DisplayPort 1.2 output because the pins that would normally be used for the other half, e.g. if you were using a USB-C to HDMI/DP adapter are instead being used to carry USB 3.1 traffic to enable the ports on the dock -- see my thread about this here.  If you had a 1080p display on your mDP port you'd be fine because there would be enough bandwidth left for your VGA display, and mirrored mode works because the PC only has to send the image out once and rely on the dock to do the mirroring. If you wanted enough bandwidth to run a 1600p display AND a 1080p display simultaneously, you'd need a Thunderbolt Dock, which gives you two full DP 1.2 outputs.  That's double what's available on a USB-C port when used solely for display and 4x what you have available when using a USB-C dock.

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jphughan
5 Rhenium

RE: Dell WD15 Dock - problem connecting VGA monitor

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Technical explanation above, but the real answer is that the WD15 FAQ page clearly shows that if you use a 2560x1600 display, the WD15 can only handle 1 display. But again, given that you own a Precision 5520, which comes with Thunderbolt 3 as standard, you would have the option to get the TB16 dock instead, which will enable the setup you're trying to achieve and even higher-bandwidth display configurations. However, it will not work AT ALL with systems that only have USB-C (and not Thunderbolt) if interoperability with non-Thunderbolt systems is a concern.

pkolaczk
1 Nickel

RE: Dell WD15 Dock - problem connecting VGA monitor

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Thank you very much. That's very useful and saved me much time asking on Ubuntu forums Smiley Wink

Do you know what is the state of TB16 dock on recent Ubuntu with updated kernel? Last time I checked many people said that dock was close to unusable on Linux due to broken/incomplete drivers.

Even WD15 still shows some minor instabilities from time to time, particularly with Ethernet connectivity, despite updating the firmware and using recent Linux kernels (4.11 - 4.13).

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jphughan
5 Rhenium

RE: Dell WD15 Dock - problem connecting VGA monitor

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Not being a Linux user, unfortunately all I know about how hardware works on that side is what I pick up in passing, and I haven't kept up with the TB16 on Linux.  Sorry!

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