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I have the following equipment:

I'd like to be able to connect the laptop to the Dell monitor with a single USB-C cable, and then connect the Dell monitor to the HP monitor with a single DisplayPort cable. This should also charge the laptop. When I do this, though, Windows won't let me set the HP monitor to its full resolution (it just doesn't show anything higher than 1920x1080 in the list). For now all I can do is connect each monitor to the laptop with a separate USB-C cable, which does work, but I would prefer to only connect one cable to the laptop (for convenience). Is there any way?

Robert Siemer
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matthewk
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    Your monitors were intended to be used with a [DisplayPort Daisy Chain](https://www.dell.com/support/article/en-si/sln293813/how-to-daisy-chain-multiple-monitors-using-displayport-multi-stream-transport-mst?lang=en), and that's the right way to use the two with a single cable to the laptop. This will give maximum resolution of 2560x1600 which can fully use the HP monitor. Charging will be separate. – harrymc Apr 24 '20 at 20:44
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    The daisy-chaining and charging with a single USB-C cable does work. It just won't let me select the maximum resolution (2560x1440) on the HP monitor. I'm trying to find out why not. – matthewk Apr 24 '20 at 21:34
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    Daisy Chainning with DisplayPort will allow maximum resolution. You need **two** DisplayPort cables for this. – harrymc Apr 25 '20 at 10:25
  • Anyway, your computer comes with the Intel HD Graphics 620 which doesn't have a USB-C port. You have not given full details for your setup. – harrymc Apr 27 '20 at 08:45
  • The laptop specification that I linked to says it has "External ports 1 headphone/microphone combo; 2 USB 3.1 Type-C™ Gen 2 (Thunderbolt, Data up to 40 GB/s, Power Delivery, DP1.2, HP Sleep and Charge); 1 USB 3.1 Gen 1 (HP Sleep and Charge)". What else can I tell you? – matthewk Apr 27 '20 at 12:12
  • Does it have an additional display adapter than the Intel HD Graphics 620? – harrymc Apr 27 '20 at 12:53
  • No, it's exactly as described in the specification that I linked to. – matthewk Apr 27 '20 at 12:53
  • This means that there is some adapter built into the motherboard that interfaces to the USB-C ports, but there is no way to find out its specifications. All that we know about it is that it limits the resolution to 1920x1080. The only answer I can provide is the description of the DisplayPort Daisy Chain, if that interests you. – harrymc Apr 27 '20 at 13:06
  • No, that's not right - as I said in my question, if I connect each monitor to the laptop with a separate USB-C cable I can select the 2560x1440 resolution. It's only when I daisy-chain via the Dell monitor that it is limited to 1920x1080. – matthewk Apr 27 '20 at 13:11
  • That's logical, so I'll explain the problem in my answer. – harrymc Apr 27 '20 at 13:48

2 Answers2

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DisplayPort over USB-C often does not have the required bandwidth for more than two 1080p screens. This mainly depends on two factors: DisplayPort version and link configuration (number of DisplayPort lanes).

High Bit Rate 2 DisplayPort (specified in DisplayPort 1.2, what you have) supports up to 5.4 GBit/s per lane.

USB-C DisplayPort Alt Mode features 1, 2 or 4 DisplayPort lanes (but no USB SuperSpeed at 4, only High Speed).

One 2560x1440@60Hz screen requires 5.63 GBit/s, meaning it requires two DisplayPort 1.2 lanes (but only 1 at DP 1.4). Adding a 1080p screen would require another lane, requiring the maximum amount of 4 lanes in the USB-C link.

Your 1080p screen indicates in its specs that it features "USB Type-C (Alternate mode with DisplayPort 1.4, USB 3.1 upstream port, Power Delivery PD up to 65 W)". The description strongly hints at it only using 2 DisplayPort lanes. This is enough for at least two screens of this type. DisplayPort 1.4 doubles the available bandwidth, allowing for more and larger displays.

There is of course a workaround: Thunderbolt. Because it uses a different type of link, it can support (at least) two 2560x1440@60Hz screens over a single cable. The drawback is of course that Thunderbolt docks are very expensive.

Daniel B
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  • OK, this looks rather more plausible to me, thank you. So the last part of my question was "Is there any way [to connect both monitors to the laptop with a single cable and use their maximum resolutions and charge the laptop at the same time]?" I've been looking at this https://www.ebuyer.com/878748-startech-com-dual-monitor-usb-c-multiport-adapter-dk30c2dagpd which says it supports "dual 2K 60Hz". So, do you think that would do what I want? – matthewk Apr 27 '20 at 15:14
  • According to your answer, lowering the frequency below 60Hz should solve the poster's problem. – harrymc Apr 27 '20 at 15:35
  • @harrymc Yes. That is, if the monitor advertises this mode. From the manual, it apparently does not. – Daniel B Apr 28 '20 at 06:58
  • @matthewk Nice find. Yes, because it supports 4-lane Alt Mode it should work. Hopefully your laptop supports it too! – Daniel B Apr 28 '20 at 07:05
  • The StarTech dock I found does seem to do the trick. One cable from it to the laptop has both screens running at full resolution and power to the laptop - exactly what I wanted. – matthewk Apr 29 '20 at 19:14
  • @matthewk: Which cables are you using for the monitors? – harrymc Apr 30 '20 at 06:42
  • DisplayPort cables from the dock. – matthewk Apr 30 '20 at 08:18
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It's about the bandwidth of the usb-c connection. Change the USB settings from USB 3.0 to 2.0, if available, on the 1st monitor connecting to your laptop and see if you can increase the resolution. You may need to disconnect and reconnect the usb-c cable for it to work.

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  • Welcome to SuperUser! How is this answer better than the old, accepted one? – DarkDiamond Jun 24 '23 at 09:06
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