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In my workplace, I have a Lenovo T14 laptop(1), connected by USB-C to a 1080p monitor(2) which functions as a docking station - specifically, it's connected to another monitor via DisplayPort out (set in the monitor as "DP Out Multi-Stream=Extend").

This works, but the additional monitor is limited to 1920×1080. When I connect it to a 2k(2560×1440) monitor (HP Z27), I can only set it to 1920×1080, and then the display looks bad because it's not its native resolution.

Question: Is this a limitation of the laptop itself? Or of the monitor? Anything I can do to get the laptop to support these external monitors properly?

What I tried:

  • Disable the laptop's built-in 1920×1080 display - same result
  • Connect the laptop directly to the 2k(2560×1440) monitor using USB-C - works well. Also tested with 4k(3840×2160, HP Z32).
  • Install monitor driver - same result
  • Update latest firmware for the 2k monitor (1.0.0.18 Rev.A, shows up on the monitor as LIM293) - same result
  • MonitorInfoView correctly shows the max resolution of 2560×1440.

(1) Lenovo T14 Gen1 20S1S3VT1N, with i5-10310U CPU and Intel UHD Graphics (latest drivers 31.0.101.2114), built-in 1920×1080 display.

(2) Philips 243B1, 1920×1080. It connects via USB-C, and then it:

  • display the output of DisplayPort-over-USBC on the monitor. Note that it does not include a separate GPU like DisplayLink.
  • outputs DisplayPort to an additional monitor via DisplayPort 1.2 daisy-chain
  • Has sound-over-USB, Ethernet-over-USB, and a few additional USB ports

Additional Research

I recorded the USB setup using USB Device Tree Viewer

Scenario 1 - second monitor limited to 1920×1080

Lenovo T14 gen1 (Intel UHD Graphics)
Dock monitor: Philips 243B1_27 1920×1080, USB set to USB 3.2
Second monitor, connected to dock monitor using DisplayPort-Out: HP Z27n G2 2560×1440 <<<<< Limited to 1920×1080
|---Intel(R) USB 3.1 eXtensible Host Controller - 1.10 (Microsoft)
|   +---USB Root Hub (USB 3.0)
\---Intel(R) USB 3.1 eXtensible Host Controller - 1.10 (Microsoft)
    \---USB Root Hub (USB 3.0)
        |HS-[2-1]
        |---[2-2]: Generic USB 2.1 Hub
        |   |---[2-2-1]: Generic USB 2.1 Hub
        |   |   |LS-[2-2-1-1]: Avago Pte USB Optical Mouse   <<<<< Monitor's USB ports, with some devices connected
        |   |   |LS-[2-2-1-2]: Lenovo Traditional USB Keyboard
        |   |   |HS-[2-2-1-3]
        |   |   |HS-[2-2-1-4]
        |   |   \HS-[2-2-1-5]
        |   |HS-[2-2-2]
        |   \HS-[2-2-3]
        |FS-[2-3]: MOSART Semi 2.4G INPUT DEVICE - Keyboard, Mouse, 2× HID
(...)
        |SS-[2-13]
        |---[2-14]: Generic SuperSpeed USB 3.2 Hub
        |   +---[2-14-1]: Generic SuperSpeed USB 3.2 Hub
        |   \SS-[2-14-2]: Realtek USB GbE Family Controller - Net   <<<<< Monitor's network controller - on USB3
(...)

Scenario2: Monitor switched to USB2, second monitor running at full res

Lenovo T14 gen1 (Intel UHD Graphics)
Dock monitor: Philips 243B1_27 1920×1080, USB set to USB 2.0
Second monitor, connected to dock monitor using DisplayPort-Out: HP Z27n G2 2560×1440   <<<< Running at full res
|---Intel(R) USB 3.1 eXtensible Host Controller - 1.10 (Microsoft)
|   +---USB Root Hub (USB 3.0)
\---Intel(R) USB 3.1 eXtensible Host Controller - 1.10 (Microsoft)
    \---USB Root Hub (USB 3.0)
        |HS-[2-1]
        |---[2-2]: Generic USB 2.1 Hub
        |   |---[2-2-1]: Generic USB 2.1 Hub
        |   |   |LS-[2-2-1-1]: Avago Pte USB Optical Mouse   <<<<< Monitor's USB ports, with some devices connected
        |   |   |LS-[2-2-1-2]: Lenovo Traditional USB Keyboard
        |   |   |HS-[2-2-1-3]
        |   |   |HS-[2-2-1-4]
        |   |   \HS-[2-2-1-5]
        |   |HS-[2-2-2]: Realtek USB GbE Family Controller - Net   <<<<< Monitor's network controller - on USB2
        |   \HS-[2-2-3]
        |FS-[2-3]: MOSART Semi 2.4G INPUT DEVICE - Keyboard, Mouse, 2× HID
(...)

Scenario3: Stronger laptop, second monitor running at full res

Laptop: Lenovo T14 Gen2i (Intel Iris XE)
Dock monitor: Philips 243B1_27, USB set to USB 3.2
Second monitor, connected to dock monitor using DisplayPort-Out: HP Z27n G2 2560×1440   <<<< Running at full res
|---Intel(R) USB 3.10 eXtensible Host Controller - 1.20 (Microsoft)
|   \---USB Root Hub (USB 3.0)
(...)
|       |HS-[1-5]
|       |---[1-6]: Generic USB 2.1 Hub
|       |   |HS-[1-6-1]
|       |   |HS-[1-6-2]
|       |   |LS-[1-6-3]: PixArt Lenovo USB Optical Mouse   <<<<< Monitor's USB ports, with some devices connected
|       |   |LS-[1-6-4]: Dell USB Keyboard
|       |   |HS-[1-6-5]
(...)
\---Intel(R) USB 3.10 eXtensible Host Controller - 1.20 (Microsoft)
    \---USB Root Hub (USB 3.0)
        |HS-[2-1]
        |SS-[2-2]
        |SS-[2-3]
        |---[2-4]: Generic SuperSpeed USB 3.2 Hub
        |   |SS-[2-4-1]
        |   |SS-[2-4-2]
        |   |SS-[2-4-3]
        |   |SS-[2-4-4]
        |   \SS-[2-4-5]: Realtek USB GbE Family Controller - Net   <<<<< Monitor's network controller - on USB3
        \SS-[2-5]
Jonathan
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  • What dock do you use for the connection? – harrymc Feb 06 '23 at 09:40
  • No separate dock - the monitor itself functions as a dock. You connect it via usb-c, and it displays via DisplayPort, has a DisplayPort-out for another monitor, and some extra sound/USB/ethernet as well. – Jonathan Feb 06 '23 at 09:47
  • So you connect a 2K monitor behind a 1K monitor/dock? – harrymc Feb 06 '23 at 09:52
  • That's what I'm trying, yes – Jonathan Feb 06 '23 at 09:53
  • Check if the Z27 has a driver [here](https://support.hp.com/us-en/drivers/products). – harrymc Feb 06 '23 at 10:07
  • It does, and I tried that - same result – Jonathan Feb 06 '23 at 10:10
  • How is the Z27 reported in the Device Manager? – harrymc Feb 06 '23 at 10:15
  • Device manager says `HP Z27n G2 Display`. [MonitorInfoView](https://www.nirsoft.net/utils/monitor_info_view.html) correctly shows the max resolution of 2560×1440. I just installed the latest firmware for the monitor (`1.0.0.18 Rev.A`, shows up on the monitor as `LIM293`) - same results. – Jonathan Feb 06 '23 at 11:26
  • Smells like an issue with the monitor you're using as a dock, that either doesn't support MST properly or is not correctly negotiating the link speed. I would update **that** monitor's firmware first and foremost, if possible. – Ian Kemp Feb 06 '23 at 19:20
  • @IanKemp: That was my guess too, but unfortunately I couldn't find any firmware update for the dock monitor ([support link](https://www.usa.philips.com/c-p/243B1_27/lcd-monitor-with-usb-c/support)) – Jonathan Feb 07 '23 at 11:35
  • Can you reduce the frame-rate to the larger monitor, to manage 2560x1440 within the bitrate available in a single DP lane? – Toby Speight Feb 07 '23 at 16:26
  • @TobySpeight: The monitor only offers 1920×1080@60 Hz or 50Hz. I'd expect it to also offer 2560×1440@30Hz or something, but it doesn't. – Jonathan Feb 08 '23 at 11:24

2 Answers2

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This is a limitation with USB-C and DisplayPort versions and, sometimes, dumb monitors.

DisplayPort Alt Mode on USB-C is typically configured to provide two DisplayPort lanes. This allows a USB 3 connection over the same connection. It’s also possible to transport four DisplayPort lanes (like a regular DisplayPort cable does), but this means only USB 2 is possible. The configuration is a property of the USB-C adapter/dock you are using (see update below). The host can do both.

DisplayPort adds another layer of complexity. You have two lanes, which cannot be split further, so each monitor gets one. Each new DisplayPort version offers vastly more bandwidth per lane.

Monitors may also have artificial (“dumb”) limitations.

One DisplayPort lane is all your 1440p screen is going to get. Theoretically this is enough, starting from DisplayPort 1.3. The HP z27 G2 (which you are apparently referring to) does not support DisplayPort 1.3+, only 1.2. It requires two lanes for 1440p resolution.

You can use a proper Thunderbolt docking station to work around this. Thunderbolt accommodates 4 DisplayPort lanes.

Update: According to the Philips 243B1/27 manual, it has a setting for changing the DisplayPort Alt Mode configuration. In the OSD main menu, go to USB Setting, then USB, then select USB 2.0. You will then have access to four DisplayPort lanes. Note that using USB 2.0 will severely reduce the performance of the built-in Ethernet interface.

However, the Daisy-chaining section in the manual also notes that a secondary screen will only support 1080p either way. You’ll have to try.

[Update from OP]: Switching to USB 2.0 indeed works. I even tested with 4K monitor (HP Z32) - still works. However, it's still a mystery why other laptops don't have this problems - they show full resolution even with USB 3.2 (tested with Lenovo P15/nVidia Quadro T1000, and also Lenovo T14gen2i/Intel Iris XE).

Jonathan
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Daniel B
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  • If so, how come my colleague's stronger laptop (Lenovo P15, with nVidia Quadro T1000), *is* capable of supporting the 2 monitors? – Jonathan Feb 06 '23 at 16:01
  • Without details about the test setup, I cannot tell, sorry. Maybe each monitor was connected individually. // However, it appears you are in luck. According to the manual, you can switch to 4 lanes in the Philips monitor OSD. I will update my answer shortly. – Daniel B Feb 06 '23 at 16:27
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    I set the monitor to USB2.0 and indeed, my laptop supports the second monitor propertly. Yay! I can take the hit for the network... However, my colleagues' laptops (tested with P15/nVidia Quadro T1000, and also T14gen2i/Intel Iris XE) connect the exact same way, to a monitor set to USB3.2, and they still support the full resolution. So I wonder how that works? – Jonathan Feb 07 '23 at 11:10
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    This will not change anything though: One lane per screen, z27 G2 is DP 1.2. Until the entire setup is explained in detail and with all devices involved with exact make and model, a statement cannot be made. – Daniel B Feb 07 '23 at 14:40
  • Oh yes, right, I misread that part of the answer, so I thought it was the laptop that is v1.2 only. I have deleted my misleading comment (and will do the same with this one soon) – TooTea Feb 07 '23 at 19:42
  • @DanielB: I understand why this should not work, but yet it does... I edited the question and added output of UsbTreeView, maybe that would help? (Question is Scenario 3) – Jonathan Feb 08 '23 at 12:41
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    Please put additional information in your question, not my answer. – Daniel B Feb 08 '23 at 13:23
  • @Jonathan So I finally had the time to look at this. Unfortunately, because UsbTreeView is just about, well, USB, it does not help in this case. Your GPU could be limited to DisplayPort 1.2, too, but available information is sketchy. Still, because the display is DP 1.2 anyway, this shouldn’t matter. But then, maybe I’m wrong about how DP MST hubs work. – Daniel B Feb 09 '23 at 19:57
  • Situation is the same when the second monitor is 4K (HP Z32). – Jonathan Feb 16 '23 at 08:34
0

Some computers using docks such as HP USB-C Dock G4, G5, require the following setting in the BIOS : "Enable High Resolution mode when connected to a USB-C DP".

Check the BIOS in your computer for a similar option.

harrymc
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