Dell P3421W - USB-B with USB-C

robhung on 25/01/2022 - 09:31
Last edited 25/01/2022 - 14:40 by 1 other user

Hi everyone! Just purchased this monitor and was wondering if anyone could help clarify the USB-B port that's connected via USB-A. (I'm using a MacBook 16" connected via USB-C to this monitor).

Firstly – does the USB-B port serve any purpose if I'm using USB-C to connect to my MacBook already? I've done a few tests and it seems that devices (keyboard, mouse, phone, etc.) plugged into my monitor's USB ports are all detected on my laptop without needing to plug in the USB-A cable to the USB-B port.

My guess is that the USB-B port is only there on the monitor when you use HDMI / DP to connect to the monitor, and not USB-C.

I do notice if I use too many power hungry devices, the USB-A ports on the monitor can struggle to charge everything – my laptop usually gives a warning saying that I need to disconnect the power hungry USB devices.

Which leads me to my second question – does the USB-B port offer more power draw for the USB-A ports, if I connect it alongside the USB-C cable? I've just purchased a USB-A to USB-C adapter so I can plug the USB-B to USB-A cable to the other ports on my MacBook.

The monitor itself charges the laptop at 65W even though the MacBook requires 96W, but it does the job most of the time, even if I'm pushing it a little. I'm wondering, if I have too many USB devices plugged into the monitor, would it be worth plugging in the USB-B to USB-A cable to my MacBook as well, just to offload some of the heavy lifting from the USB-C cable, or does it not make a difference?

Sorry if that was all a bit confusing - I actually have no idea how to test any of this, and am hoping someone here might have a bit of experience / knowledge to explain to me.

Thank you!

Computing 34inch Dell LCD Monitor

Comments

  • avoidfullprice on 25/01/2022 - 10:36

    If I read the Connectivity Options description correctly

    5) SuperSpeed USB 5 Gbps (USB 3.2 Gen1) upstream port
    6) USB 2.0 downstream port (2) |
    7) SuperSpeed USB 5 Gbps (USB 3.2 Gen1) downstream port (2)

    The USB ports act as hub. USB B port (#5) connects to your laptop. You connect mouse and other things to other downstream USB ports (#6, #7) The USB C port (#7) doesn't carry video signals. You still connect your MacBook to monitor via HDMI/DP.

    Edit: however that contradicts the Connectivity information that it has USB C upstream port alternate with DP. I am confused as well. My guess is that USB C upstream port doesn't exist!

    Edit 2: no I misread. That USB-C video port is (#4). So that means USB-B port is used as a USB hub as I described above. Which means your assumption should be right (you have the actual monitor to test, I don't). To this end no idea if it offers you more power.

    • robhung on 25/01/2022 - 11:06

      Yeah, I've tested it with the USB devices plugged into the monitor and they all continue to work without the USB-B to USB-A cable (just using USB-C).

      I guess the gap in my understanding is whether there is a maximum power draw for each USB-C port on my MacBook, and if so, will my USB devices get more power if I use the USB-B to USB-A cable.

      (And finally, would I need to plug in the cables in any particular order for the USB hub in the monitor to correctly detect where to draw power from)

  • toomuchdogfur on 25/01/2022 - 14:58
    +2

    My guess is that the USB-B port is only there on the monitor when you use HDMI / DP to connect to the monitor, and not USB-C.

    Yes that'd be it

    does the USB-B port offer more power draw for the USB-A ports, if I connect it alongside the USB-C cable?

    I'd doubt it - if everything is configured sanely the monitor should be acting as a powered hub, so its power budget would be coming from its own power supply (not your laptop's).

    I'd theorize that your monitor has a limited power budget that is being taken mostly by your Macbook being charged on USB-C, hence only a small amount of power is given to the USB-A ports (which are only meant to be used for a keyboard and mouse, after all).

    127W max power draw (taken from Dell spec: "Max brightness and contrast setting with maximum power loading on all USB
    ports"    )
    - 13W (assuming 10% efficiency loss)
    - 32W for the monitor (Dell spec for "On" mode, assuming this is power draw without peripherals connected)
    - 65W to charge your MacBook
    = 17W for the USB-A ports

    You could try switching your MacBook to DP/HDMI + USB-B and see if the monitor reshuffles its power budget to your USB-A ports?

    • robhung on 25/01/2022 - 15:05

      Wow, this is exactly the answer I was looking for - thank you @toomuchdogfur ! Will conduct more tests with your suggestions.

    • robhung on 25/01/2022 - 15:15
      +1

      Okay did a test, and it doesn't reshuffle the power to the USB-A ports. Looking at the manual, it seems to suggest the USB-A ports have a set power rating (5V / 0.9A for USB 3, 5V / 0.5A for USB 2).

      It's a shame they're not given more power, as I can't even plug in my iPad 12.9" and take advantage of the USB 3 speeds!

      • toomuchdogfur on 25/01/2022 - 17:19
        +1

        Ah there you go… Turns out the real answer all along was even further in the manual! I didn't think they'd bother putting dynamic power smarts in there, least you have your answer now.

      • kommy on 26/01/2022 - 06:52

        If you need more power from the usb A port on the monitor, you could use one of these y cables from Amazon. Be aware that this cable will occupy 2 USB ports and will only support usb2.0 speeds. https://www.amazon.com.au/HIGHROCK-Enhancer-Female-Charge-Ex…

      • yokel on 26/01/2022 - 08:31

        For the output on the ports to be higher then the power supply to the monitor needs to be beefier along with other design choices. They have kept the cost down. I use usb on my Dell monitor for the web cam that sits on top and a head set. I find it’s useful for reducing the clutter.

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