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As I need to buy a new display anyway, I wanted to make my life easier with an USB-C one cable connection to my laptop (Razer Blade Stealth, it shouldn't matter for the question though).

Let say I would pick DELL S2718D display. A relevant piece from its user manual: enter image description here

I would like being able to switch the display between the laptop and a desktop PC with the least cable swapping possible whilst peripherals (mouse and keyboard) stay connected to the monitor's USB downstream ports.

Now comes the tricky part, desktop PC doesn't have an USB-C port. Input source is not an issue as desktop would be connected over HDMI.

How about the peripherals though? Is there any chance it would work if a female USB-C to male USB 3.0 adapter or cable was used between the desktop and the display? If there is no chance, any other solution?

An example adapter:

enter image description here

From its description:

USB-C USB 3.1 Type C Female to USB 3.0 A Male Data Adapter for Macbook Tablet Mobile Phone

  1. This adapter can convert the data which is USB-C head used into the ordinary USB 3.0 data cable.

  2. Type C connector is the new design for USB 3.1 and can support charging and data transfer.

  3. Type C connector can support Reversible plug orientation and Cable direction.

Perhaps it's worth mentioning, this display supports charging over USB-C (45 Watt) and I would like to use this feature for charging laptop, not sure wouldn't it 'try charging' the desktop PC over the adapter and damage it.

EDIT: A relevant piece from LG 27UD88 display user manual for the discussion in comments.

enter image description here

EDIT2: I've got an answer from DELL customer service so if anyone was interested it's no.

USB-C upstream port on the S2718D display is not capable of acting as a Hub

And as I've found out the LG could do it, I can turn to pick this one. There is an other risk though. The predecessor of my laptop was not charging when connected to this display model. This would surely exclude it.

Conclusion: such a setup is possible, it's wrong though to assume an USB-C upstream with USB 3.0 downstream ports built in display imply the display does have an USB hub. This was my initial assumption as this was always the case when a built in display upstream USB port was of an older type USB-A (regardless 2.0 or 3.0).

2020 EDIT: I'd gone with Lenovo ThinkVision P27h monitor for quite a while and it worked as I wanted it to. One cable switch. Now I've started using Dell U3419W and it's even better. Overall similar in terms of ports but it has a built-in kvm switch so I have to change no cables at all.

Community
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topr
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  • “Now comes the tricky part, desktop PC doesn't have an USB-C port.” Then I strongly believe it won’t work. A protocol is a protocol and USB 3 will not magically be able to deal with data required for USB-C in the way you are thinking it will. – Giacomo1968 Nov 01 '17 at 03:13
  • You don’t need an adapter, just use a USB cable with USB-C on one side and USB-A on the other side. The Monitor might even come with one. It might only use USB 2.0 or 3.0 speed then. – eckes Nov 01 '17 at 05:38
  • If your notebook has no USB-C connector it is unlikely that you can charge it via USB – eckes Nov 01 '17 at 05:42
  • @eckes notebook has USB-C, obviously. It's the desktop which doesn't. Thanks for the advice about the cable. JakeGould has a point though DELL doesn't mention anything in the manual it would work (I know it should, but there is a LG monitor which has an explicit info about use either C-C or C-A cable, perhaps this DELL would handle this too). The adapter would make it even one step easier as I wouldn't need to disconnect the cable from the back of the screen, just unplug from the adapter (desktop) and plug directly to laptop. Without adapter I would need to change cables. – topr Nov 01 '17 at 11:21
  • @JakeGould ok, let's say I would pick LG 27UD88 display (just added a relevant image from its manual to the question). Would it work with such adapter and the same C-C cable then? – topr Nov 01 '17 at 11:25
  • @topr My general attitude towards anyone asking these kinds of “I would like to save money and do this thing that doesn’t sound like it will work.” questions is stop asking me (and strangers) for advice but just buy it if you think it’s such a great idea. Sorry if I sound condescending, but the reality is if you are asking questions like this you know already in your mind it *won’t* work and you are asking people to prove otherwise. – Giacomo1968 Nov 01 '17 at 12:00
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    @JakeGould it's totally not about saving the money. I'm trying to find the most convenient way of switching between desktop and laptop on the same screen without the need of reconnecting all the peripherals. I'm willing to spend as much as it needs to achieve that. There is no rationale in assuming it wouldn't work as display built-in USB hubs are as old as the USB port itself and still very common with pre-USB-C ports. Moreover, as proven the mentioned LG screen is capable of such a connection. It's just I like DELL's more and I read that this LG wouldn't charge my laptop which excludes it. – topr Nov 01 '17 at 14:27
  • @topr Okay, fair enough. – Giacomo1968 Nov 01 '17 at 14:30

3 Answers3

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The manual of the DELL S2718D says on p. 9

1x USB Type C (Alternate mode with DP1.2, USB 3.1 upstream port, Power Delivery PD up to 45W)

So it's not only a USB-C type port, it also needs alternate mode, so the USB host adapter in the computer you plug it into must be capable of this mode (and not all host adapters are).

Now it might be possible that the USB controller in the monitor is really really smart, and also supports an USB 2.0 connection, without power delivery or display port signals, where it just acts as a simple USB hub. In that case, an USB 2.0 to USB-C adapter will work.

But there's nothing in the manual that indicates it will do that, so the only way to find out is to either ask Dell, or try it out.

Edit

The manual of the LG 27UD88 says on p. 17:

The USB port on the product functions as a USB hub. Please connect the USB C-C or USB C-A cable to the PC.

So yes, for data only, it can use USB 2.0. But to display an image, you'll also need a PC capable of USB C alternate mode (even though this isn't explicitely mentioned in the LG manual, but that's the only way DisplayPort can be transported via USB C).

dirkt
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  • Thanks for the answer. None of the points quoted about the adapter shows it could support the alternate mode, does it? By the way, how about the LG display I've just added to the question. Is that the kind of 'smart' USB controller you've mentioned? – topr Nov 01 '17 at 11:32
  • The *manual* says it *needs* alternate USB mode - that's the mode of operation to get it to display something. So it *does* support alternate mode for DP 1.2, USB 3.1, as quoted from the manual. The question is whether it also supports USB 2.0 as a fallback. And it doesn't matter what adapter you use, that's not the problem. – dirkt Nov 01 '17 at 11:36
  • Thanks for clarification. I never thought it would pass video signal over USB C-A cable. That's OK though as when such cable is linked for the sake of USB hub only, video signal would go over a separate HDMI (or DP) cable. By the way, you're saying about fallback to USB 2.0. Could it not fallback to USB 3.0? Not like it would matter for a mouse connection, but for a external hardd rive it would. – topr Nov 01 '17 at 12:01
  • The point I'm trying to make is that the cable is not very important. The capability of the USB host controller on your PC is. You can have an USB-C port on your PC, and it still won't work. – dirkt Nov 01 '17 at 12:07
  • Oh, I thought you meant the USB controller on the display side. Now I'm confused again :P How on earth PC would not be able to have an USB hub connected to its USB port. I mean, even if it's technically possible It was a basic feature since I remember. Does introduction of USB-C makes it uncertain anyhow? – topr Nov 01 '17 at 12:50
  • Keep in mind that those USB Hubs/Ports are designed for a wide range of equipment. If you connect the desktop with USB 2.0 it will work (obviously not for charging or display).if you connect a USB-C it can (but does not have to) use Power Delivery or Display output. However I wonder in terms of convenience why you would want to switch. Just keep the desktop on hard wired cables, then you have the USB-C cable for the Notebook. (Only two ports at the back of the monitor does not really help a lot) – eckes Nov 01 '17 at 17:59
  • Don’t worry, only cheap and odd hardware may introduce complications with C-Types, all others just work for basic services like USB 2.0 connections. – eckes Nov 01 '17 at 18:03
  • @eckes oh, I wish being able to hardwire! No hassle to do it for the video signal, how about the USB devices though (keyboard and mouse attached to the display)? For this, there would need to be two upstream USB ports (ideally one USB-C and the other USB-A with hub). Considering we speak about any decent screen not necessarily these two models I listed in the question, do you know any like this? – topr Nov 01 '17 at 22:58
  • Well, not sure if I would want to switch the keyboard between two computers, that is then indeed a problem (could use a usb switch box but somehow I doubt that’s a solution) – eckes Nov 02 '17 at 01:54
  • @eckes The use case is, a single desk setup with a single screen and peripherals. Usually the desktop is connected (home PC). Sometimes for working from home a laptop gets connected (work PC). Connecting the laptop to the whole desk setup by (re)attaching a single cable is just a matter of convenience. – topr May 22 '18 at 16:01
  • @dirkt now I understand the alternate mode is only for video signal, correct? I've never meant this to work over USB-A 3.0. The desktop PC uses HDMI for video signal so no cable switching needed for that. When laptop gets connected it's just a matter of selecting relevant display input source (usually auto-detected anyway). Ideally if the display had two USB-C and USB-B inputs for the built-in USB hub and a KVM switch. No cable switching needed at all then. I've found only one display model (ViewSonic) capable of this and it's neither easily available nor reasonable in cost (about £1k). – topr May 22 '18 at 16:04
  • Just to wrap it up, the former question is - given a display has a built-in USB hub with a several USB-A downstream ports but only a single USB-C upstream port. How to connect the display USB hub to a PC which has only USB-A ports? A sub question is, given it would work (but would it?) as well with an USB-C to USB-A cable should it work with an USB-C to USB-C cable plus an USB-C female to USB-A male adapter on one end? Describing the switching between two computers only clouded what's the crucial part of information I'm trying to find out, sorry about that. – topr May 22 '18 at 16:10
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This is a little bit late (this post is Oct 2018) but perhaps it will be useful for others who might be researching this question.

I own a LG 38WK95C which has a USB-C upstream and 2x USB 3.0 downstream behind the monitor. There is also a included USB-A and USB-C adapter.

Two scenarios:

  1. I plug keyboard and mouse into the USB 3.0 behind the monitor. I plug the USB-C cable into USB-C port in the MacBook. It charges the MacBook, it display the MacBook screen as a secondary display (via the USB-C) AND the keyboard and mouse work in the MacBook. It also carry sound, so the monitor speaker will play sound from the MacBook. I believe the USB "hub" is probably running in Alternate Mode which allows display, sound and data through the hub.

  2. I unplug the USB-C from the MacBook and then plug into the C-A adapter, and then plug this cable+adapter into the PC USB port. The monitor display have to be powered by a separate cable connecting the monitor's DisplayPort port to the PC nvidia card (DP power the display and sound). The USB-C cable then enable the USB-C port behind the monitor to act as a upstream port, powering the hub and thus allowing me to use the same mouse and keyboard in Windows.

So, this hub works for both PC and Mac. The only inconvenience is that you have to physically unplug one cable from the PC to the Mac. But at least, the solution works and you only have one keyboard and mouse set connecting to the monitor.

Habibi
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    What you describe works only due to the hardware you have. The author has completely different hardware than you. While the adapter would work for any compatible device, the author does not have a device that is compatible, what you describe would not function for the author how you describe it. – Ramhound Oct 19 '18 at 04:03
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    Oh really? He seemed to point to the manual in Dell and I quote “The manual of the DELL S2718D says on p. 9”. And that, to me, made it sound like the Dell monitor supported it. I could be wrong. – Habibi Oct 19 '18 at 04:10
  • You have a MacBook, you have entirely different ports then the author's PC, I don't doubt the monitor supports it. – Ramhound Oct 19 '18 at 04:41
  • Actually my setup is not much different. I have no Macbook but my laptop has Thunderbolt 3 / USB Type C port and my PC has USB Type A 3.0 port. I ended up buying Lenovo P27h display with the same kind of USB Type C upstream situation as the DELL from my initial question and the @Ramhound LG (thanks, btw). Exactly as I thought, this is standard and it works as expected. I need to switch the USB cable between laptop/PC to use the same keyboard/mouse connected through the display's hub and keep HDMI between display and PC but that's no surprise. – topr Nov 02 '18 at 11:46
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It appears the person asking this question originally found an answer but this is for anyone that comes with a similar question.

These adapters shown are dangerous, do not use them. They violate the USB specifications by allowing unsafe cable connections. Do not use them, destroy them and dispose of them before you damage expensive hardware.

Dangerous USB-C to USB-A adapters

The proper way to connect USB devices into a USB-A port is with a cable that has a USB-A connector already on it. If the device has a captive USB-C cable, captive as it cannot be removed without breaking something, then it is not to be plugged into a USB-A port. Using a USB-C female to USB-A male adapter like the one shown is dangerous and will not provide the connections needed for the USB-C device. If the device did not need all the connections that USB-C allows then it would have come with a USB-A connector on the captive cable.

Displays with USB-C input will often provide power output from this port to a laptop and take USB and video input into this port from the connected laptop. Using a full featured USB-C to USB-C cable is the only to get power, video, and USB through this connection. Use of a USB-C to USB-C cable to the display allows the display to act as a USB hub for the computer, and is the specified safe means to make these connections by the USB spec. This was in the image included above.

Safe to use USB-C to USB-C and USB-C to USB-A cables

Using any other kind of cable than those shown will either be dangerous or nonfunctional.

I have seen quite a few monitors that allow use of a USB-C input from one computer, a combination of USB-B and some other video connection to another computer, with the monitor having an internal KVM switch to select which of the two gets access to the display and attached keyboard and mouse. Don't try to "outsmart" the display with nonstandard cables to get it to do something other than it was designed to do. With the correct cables often costing less than $10 each it would be wise to buy the right cables than put $1000 or even $100 hardware at risk of damage by using out of spec cables. USB cables that meet the spec will have the USB trademarked icon. This is true of most every cable, those trademarked icons are not just to indicate the cable function but also to indicate it was tested to meet safety standards.

Cables lacking trade iconography aren't necessarily nonfunctional or unsafe but that's the way to bet. The "right" icons aren't always a safe bet either as there can be nuance of cable specifications that was not made clear in the documentation, or the cable has been damaged in a way not visibly obvious. Use good judgement when hooking up expensive equipment with cables capable of carrying many watts of power. The wrong cable, or a damaged "right" cable, can be a fire hazard. Those adapters to plug USB-C cables to USB-A ports are the wrong cables and a fire hazard.

MacGuffin
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