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I need to connect about 10 different experimental devices (lasers, cameras, motorized stages etc.) to two PCs, such that you can choose to control the whole setup from either one of those PCs.

My first thought was to get a 10-port USB 3.0 hub, connect all devices to it, then use a 2-port USB 3.0 switch to connect the hub to the PCs:

              ,-------,
Device 01 ----|       |
Device 02 ----|       |
Device 03 ----|       |    ,-------,
Device 04 ----|  USB  |    |  USB  |---- PC 01
Device 05 ----|  3.0  |----|  3.0  |
Device 06 ----|  HUB  |    | Switch|---- PC 02
Device 07 ----|       |    '-------'
Device 08 ----|       |
Device 09 ----|       |
Device 10 ----|       |
              '-------'

I need USB 3.0 due to the bandwidth of the cameras I'm using.

I've read that it's best to use externally powered hubs and switches in order to ensure that all devices get enough power, so I was considering something like this (Renkforce RF-UH-A10 10 ports USB 3.2 1st Gen (USB 3.0) hub Aluminium casing Black) for the hub.

But I'm uncertain about the switch, as many product descriptions seem to imply the support of only a single device and not, as required here, up to 10 connections coming through the USB hub. For example, the description of the following model (Digitus DA-73300-1 2 ports USB 3.2 1st Gen (USB 3.0) changeover switch Black), says "Up to two PCs have shared access to one terminal (printer, scanner, etc.)".

But to my understanding, the USB standard allows for up to 127 devices bidirectionally connected and converging to a single host line, with daisychaining of up to five hubs (i.e. a total of 7 "levels" including the host and client devices).

Question:

  • How can I tell for certain how many devices a switch supports, so I can get the right one for my application? I.e. do all USB controllers inside these hubs and switches support the full network size (i.e. 7 levels, 127 devices) specified in the USB standard and, if not, how does one tell? For example, how can you tell whether or not the 2-port switch linked above can handle 10 device connections coming through the single line from the hub?

PS.: These are the specifications of the two PCs in question:

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  • You are going to cap your bandwidth resources well before 127 devices.The standard might allow 127 devices, but that is more like 127 keyboards and mice, not 4K web cameras, 2.5 GB Ethernet adapters, and WiFi 6E adapters.USB 3.2 GEN 2 resolves some of those concerns, but a single USB host, is still going to reach its limitations well before 127 devices. I ran into this problem, wanted to have external USB HDDs, microphones, headsets. The length of the USB cable, the number of those cables and their length, all were consideration before Windows would generate a notification about USB resources.
    – Ramhound
    Oct 17, 2022 at 4:51
  • @Ramhound Yes, but fortunately I will never get anywhere near that scenario of having to connect over 100 devices. :) In my case, it's going to be one or two scientific cameras (sCMOS/CCD/EMCCD), a few laser controllers, and mostly controllers for motorized stages, piezo elements etc. which don't require much bandwidth. But what do you mean by "before Windows would generate a notification about USB resources"? Does Windows recognize signal loss or bandwidth issues?
    – srhslvmn
    Oct 17, 2022 at 18:53
  • If you exceed the capabilities of a USB port (HUB) you will receive a Windows toast notification indicating that there are not enough USB resources available for the device to function properly.
    – Ramhound
    Oct 17, 2022 at 19:42

1 Answer 1

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If that switch is indeed a simple electric switch that connects the USB data lines of either one or the other PC to the device-side port, then there should be no limit to the number of devices it can support via USB hub(s).

But if the switch participates to the data traffic of the USB bus in any way (e.g. by trying to pretend the device is connected to both PCs, or even trying to auto-switch between the PCs according to which PC is trying to use the device) then plugging in a hub with many devices could cause problems.

A simple test would be to plug in just the switch to one computer, with no device attached to the switch. If the computer does not recognize the switch itself as a plugged-in USB device, then the device is essentially the equivalent of a dumb USB cable, and there are no "smarts" inside that would affect the content of the USB traffic in any way. If you then plug in a device and flip the switch from "computer A" to "computer B" and back, you should see the device attached to the switch detected as "hot-plugged" and "unplugged" according to the position of the switch.

Of course, this also means that when you plug in a hub with many devices to the switch, if you flip the switch while one or both PCs are running, you are likely to get a cacophony of hot-plug/unplug notifications...

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  • Hi telcoM, is there any way to somehow discern "dumb" from "smart" USB switches from the specs/datasheets or would I have to buy and test switches one by one? For example, what about the one linked in the question, do you see any indication that this is anything more but a "glorified cable"?
    – srhslvmn
    Oct 16, 2022 at 22:19
  • It does indeed look like a "just a glorified cable": there is no software to download, and the button on the top of the switch seems to be the only way to switch the devices from one PC to the other.
    – telcoM
    Oct 16, 2022 at 22:25
  • ...so I can safely get that switch, hook it up to the PCs, and simply "squeeze" 10 different device connections through it into e.g. the frontpanel USB 3.0 port of the PCs?
    – srhslvmn
    Oct 16, 2022 at 22:28
  • (By the way, I've updated some information on these two PCs to the question in case this is of any significance)
    – srhslvmn
    Oct 16, 2022 at 22:31
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    Note that the manual of the hub says its PSU can supply the full 900 mA current to only up to four USB ports, so it's up to you to ensure you won't overload the hub's power capacity. Otherwise it looks doable to me.
    – telcoM
    Oct 16, 2022 at 22:38

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