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I need to connect about 10 different electronic 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'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 for the hub:

https://www.conrad.com/p/renkforce-rf-uh-a10-10-ports-usb-32-1st-gen-usb-30-hub-aluminium-casing-black-1642425

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,

https://www.conrad.com/p/digitus-da-73300-1-2-ports-usb-32-1st-gen-usb-30-changeover-switch-black-2258896

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:

DELL OptiPlex 7040 MT XCTO

https://www.dell.com/support/manuals/en-us/optiplex-7040-desktop/opti7040mt_om/specifications?guid=guid-f058d593-d332-479e-9d55-6c6031fa6cba

DELL OptiPlex 7060

https://www.dell.com/support/manuals/en-us/optiplex-7060-desktop/opti_7060_tower_setup_specs_manual/system-specifications?guid=guid-ab67ed37-0818-4592-a25c-f04b3a73c18d

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  • \$\begingroup\$ PS.: I need USB 3.0 due to the bandwidth of the cameras I'm using. \$\endgroup\$
    – srhslvmn
    Oct 16, 2022 at 21:08
  • \$\begingroup\$ Another crucial limit is the number of USB lanes supported by the host - usually the PC motherboard. I have too too many USB hard drives connected to my system (40 TB or so). An older AMD system supported these but a faster newer Intel system had far fewer lanes and would bit support all drives. \$\endgroup\$
    – Russell McMahon
    Oct 16, 2022 at 22:19
  • \$\begingroup\$ @RussellMcMahon Hi, could you explain this point a little further? Does this mean that I cannot simply plug the single USB cable coming from the switch into one of the USB ports of the PC (because that one port on the mainboard would have to deal with 10 simultaneous connections)? I.e. would this make a second 10-port hub necessary in order to fan out those 10 connections to 10 individual different USB ports on the PC? \$\endgroup\$
    – srhslvmn
    Oct 16, 2022 at 22:23
  • \$\begingroup\$ Devices may use more than one lane. You can hit the limit with far fewer devices than lanes. 127 devices would be exceedingly unlikely. I can run somewhere in the 10-20 range total on the pc due to lane limitations. \$\endgroup\$
    – Russell McMahon
    Oct 16, 2022 at 22:23
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    \$\begingroup\$ @RussellMcMahon (...and "lanes" meaning PCIe lanes, right?) \$\endgroup\$
    – srhslvmn
    Oct 16, 2022 at 22:26

2 Answers 2

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A USB hub is formally a "USB device", with special features. The description of your switch is simply overly restrictive, likely out if ignorance of their marketing. You should not worry about the number of devices behind this link to a hub, provided that the switch is designed correctly, and the tree of USB devices behind it is properly powered, and cables are of good quality.

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The number of devices is an addressing limitation, so any host will be able to address 127 devices (including hubs).

Normally the limitation is either power (which you can solve by adding powered hubs) or bandwidth (which can be calculated from the USB descriptors of the devices -- that's what the USB stack does when it determines whether a device can be activated).

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Voltage Spike
    Oct 17, 2022 at 23:41

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