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I have an USB C type dock that has 2 USB A ports. The dock operates the monitors, the UTP and event the power with a USB C power adapter.

The dock provides USB 3.0 ports and the hub has USB 2.0 ports. The dock is a Targus DOCK423E USB-C Dual HDMI 4K Docking Station with 100W PD Pass-Thru.

When I use the 2 USB A ports directly, it works all right with my mouse and keyboard but I have 4 USB devices to connect.

So I bought a 4 port USB A hub and plugged it into the dock. Windows 11 shows the the USB device is unknown and can't operate it.

When I plug the hub directly to a port on the computer, it works normally. The laptop is a Dell Latitude 3520.

So how can I make the USB A dock work with the USB C hub? I think the dock has the necessary power to operate 2 devices (if I use the directly it works) so I don't really understand why it can't work through the dock.

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  • Theoretically USB should work as you can cascade USB hubs up to 7 levels (including root hub). So assuming the dock contains an USB hub you could add another 4 hubs cascaded after another. What you forgot to mention is if the ports the hub provides are USB 2.0 (white) or USB 3.x (blue) and also the same for the hub.
    – Robert
    Jul 16, 2022 at 12:37
  • @Robert I added the necessery information. I assume, an USB 3.0 ports provide enough power for a lower speed USB 2.0.
    – Nestor
    Jul 16, 2022 at 13:13
  • Is the 4-ports USB hub self-powered?
    – harrymc
    Jul 16, 2022 at 13:17
  • @harrymc No, it is just a simple splitter.
    – Nestor
    Jul 16, 2022 at 13:19
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    @Giacomo1968 I added the laptop model in the description.
    – Nestor
    Jul 20, 2022 at 18:52

2 Answers 2

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From another answer by @harrymc:

A USB 2.0 port gives 500 mA of power, USB 3.0 gives 900 mA.

It's more complicated than that. A USB 2.0 port on a device that is plugged into a host with mains power is required to provide a minimum of 500 mA of current to comply with the spec. A battery powered USB host is permitted to supply less power, but that's a path we don't need to go down now.

The maximum current from a USB 2.0 or USB 3.x host can be 1.5, 2.1, 2.4, or possibly 3.0 amps depending on the revision of the USB spec it follows. A hub port must be able to supply a minimum of 100 mA to comply with the USB spec. That's to allow for a 4-way hub that has 100 mA per port and have 100 mA for the hub electronics when plugged into a host supplying 500 mA. 100 mA is often more than enough power for mice, keyboards, and powered devices like printers and scanners.

Unless the hub is specifically made to support high power devices it must supply only 100 mA to comply with the spec. Some devices want more power than this, and to get the power it wants it might not be in compliance with the spec. One device that takes more power than it is allowed in the spec could trigger a protection circuit, pull the voltage too low for everything to work, or cause some other undesirable behavior.

Because of the complexity on how USB works it can be difficult to find which device is at fault without considerable experimentation and digging into the device specifications. The Targus dock mentioned does claim to support USB-BC power up to 1.5 amps out of the USB-A ports. That should be more than enough power for a hub to have most typical devices plugged into it. The problem is that unless the hub "speaks" USB-BC the hub can't know it has 1.5 amps available to it. It may assume it has only 100 mA available because it is told that it is plugged into another hub. Plug this hub into a port directly on the host and it sees that it is the first hub in the chain and therefore assumes 500 mA is available and so works normally.

How does one resolve this issue? There's many options. One is to do what you have been doing and plug in the dock and the hub directly into the host computer. Maybe a newer USB 3.x hub will recognize that there's more power available on the dock port and so will allow more power to downstream devices. What I offer is a best guess on why your current setup is not operating as desired.

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  • How does this answer improve on the other answer you reference? It seems like this is just a very long, and sorry to say rambling, comment to the other answer you reference and not a new answer at all. Jul 17, 2022 at 18:43
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    "How does this answer improve on the other answer you reference?" For one it corrects the error that the dock can't provide more than 500 mA from the USB port. The dock spec shows the port is capable of providing 1.5 amps. An unpowered hub might work from this, or it might not. It depends on if it is compatible with USB-BC for power. Adding a powered hub may not resolve the issue either because a device may need USB-BC to work and the hub does not support that. I tried to explain the how and why so that the reader could more easily and quickly determine compatibility.
    – MacGuffin
    Jul 19, 2022 at 3:48
  • Your answer is in error because you have taken only part of my answer and the dock specifications. First, you have ignored the link in my answer that specifies which USB Type-C ports can give more than 900mA - you must have at least a "Multi-lane SuperSpeed (USB 3.2 Gen 2)" port on the computer to supply 1.5A. If the computer doesn't have such a port, then 900mA is the limit. [...]
    – harrymc
    Jul 19, 2022 at 14:26
  • [...] Second, the dock specification says "Supports BC 1.2 at 1.5A charging (when USB-C power pass-thru port and laptop are connected)", which only means that the dock has one power pass-thru port and with it the dock can charge one device at 1.5A. But it still needs to get that much power as input...
    – harrymc
    Jul 19, 2022 at 14:27
  • @harrymc The Targus specifications state that the dock can provide 1.5A to the USB-A ports, so I am not in error to correct that. The USB spec allows 1.5A from a USB-A port if the port also supports USB-BC. The USB 2.0 spec has 500mA as the minimum, not maximum, and that only applies to hosts. USB 2.0 hubs are required to supply a minimum of 100mA from downstream ports, and a USB-C dock can qualify as a USB 2.0 hub in the spec. USB 3.2 gen 1 (as in this specific dock) does not allow for 1.5A unless the port also supports USB-BC (as in this case) or USB-PD.
    – MacGuffin
    Jul 19, 2022 at 15:09
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A USB 2.0 port gives 500 mA of power, USB 3.0 gives 900 mA. (see Wikipedia USB Low-power and high-power devices for more recent USB ports).

This is used to power the dock and hubs and the rest is divided over all the connected devices. This means that practically speaking, there is a limit to the number of devices that can hang from a single port.

To avoid this problem, use a self-powered hub. With mains power, all the ports can be fully powered.

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