Are there issues with stacking USB hubs?

Question

Would attaching a 4 port USB-3 hub to a port on a USB-C to USB-3 hub cause problems?

Answer 1

You can daisy-chain hubs, but there are limits. As long as you don't exceed the limits, you're fine.

• You can have a maximum network of 127 ports on a USB controller. That includes all the ports used to create the network tree.
• You can have a maximum of 7 tiers, including the root hub and the device at the end of the chain, leaving a maximum chain of 5 daisy-chained external hubs.

• If you're working with self-powered hubs, that's about it as long as you're within limits on the other USB requirements (like cable lengths; the length limitations apply to each leg of the chain).

  However, the USB specification requires that bus-powered hubs may not be connected in series to other bus-powered hubs.

• If you choose to ignore the USB specification, you might get away with connecting a bus-powered hub to a bus-powered hub as long as you don't try to exceed the power available from the upstream connection, depending on the specific hardware involved (e.g., how much power the USB-C to USB-3 hub can negotiate for, the loads on that hub and the USB-3 hub, whether the USB-3 hub will connect to the upstream hub if less than 6 unit loads are available, etc.).

Some general discussion here, here, and here, but if you want to daisy-chain bus-powered hubs, you will need to try it to see what might work with your specific equipment..

Answer 2

An USB 3 port delivers at most 900 mA. If you connect a hub to that port, than this hub will needs some power for itself and the minimum amount of power that is reserved per USB 3 device is 150 mA. So your first hub only has 750 mA that it can distribute to devices you connect to it. If you plug another 4 port USB hub into it, another 150 mA are reserved for it and now both hubs only have 600 mA to distribute for devices. That means you can at most connect 4 more devices to either of these hubs and only if these devices are fine with getting the minimum of 150 mA. If you plug in a SSD adapter, this adapter itself probably requires 900 mA for the controller chip and the SSD attached to it.

What will happen if you ignore these power constraints?

• In the best case, the operating system will notice that, refuse to activate the device, as well as informing you about this problem. The one device requiring more current than available will not work at all, the rest of the bus won't be affected.

• A lot worse would be if the system ignores it, the hubs ignore it and the device just draws as much power as it wants, causing a current limiter to kick in. As a result, the entire bus is unpowered and temporarily stops working. While this seems pretty bad, it protects your hardware from damage and at least will notice that there's a serious problem.

• Yet it can get even worse than that. If the current drawn is not enough to make the limiter kick in, yet still more than allowed and expected, the voltage may drop lower than allowed and this will lead to subtle malfunctions, e.g. only some devices may fail (as they are more sensitive to low voltage than others) or they may only fail some of the time (as the voltage fluctuates and how much they need depends on what they are actually doing at the moment). This can cause very hard to trace, odd malfunctions, as well as data loss, and even hardware damage.

Things will change if your second hub has an own power supply, though. In that case it will power its ports from that power source. The first hub still only has 600 mA for its remaining ports, though, that's because it still must provide 150 mA to the second hub, despite it having an own power source but devices plugged into the second hub won't count towards the 600 mA limit of the first one. And given that the power supply has sufficient power to offer, the second hub will not get another 900 mA to share between its 4 ports, actually each of its 4 ports will be able to offer 900 mA.

Not part of your question, but maybe interesting for other readers to know: With USB 2 things were a lot worse than what I just described above. The key differences are:

• A "workload unit" is the minimum amount of current that an USB bus must reserve per connected devices, regardless how much power the device will later actually need. In USB 3 this unit is 150 mA, in USB 2 it is only 100 mA.

• A USB 3 port offers up to 6 workload units if a device requests that (6 * 150 mA = 900 mA) whereas a USB 2 bus offers at most 5 (5 * 100 mA = 500 mA).

• A bus powered hub in USB 2 may only distribute 1 workload unit to each of its ports. So when connecting a USB device to a bus powered USB 2 hub, this device cannot get more than 100 mA. In USB 3 the available power can be distributed freely among ports, as long as every port gets at least one unit (if 750 mA are available, a 4 port hub must reserve 150 mA for each port, but can give one port 300 mA instead, USB 2 doesn't allow this; even if the hub only has 2 ports and still 400 mA available, every port only gets 100 mA at most and the other 200 mA go unused).

• Thus chaining two bus powered hubs is not even possible with USB 2, as the first one may only distribute at most 100 mA to every port and the second one itself consumes 100 mA for operation, leaving no power at all to distribute to its port.

You have violated any of the rules above and it did work?

Well, interestingly enough, systems are not very strict about following the rules standards. Often they are prepared to deliver more than 900/500 mA per port, e.g. for quickly charging devices and knowing that plugs, ports, and cables are able to transport up to 2000 mA, limiters are not set too strictly either.

So if you plug a bus powered hub into a bus powered hub and then an USB stick in the second hub, this stick may require 100 mA, causing the second hub to draw 200 mA from its upstream hub, despite only being allowed to draw 100 mA, yet neither will the current limiter of the first hub kick in at 200 mA, nor will the voltage drop because of that and the operating system will also think it is fine. Thus despite being totally out of the specs here, nothing terrible will happen and most likely it will just work.

In theory you could damage the first hub as if this hub was built to always be bus powered, it doesn't have to be prepared to ever deliver more than 100 mA per port and drawing 200 mA is twice as much as it has been built for. In practice though, this hub uses standard ports that are capable of handling 2000 mA, the conducting paths on the board are all the same and the chips used in these hubs are the same ones used in self powered ones, thus they are built to handle the maximum current to expect in such a situation. Damage is very, very unlikely.

Also a lot of Asian vendors came up with another trick: Instead of just drawing the allowed amount of current from a port, the hub simply claims to be an active hub (it isn't but it reports to be one) and then draws as much power as it requires to act like an active hub. This is a total violation of the spec and yes, it can lead to unstable buses (I've experienced that plenty of times) and even damage hardware but those vendors just don't care.

One big downside if that USB hubs don't require any certification that proves they are standard conform; more than 9 out of 10 USB hubs would never be able to get one and that's why so often people struggle with really strange USB bus problems. There are a few vendors that voluntarily got a certification by the USB consortium, meaning they totally follow the standards and that's why those hubs are easily 3-4 times the price of what you are used to.