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?

Solution

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..

Comments

[Ravindra Bawane]

What problems do you anticipate? Have you tried it? USB has always supported daisy chaining hubs.

[barlop]

from what I recall connecting usb hubs in the past and it may still be so today, they may need to be be 'active hubs' i.e. hubs that you plug in., that way the power gets maintained.. I don't think it'd be any different today.

[Christopher Hostage]

Agreed with @barlop , the limitation is amount of power available. Also, mixing make/model may lead to weird problems. I've personally daisy-chained three USB2 hubs without issues. I don't believe that they took the ability away with USB3.

[Ignacio Vazquez-Abrams]

Don't go above 127 USB devices and hubs total on a single bus though; USB will not be happy if you try.

[sawdust]

All USB hubs are "active", i.e. they all have active circuitry (electronics) to function. But the electrical power can be from the upstream connection, i.e. the host, or from an external PSU. Just because there's no external PSU doesn't make it a "passive" device. Cascaded hubs could introduce excessive latency. Latency is (one of) the reason for USB cable length restrictions.

[barlop]

@sawdust maybe it's a colloquial thing but the usb hubs that don't have external power are often referred to as passive hubs e.g. see speedron.com/blog/hardware/passive-vs-active-usb-hubs

[barlop]

@Metaphor if somebody gives you useless info or no info, why are you goading them to try to get more, why don't you just pay attention to the good info that other people give you, and just let the people that give you useless or no info go away, instead of encouraging dialogue with them as you are.

[Ravindra Bawane]

"Encouraging dialogue" is a nice way to put it, @barlop. There are many problems with this question. First, it's contextually theoretical. It's asking IF something works one way. It is not saying "I tried to get X to do Y, but Z happened instead". Generally, this indicates what information is out there about X and Y has not been searched for, and means OP failed to do the bare minimum of due diligence before asking. My question, characterized as being that of a "busybody", was seeking to improve the question by narrowing it down to an actual problem, and encountered error.

[Ravindra Bawane]

I was seeking to walk you through improving your question, Metaphor. Accusing people is a sure way to make it less likely you'll get good information.

[plugwash]

Something to note is that both hosts and devices may have internal (real or virtual) hubs and that some external hubs actually contain multiple daisy-chained hubs.

[barlop]

what about cable length, is that relevant? many times people daisychain to get more length anyway.. and perhaps they might get a usb cable that is too long?

[fixer1234]

@barlop, yeah. I mentioned that in the 3rd bullet. Didn't want to get sidetracked on a whole discussion of cable length because that's covered in other threads.