Why are usb hubs only available up to 7 ports?

Why are usb hubs only available up to 7 ports? Is there something technical that prevents that? I thought USB addressing allowed 127 USB devices to be connected, so that's not it.

EDIT: Thanks for the links for hubs with more ports. There are a lot of them with 7 ports, and I'd never seen one with more. I didn't think 7 was the technical limit, I just wondered why 7 seemed to be so universal.

Here's what I have that plugs into USB:

  1. mouse
  2. keyboard
  3. ipod
  4. ipod shuffle
  5. camera
  6. joystick
  7. rudder pedals
  8. throttle quadrant
  9. gps
  10. external HD
  11. memory stick
  12. cell phone

Replay

Why should the theoretical port number be limited to only seven ports? There are hubs with more ports on the market.

Look at this monster example: 49-port USB hub is just plain practical

Why are usb hubs only available up to 7 ports?

I think the main issue is power.

I'm not going to look up the technical numbers and figures, but if you put too many devices on a hub, The USB port won't be able to supply enough power to all the devices.

Here is a USB hub with 16 ports (8 on each side) and it uses an external power source.

edit: the 13 port hub posted earlier also has an external power supply...

The 7 comes from the spec. The theoretical limit for USB is 127 devices which is (2^7)-1. It would be 128 because there are 7 bits for the address, but address 0 is reserved. So we have 127. Did you notice how the 49 port hub is a multiple of 7? Also, there is an upper limit of 7 tiers of devices which is explained in these articles:

http://www.totalphase.com/support/kb/10047/

http://www.usblyzer.com/usb-topology.htm

Here is a 13 port usb hub.

13 port USB hub

Maybe because no one (as in 99.9999%) has a use for more then 7 ports ?

There are hubs with more then 7 ports, I've seen hubs with 10, 13 ports usually. However, a USB host can have only 7 devices attached to it. Therefore, when there are more ports then 7 in a USB hub, there are several logical hubs inside.

So, I think, the 13 port hub internally is organized as two hubs connected one to the other:

a) primary 7 port hub, of which 1 port is used for uplink for the secondary hub, so has 6 free ports

b) secondary second 7 port hub, with 7 ports.

Together, it is 6+7=13 ports.

Therefore, the 13 ports are not equal, 6 are closer to the root hub, and 7 ports are 1 level further away. This can be important in some cases when many hubs are daisy chained. Recently I got a warning in Windows 8 saying I have too many USB hubs daisy chained, and devices more then 5 level away from the root will have problems, so I had to reconfigure them.

The practical consequence of this is that it's better to have one hub which acts as a "source" for all other hubs, instead of plugging one hubs sequentially, to minimize the distance from the root.

I have tried many USB 2.0 hubs recently, in the end I have settled with an EXSYS EX-1177. It provides sufficient power for external HDD drives, and non-powered USB hubs. I use it as the lowest level hub, and other hubs are attached to this one.

I suspect that power is a concern, as suggested, but also that the market for hubs over a certain size isn't big enough to make it worth pushing them as a common item.

Category: usb Time: 2009-06-04 Views: 1
Tags: usb

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