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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

7 Answers 7

23

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

alt text
(source: crunchgear.com)

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  • 3
    Why did this get voted down? I'm new here so I'll admit I don't understand the culture completely but this answer showed us that there are hubs larger than 7 ports and it entertained a bit.
    – kbyrd
    Jun 4, 2009 at 13:58
  • 1
    Not a clue! I thought it was good and gave it a +1 :)
    – cagcowboy
    Jun 4, 2009 at 13:59
  • Perhaps it was marked down because it's not available yet? I think it's awesome though.
    – GoatRider
    Jun 4, 2009 at 16:49
7

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...

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  • This one doesn't seem to be available yet either.
    – GoatRider
    Jun 4, 2009 at 16:50
6

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

2

Here is a 13 port usb hub.

13 port USB hub

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

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  • 1
    It's getting pretty ridiculous how many USB ports we need. I have 1)2) 2 USB Thumb Drives. 3) A USB Mouse. 4) A USB Keyboard 5) USB Speakers 6) A USB Headphone/microphone. 7) A USB Webcam. 8) A USB Printer 9) A USB CardReader (CF, SD, etc..). 10) A USB Cable Modem. 11) iPod. 12) Digital Camera. That's 12 devices, which I need to mostly all plug in at once, and I don't think i'm all that atypical. Jun 4, 2009 at 16:50
  • You should try Ethernet for your cable modem. It's faster than USB and could be run through a router and shared.
    – Brad Bruce
    Jun 21, 2009 at 0:17
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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.

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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.

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