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