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I have a really "weird" question.

We have to connect a lot of USB's devices (Android phones) on a PC thus we are using three USB hubs (we are also using adapters to power the hubs) and three USB ports for the HUBS. We also use another USB port for keyboard and another one for the mouse.

The problem is that when I try to connect more than 15 phones on the hubs the PC become unstable. ( I tried first with Windows 7 and then CentOS as the host OS) Even if the devices perform really well on CentOS I cannot connect more than 15.

The question is:

How can I check if there is a hardware limitation regarding the number of USB devices I can connect to the PC? And if there is a limitation can I overcome it?

Finally, I would like to ask if there is any software that I can use to log/monitor any problems regarding the USB connection.

Thanks

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By desgin, you can connect a maximum of 128 devices (including hubs and a "USB root hub" on your motherboard) to a USB controller. I am not sure if all motherboards has this tested, but I guess recent versions of Windows and Linux should have this capability. I guess the problems could be caused by power shortage, buggy USB protocol implementation in the devices, faulty hubs or even buggy driver when handling multiple devices. Could you specify the model of your hubs and phones and if you have installed any relevant drivers. –  billc.cn Oct 6 '12 at 17:26
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Can we get your system specs? –  user88311 Oct 6 '12 at 17:32
    
A little clarification: If you have a second USB host controller, I believe you would get an additional 128 devices. Like these folks above said, each USB hub and controller are themselves devices, so each one uses a USB spot. –  UtahJarhead Oct 6 '12 at 20:43
    
There are at least 3 constraints. (1) The USB protocol can only address 127 devices; you cannot "overcome" that. (2) The host PC has limits on the power delivered on each USB port; you have "overcome" that by using powered hubs. (3) The USBus is hosted by a single PC and has finite bandwidth; you cannot "overcome" that. IOW it's not just the number of connected USB devices, it's also about how much bus traffic each device requires before the host PC becomes overloaded/nonresponsive. Given that it's the PC that struggles, it's the PC capabilities that should be questioned, not USB capacity. –  sawdust Oct 6 '12 at 20:46
    
@UtahJarhead - It's very likely that multiple USB host controllers are being used. (The PC mobo I'm using now has 4 USB controllers and 4 hubs, for a total of 8 USB ports.) Multiple USB controllers are good for the USB devices, since there'll be more USB bandwidth for each device. But it is a bigger I/O load for the PC. Win7 could allocate 20% of "System bandwidth" to "Enhanced" USB controllers and 10% to OpenHCD controllers, or potentially 60% of "System bandwidth" on my PC. –  sawdust Oct 6 '12 at 21:30

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