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Oh the dream of connecting 127 devices at once! It'd be a dream if I could connect even...say..more than four...

I have a Belkin USB hub with 7 ports on it. At any given time I can get about four of them to work. (Actually, all the ports work, just not all at the same time...)

Could this be somekind of a memory issue? I have a teribyte-and-a-half hard drive hooked up to it, and when I plug in my measly 8GB thumb drive it doesn't recognize it and I have to plug it into a port directly on the machine to get it to work.

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4 Answers 4

up vote 3 down vote accepted

As an insult to the existing injury...each hub counts as a device, so you'll burn quite a few device addresses on your quest to get to 127.

A "powered hub" actually has a power supply that plugs into it. A lot of them are made this way.

If the hardware conforms to USB spec, and the operating system is decent, you will get message pop-ups for actual faults. If the current draw is too much you would get "power limit exceeded" message. If a device just stops responding there's no message, but there is that "disconnect sound" if you use Windows.

A lot of those "USB toys" made overseas patently ignore anything about power requirements. They also work a lot of the time, which tells you how faithfully most computers follow the spec.

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I think you are running afoul of the maximum power draw for a single USB device.

You can chain more hubs to get more ports but you must supply more power than the computer can via the USB port to run them all.

"A maximum of 5 unit loads (500mA) can be drawn from a port in USB 2.0," - http://en.wikipedia.org/wiki/Universal_Serial_Bus#Power

Unless you supply more power by plugging the hub into a wall wart you will not be able to draw more than 500mA via the computer's USB port.

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+1 - That's pretty much the gist of it. You need powered hubs if you're going to be doing this. Aside from that, even though the spec says 127 devices I'd be surprised if you can even hit half that number. There's the spec, and then there's the reality of poorly manufactured products that skirt tolerances in the name of cost. –  Evan Anderson Jun 6 '09 at 3:05

Is the hub powered? You might be drawing too much power from the bus.

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Well I plug the hub in if that's what you mean...You're actually supposed to be able to stack multiple hubs like this together...(according to Belkin...) Also the HDD is powered, and plugged into the wall. The only other two things I have plugged in are a keyboard and mouse...they can't draw that much current.. –  leeand00 Jun 5 '09 at 21:55

I'd wager it has something to do with the (poorly-conceived) way that USB works, which is essentially a round-robin queue. The host bus controller asks each device in turn if it needs to do any sending or receiving, then after that one moves on to the next. The more devices you have on the bus, the more latency you're going to have.

Especially with USB hard drives, it's really nice if you can give them their own bus so that the host never has to interrupt them to see if the mouse has moved at all. How much of a difference this makes in practice I haven't bothered to measure.

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1  
hmm I wonder how one would measure this... –  leeand00 Jun 5 '09 at 22:54
    
I guess you could plug a USB HD into the bus and do some tests, then plug it in alongside as many USB devices as you can cram together on the bus and see if it degrades. –  Dan Udey Jun 5 '09 at 23:21

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