USB Capacity

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How many iMic sound devices and other USB devices can I run per computer? When do I need a powered USB hub?

In part, it depends on the computer. In part, it depends on the hub. In part, it depends on the device.

You have a power budget and a bandwidth budget to consider.

Go to your Windows Device Mangler.

Expand the USB tree. There you will see how many USB Root Hubs and Host Controllers you have. Each Root Hub and Host Controller may support one or more discreet USB ports on your computer. Most newer computers have just one port each, so there's no inherent contention unless you connect an external hub to the port. If you want to plug in more devices than you have ports, you'll need to get a hub.

Hubs can be powered or unpowered. Powered hubs require a power adapter (from AC or a DC car plug). You need a powered hub when your total power consumption exceeds that available at the computer.

Find the USB Root Hub entries by expanding the USB tree. Find the one your hub is plugged into by finding the one that shows some power consumption in the Power tab. Note that you should have 500mA available per USB 1.1 root hub, then note the power consumed. For iMic2 it's 100mA, so in theory you should be able to plug in 5 iMic2 devices, except for that the unpowered hub consumes some power.

Different USB devices consume different amounts of bandwidth. Verify that the total bandwidth consumed does not exceed what is available per USB Host Controller.

Find the correct USB Host Controller entry and go to the Advanced tab. Find the one you're using by noting one that is using more than the default 10% bandwidth when active (you must be actually using the iMic with the app at the time). For an iMic2, it's 16% each, so in theory you should be able to run up to 5 iMic2s (5 x 16 + 10 = 90).

The above numbers are based on USB 1.1 and use of iMic2 device. YMMV with different devices, so check these stats on your own computer.

-- GaryHahn - 20 Mar 2009