USB is a hardware standard for physically connecting devices to a computer. Basically every peripheral, whether a mouse, keyboard, printer, or even network card can be attached via the USB port. However, there is a limit in the number of devices you can connect to a computer. More specifically, there is a limit of the number of devices a single USB port can support. Why? Power, simply. Each port can only sustain 500mA of electricity. Therefor, devices such as mice, keyboards, or even scanners usually draw their current via USB.

Savvy users are now wondering why any of this matters since one USB port can only physically fit one device, right? Not so – typically there are 2 USB ports to one controller – it’s on the controller that the electrical limitation occurs. This is why you see USB connection in pairs. It can happen that two devices on the controller can exceed the power available, at which point all sorts of erratic behavior can occur. The solution is pretty logical, split the devices between two USB pairs!

Actually, you can be a bit more intelligent in determining the layout of your connected devices rather than resorting to the haphazard approach above – you just need to know the power draw of each device.

USBDeview is a free utility from NirSoft that can unearth all sorts of information from every connected USB device, including power requirements for our situation. Executing the application, no installation necessary, will inventory the current system and display all the detected USB related devices, drivers, services. It can be a bit difficult to identify which device is what line but usually the driver description is sufficient and it just so happens to be next to the power column. Here you can a lone connected device, a wireless Microsoft mouse, that draws 100mA; a mere 20% of the total power available. The adjacent USB port should have plenty of left-over juice for another peripheral.

Download USBDeview