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156. All About USB

By Andrew D. Wright

Back in 1994, connecting anything to your computer required a bit of fiddling. You had a bewildering array of PS/2, serial ports, parallel ports, SCSI buses and more. Adapters could be male, female, 9 pin, 25 pin, or some other form not available to you when trying to hook something up. If some device was using the only available adapter, you had to add another adapter on an add-on PCI card to use a second device at the same time.

When you did physically plug something in you then had to allocate the correct resources to the device - more gobbledygook about IRQs and DMAs and avoiding resource conflicts while manually installing device drivers.

[Graphic: USB Logo] A group of software and hardware companies got together to create the Universal Serial Bus standard. The now-familiar rectangular plug with its three tine forked logo was given some basic support in Windows 95B, the next-to-final release of Windows 95.

USB 1.0 could send data at 12 Mbits per second and enabled different devices to be connected to the computer with one kind of adapter. The USB logo is on the top of the plug so users could tell at a glance which way to plug it in. On vertical USB slots the USB logo faces right.

USB devices are broken down into classes with basic driver support for most of the classes built-in to modern computer operating systems. This is how you can plug a new flash memory stick into a Windows XP computer and have it work without having to download any new drivers for it.

By 1998 a USB 1.1 standard had been agreed upon, setting up 1.5 Mbit per second low speed access for slower devices in addition to the regular 12 Mbits per second speed.

It wasn't until 2002 and the adoption of the USB 2.0 standard that USB truly came into its own with 480 Mbit per second access speed.

USB differs from the traditional connection methods in several ways. Devices can be connected through USB hubs, so up to 127 devices can form a chain all connecting to one USB port on the computer.

USB allows hot-swapping of devices, where a device can be plugged in and unplugged without having to restart the computer. USB carries 5 volts of electrical current with a load of 100 - 500 milliAmps so devices with moderate power demands can be powered through the USB cable alone.

USB 2.0 can stretch out for five metres between hubs, which can be chained - up to five hubs on one USB connection. When looking for a USB hub, getting one that has its own power supply is a good idea for maximum convenience when plugging in a lot of devices, so anything needing power can have it.

While most new hubs are USB 2.0, some cheaper ones still run at the slower speed. It is worth double checking this before purchase. Naming conventions for USB can be confusing.

USB 1.1 and its 12 Mbit/sec speed is referred to as "full speed" USB. USB 2.0 and its 480 Mbit/sec speed is "high-speed" USB. Some older computers have BIOS-based switching that can select the speed of the motherboard-based USB slots - full speed or high-speed. The idea is that system resources can be saved by assigning the lower speed USB to slots with devices not needing faster speed like keyboards or mice.

USB 3.0, or "SuperSpeed" USB will allow speeds of 5 Gbits per second. The specification was agreed upon in 2008 and the first USB 3.0 devices should start showing up late in 2009 or early 2010.


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Originally published 7 August 2009


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