USB 2.0 - A Write Up

USB allow you to connect external peripherals to PC, such as digital cameras, keyboards, mouse, printers, DVD Writers, Back-UP hard disks etc, through a standard connector available in the computer's motherboard or through an extra board added to the computer (if you don't have a high-end motherboard with this kind of bus).

Firewire (also known as IEEE 1394) is an external bus to connect external peripherals to the computer, similar to USB, which has as great attractive a high transfer rate: 400 Mbps (that is approximately 50 MB/s).

The USB 2.0 version has a maximum transfer rate of 480 Mbps (approximately 60 MB/s), that is, a higher rate than the FireWire and much higher than its previous version (called 1.1), that is the version in the computers, until recenlty and that allows the connection of peripherals using transfer rates of 12 Mbps (approximately 1.5 MB/s) or 1.5 Mbps (approximately 192 KB/s), depending on the peripheral.

The great problem of the USB was its transfer rate. Just remember that most hard disks available nowadays on the market work at a rate of 66 MB/s. As the USB used at transfer rate of 1.5 MB/s, an external hard disk connected to the computer through USB is extremely slow. For more common applications - such as printers, scanners and video cameras - the USB transfer rate is satisfactory. The real problem is the connection of peripherals that demand high transfer rates, basically data storage systems, such as hard disks, CD recorders and Zip-drives.

The USB 2.0 port continues 100% compatible to USB 1.1 peripherals. When initializing the communication with a peripheral, the port tries to communicate at 480 Mbps. If it does not succeed, it lowers its speed to 12 Mbps. If the communication is still not established, the speed is then lowered to 1.5 Mbps. So, the users should not worry about the USB peripherals that they already have: they will continue being compatible to the new pattern.

A very important detail is that USB 1.1 hubs cannot establish connections at 480 Mbps to peripherals connected to them. For example, if you have a USB 1.1 keyboard which has a built-in USB 1.1 hub, USB 2.0 peripherals connected to this keyboard will only get to communicate with the computer at 12 Mbps tops, and not at 480 Mbps. So, you should pay close attention to this detail.

The great advantage of the USB 2.0 over the FireWire is, therefore, the compatibility with the already existing USB peripherals.