USB Hard Drives and Portable Hard Drives

    Personal computers are used for a wide range of applications and many new devices have been developed since IBM's original two serial port personal computer some twenty years ago. Scanners, portable hard drives, Zip drives, and force-feedback joysticks are just a few examples of devices appearing on the desktop. Although attempts have been made to provide more serial ports on a single PC, there is no real standard that has gained widespread acceptance. SCSI seemed to be the solution of choice for many vendors of scanners and external drives, however, interface cards and devices were expensive and the standard lacked unity. Therefore, many began work on specifications for new interconnecting solutions, such as firewire and USB.

USB Hard Drive Interface

   The Universal Serial Bus (USB) is the solution touted by seven leaders of the PC and telecom industry: Compaq, DEC, IBM, Intel, Microsoft, NEC and Northern Telecom (now, Nortel Networks). Though slower than FireWire, USB still boasts a data rate of 12 Mbps (mega-bits per second) and allows you to connect up to 127 devices to your PC! It is designed to support modems, keyboards, mice, 4x – 6x CD ROM drives, joysticks, tape drives, floppy drives, hard drives , scanners and printers. In addition, a new wave of peripherals such as telephones, digital speakers, digital snapshot and motion cameras, data gloves and digitizers are to take advantage of this exciting and versatile new interface.

How Does USB Work?

There are three physical parts to the USB system. These include the host (computer), hubs, and devices including hard drives. All connectors are one-size-fits-all, so a device like an external USB hard drive can be plugged directly into the host, or into a hub, which in turn is plugged into the host.

    The USB cable is thin (four wires) and carries enough power for low-power devices, like keyboards and mice. The maximum bandwidth is 12 Mbps, which is shared amongst all devices on the USB network. Since devices are organized in a tiered fashion, not every device needs a direct connection to the host. A hard drive can be plugged into a hub, into another hub, and then the host, thus avoiding a clutter of wires behind the computer.

When you Plug in a USB Hard Drive or other device, what happens?

    Whenever you plug in a device (like a USB hard drive), the host senses voltage differences in the USB network and proceeds to query the device for type, vendor, functionality and bandwidth required. That device is assigned a unique address ID and co-exists with all other USB devices. Even if two identical devices are plugged in, they will each have a unique address and can be distinguished separately by the computer. Once enumeration is complete, the appropriate device driver is loaded by the operating system (O/S) and the user will be prompted for the driver disk if necessary. All contention of devices is handled by the host and by the software residing on the host. There is no need to configure interrupt IRQs, addresses, or DMA channels.

    When devices are detached (unplugged) from the USB network, the host computer detects the detachment, alerts the appropriate application and unloads the drivers.