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What is Universal Serial Bus (USB)? - Technical Tip - 163024




Universal Serial Bus (USB) explained.

USB stands for Universal Serial Bus. USB provides easy attachment and removal of external devices such as printers, scanners, modems, and cameras. USB is a replacement for the older serial port found and uses the features of Plug-and-Play and Hot Swapping to achieve the easy attachment and removal.

Only one device can be attached to a standard serial port. In contrast, up to 127 devices can, in theory, be attached to the USB network. There are two types of USB ports: active and passive. The USB port on the back of a computer is an active port. Active ports are able to provide a small amount of power to devices that require such power. The power supplied by an active USB port is in no way a replacement for power supplied by other sources, such as wall outlets or batteries. The power supplied by an active USB port is used to activate certain electronic components within some USB devices. Passive USB ports are not able to supply power to USB devices.

When multiple USB devices are to be attached to a computer, a device called a USB hub should be used. The USB hub is a device that attaches to the USB port on the computer with a cable, and provides several USB ports. USB hubs offer the convenience of being able to place the USB ports in an accessible location such as a desktop. Active USB hubs contain active ports. Passive USB hubs contain passive ports. Connecting a passive USB device to an active USB port will produce no adverse effects. However, connecting an active USB device to a passive port will cause the active device to function poorly or not at all. A passive USB port will not physically harm a device that requires active USB support. It is a good idea to purchase an active USB hub, if a USB hub is desired, along with cables that are rated for active devices.

There are two kinds of USB connectors. The Series A connector is a thin rectangular connector. This type of connector is the type that plugs into the back of your computer. The other USB connector is called a Series B connector. The Series B connector is somewhat cylindrical. Many manufacturers use the Series B connector on the USB devices they produce. For example, a USB cable that connects a camera to the computer might have a Type B connector on the camera end and a Type A connector on the computer end.

The option to automatically power down USB devices after a processor is idle but still in an active (not in a standby or suspended) state is not supported in Windows 95, Windows 98 or Windows 98 SE.

If a USB device is attached to laptop computers that use Advanced Power Management (APM) or Advanced Configuration and Power Interface (ACPI), the computer may experience increased power consumption leading to a more rapid drain of battery power. The polling required to maintain communications with the USB device prevents the Central Processing Unit (CPU) from switching to the C3 (Clock-Stopped) power state. When a USB device is not connected, the CPU can spend a significant amount of time in a C3 state, which substantially extends battery life. Disconnect the USB device when it is not in use to increase the battery life and avoid the need to recharge as frequently.

Installing software for USB devices varies by device. Be sure to follow the manufacturer''s instructions carefully.



The different revisions of Universal Serial Bus (USB)

Since its inception, USB has gone through two revisions. These revisions have allowed USB devices to continue to stay competitive and improve the features that are offered. Below is a list of the 3 versions (the original and the two revisions) that are offered with examples of devices that would fit in that category.

  • Low Speed (1.0) Transfer Rate: 1.5 Mbps.

    Examples include:
    1. Keyboards / Mice
    2. Game Controllers
    3. Virtual Reality Devices

  • Full Speed (1.1) Transfer Rate: 12 Mbps.

    Examples include:
    1. Broadband Devices (DSL and cable modems)
    2. Audio Devices
    3. Microphones

  • High Speed (2.0) Transfer Rate: Up to 480 Mbps.

    Examples include:
    1. Broadband Devices (DSL and cable modems)
    2. Imaging Devices
    3. Storage Devices



 
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Article ID: SLN37314

Last Date Modified: 08/23/2010 12:00 AM


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