conveylive.com

Universal Serial Bus

Universal Serial Bus

Tracy Pettigrue
Tracy Pettigrue
Sep 28, 2009
0 Comments | 1797 Views | 0 Hits


Universal Serial Bus (USB) is a serial bus standard to interface devices. A major component in the legacy-free PC, USB was designed to allow peripherals to be connected using a single standardized interface socket, to improve plug-and-play capabilities by allowing devices to be connected and disconnected without rebooting the computer (hot swapping). Other convenient features include powering low-consumption devices without the need for an external power supply and allowing some devices to be used without requiring individual device drivers to be installed.

 
Overview:
 
A USB system has an asymmetric design, consisting of a host multitude of downstream USB ports, and multiple peripheral devices connected in a tiered-star topology. Additional USB hubs may be included in the tiers, allowing branching into a tree structure, subject to a limit of 5 levels of tiers per controller. Up to 127 devices, including the hub devices, may be connected to a single host controller. Modern computers often have several host controllers, allowing a very large number of USB devices to be connected.
 
So-called "sharing hubs" also exist; allowing multiple computers to access the same peripheral device(s), either switching access between PCs automatically or manually.
USB supports three data rates:
  • A Low Speed (1.0) rate of 1.5 Mbit/s (192 KB/s) that is mostly used for Human Interface Devices (HID) such as keyboards, mice, and joysticks.
  • A Full Speed (1.1) rate of 12 Mbit/s (1.5 MB/s). Full Speed was the fastest rate before the USB 2.0 specification and many devices fall back to Full Speed. Full Speed devices divide the USB bandwidth between them in a first-come first-served basis and it is not uncommon to run out of bandwidth with several isochronous devices. All USB Hubs support Full Speed.
  • A Hi-Speed (2.0) rate of 480 Mbit/s (60 MB/s).
Cables
The maximum length of a standard USB cable is 5 meters (slightly more than 15 feet). The primary reason for this limit is the maximum allowed round-trip delay of about 1500 ns. If a USB device does not answer to host commands within the allowed time, the host considers the command to be lost. When USB device response time, delays from using the maximum number of hubs and delays from cables connecting the hubs, host and device are summed, the maximum delay caused by a single cable turns out to be 26 ns [7]. The USB 2.0 specification states that the cable delay must be less than 5.2 ns per meter, which means that maximum length USB cable is at least 5 meters long.
The USB specification provides a 5 V (volts) supply on a single wire from which connected USB devices may draw power. The specification provides for no more than 5.25 V and no less than 4.75 V (5 V±5%) between the positive and negative bus power lines.[8] Initially, a device is only allowed to draw 100 mA. It may request more current from the upstream device in units of 100 mA up to a maximum of 500 mA.
 
Author's note: USB
Keywords: USB, Universal, Serial Bus



Please Signup to comment on this article