UNIVERSAL SERIAL BUS is a serial bus standard to interface devices. UNIVERSAL SERIAL BUS was invented to allow peripheral to be connected using a single standardised interface socket, to improve a plug and play capabilities by allowing devices to be connected and disconnected without rebooting the computer. Other convenient features include powering low-consumption devices without the need for an external power supply and allowing some devices, keyboard, PDA, game pads and joysticks, scanner, printers. UNIVERSAL SERIAL BUS is also used extensively to connect peripheral such as mouse device, keyboard, PDAs, games. The design of UNIVERSAL SERIAL BUS standard by the USB implements form, an industry standard body incorporates lading companies from the computer and electronics industries. USB has one important advantage in that it is possible to install and remove devices without opening the computer case, making it useful for external drivers.
USB Supports Three Data Rates
- A low speed rate of 1.5Mbit/s(192KB/s) that is mostly used for human interface such as keyboard, mice, and joysticks.
- A full speed (1.1) rate of 12Mbits/s. full speed was the fastest rate before the USB 2.0 specification and many devices fall back to 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.
- A hi speed rate of 480 M/s (60MB/s).
- A super-speed (3.0) rate of 4.8Gbit/s. the USB 3.0 specification will be released by Intel and its partner in mid-2008.
- USB host: the computer act as host
- Multiple devices: up to 127 devices can connect to the host, either directly or by way or USB hubs.
- Individual USB cables can run as long as 5 metes
- The initial USB1.0 standard supported 12Mbps data transfer rate
- A USB cable has two wires for power and a twisted pair of wires to carry the data.
- USB controller in pc detects the presence or absence of the USB device and does allocation of electrical power. On the power wires, the computer can supply up to 500 milliamps of power at 5 volts.
- USB devices are hot swappable, meaning you can plug and play them into the bus and unplug them any time.
There are several types of USB connectors, and some have been added as the specification has progressed. The original USB specification detailed.
The USB standard uses “A” and “B” connectors. “A” connectors head “upstream” toward the computer.
“B” connectors head “downstream’ and connect to individual devices.
The USB process
When the host powers up, it queries all of the devices connected to the bus and assigns each one an address. This process is called enumeration devices are also enumerated when they connect to the bus. The host also finds out from each device what type of data transfer it wishes to perform.
A devices like a mouse or keyboard, which will be sending very little data, would choose the interrupt mode.
A device like printer, which receives data in one big packet, uses the bulk transfer mode.
A streaming device uses the isochronous mode
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