For a long time SCSI was the only universal interface for the PC, this expensive option was made only for high capacity devices.
Serial and parallel ports developed originally for printers and modems are not useful. Their data transfer rate is low (maximum 115Kbit / sec serial port, up to 400KB / sec for the parallel), and each device requires its own hardware interrupt (the IRQ), which limits the number of possible extensions.
The necessity of average speeds, an inexpensive interface that can be used for connecting virtually unlimited amount of devices eventually gave us Universal Serial Bus - USB. So today I will not lay out tough scientific material and will just tell you about USB technology that we use on a daily basis.
USB is used to connect Flash drives, hard disks, sound cards, speakers, microphones, the MIDI-keyboard, Wi-Fi adapters, modems, keyboards, mice, joysticks, webcams, printers, card readers, optical drives, and other equipment to PC.
USB 1.0 standard was approved in early 1996. It provided the exchange of data at two speeds: low - to 1.5 Mbit / s and high - up to 12 Mbit / c.
USB 2.0 was approved in April 2000 and it is the most common till now. The theoretical maximum data transfer rate increased to 480 Mbit / s, in real life, it rarely exceeds 280 Mbit / s. The voltage that goes across the board is 5 V with amperage of 150 or 500 mA.
USB 3.0 standard was approved in 2008, it increased data rate up to 5 Gbit / s, and the amperage - up to 900 mA. The USB 3.0 cable uses nine wires: two for power and seven for the data transmission.
USB uses a multilevel star topology, which looks like a tree. Where the movement is divided into two or more branches of the hub. At the end of each branch, there is a peripheral function. Word function in this context is a specific USB term.
Each physical device includes USB interface bus, logical devices, and one or more functions. The bus interface is a standard for all USB devices.
Although the physical USB card may look like a tree, logically it still looks like a star with the ability to connect up to 127 devices connected to the same hub. The client software connects directly with its device. Each device has a unique address that is assigned to it at the level of the system software of USB during setup to avoid conflicts.
At any time, the host controller has a list of operations that are waiting to be used. A transaction begins when the controller sends the packet describing the type and direction, the 7-bit USB-device address and endpoint number. This package is called a marker of packets.
Major functions of error checking and error handling have been integrated into the USB to ensure that it is a reliable way to connect peripherals to a PC.
Data transfer between USB-host and the USB-peripheral device is provided with the help of the USB-controller. The data is transmitted in short packets "question-answer". Packets go simultaneously in both directions, wherein the controller does not wait for confirmation of delivery, which enables connection with high speed.
Universal Serial Bus provides a universal, flexible way to connect a wide range of low-speed and medium-speed peripherals to a PC with a relatively low cost. Its installation, connection, and configuration mean that the installation and peripheral support is much easier if compared with devices using serial, parallel or proprietary interfaces.
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With Love,
Kate
