USB (Universal Serial Bus)

USB (Universal Serial Bus)

Universal Serial Bus (USB) is a serial bus standard that connects computer systems to external devices, and is an input/output interface technology specification widely used in personal computers and mobile devices, as well as extending functionality to cameras, digital televisions (set-top boxes), game consoles, and other related fields.

When multimedia computers first emerged, the transmission interfaces of external devices varied significantly. For example, printers could only connect via LPT ports, modems could only connect via RS-232, and mice and keyboards could only connect via PS/2. The complexity of the interface system, along with the need to install drivers and restart the system for usage, caused inconvenience to users. Thus, the creation of a unified and hot-swappable external transmission interface became an inevitable trend.

USB was initially advocated by Intel and Microsoft, with its greatest features being hot-swappability and plug-and-play functionality. When a device is plugged in, the host enumerates the device and loads the required drivers, making it much more convenient than PCI and ISA buses.

USB is significantly faster than traditional computer standard buses such as parallel ports (e.g., EPP, LPT) and serial ports (e.g., RS-232). The original standard USB 1.1 has a maximum transmission bandwidth of 12 Mbps, while USB 2.0 offers a maximum bandwidth of 480 Mbps. The recently introduced USB 3.0 has increased this to 5 Gbps.

The design of USB is asymmetric, consisting of a host controller and several devices connected in a tree structure through hubs. A single controller can have up to 5 levels of hubs, allowing for a maximum connection of 128 devices. This is because the design uses a 7-bit addressing field, where two to the power of seven equals 128. It is commonly said that USB connects 127 devices, meaning one connection must be deducted for the USB port connected to the host. A computer can have multiple controllers simultaneously. Unlike standards like SPI and SCSI, USB hubs do not require terminators.

USB-compatible peripherals include mice, keyboards, game controllers, joysticks, scanners, digital cameras, printers, hard drives, and network components. For multimedia peripherals like digital cameras, USB has become the default interface; due to greatly simplifying connections to computers, USB has gradually replaced parallel ports as the mainstream connection method for printers. By 2004, over 100 million USB devices had been produced; by 2007, high-definition digital video peripherals were the only category of devices not utilizing USB due to their higher transmission speed requirements.

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