USB


Universal Serial Bus is an industry standard, developed by USB Implementers Forum, for digital data transmission and power delivery between many types of electronics. It specifies the architecture, in particular the physical interfaces, and communication protocols to and from hosts, such as personal computers, to and from peripheral devices, e.g. displays, keyboards, and mass storage devices, and to and from intermediate hubs, which multiply the number of a host's ports.
Introduced in 1996, USB was originally designed to standardize the connection of peripherals to computers, replacing various interfaces such as serial ports, parallel ports, game ports, and Apple Desktop Bus ports. Early versions of USB became commonplace on a wide range of devices, such as keyboards, mice, cameras, printers, scanners, flash drives, smartphones, game consoles, and power banks. USB has since evolved into a standard to replace virtually all common ports on computers, mobile devices, peripherals, power supplies, and manifold other small electronics.
In the latest standard, the USB-C connector replaces many types of connectors for power, displays, and many other uses, as well as all previous USB connectors.
USB consists of four generations of specifications: USB 1.x, USB 2.0, USB 3.x, and USB4. The USB4 specification enhances the data transfer and power delivery functionality with "a connection-oriented tunneling architecture designed to combine multiple protocols onto a single physical interface so that the total speed and performance of the USB4 Fabric can be dynamically shared." In particular, USB4 supports the tunneling of the Thunderbolt 3 protocols, namely PCI Express and DisplayPort. USB4 also adds host-to-host interfaces.
Each specification sub-version supports different signaling rates from 1.5 and 12 Mbit/s half-duplex in USB 1.0/1.1 to 80 Gbit/s full-duplex in USB4 2.0. USB also provides power to peripheral devices; the latest versions of the standard extend the power delivery limits for battery charging and devices requiring up to 240 watts as defined in USB Power Delivery Rev. V3.1. Over the years, USB has been adopted as the standard power supply and charging format for many mobile devices, such as mobile phones, reducing the need for proprietary chargers.

Overview

USB was designed to standardize the connection of peripherals to personal computers, both to exchange data and to supply electric power. It has largely replaced interfaces such as serial ports and parallel ports and has become commonplace on various devices. Peripherals connected via USB include computer keyboards and mice, video cameras, printers, portable media players, mobile digital telephones, disk drives, and network adapters.
USB connectors have been increasingly replacing other types of charging cables for portable devices.
USB connector interfaces are classified into three types: the many various legacy Type-A and Type-B connectors found on hosts, hubs, and peripheral devices, and the modern Type-C connector, which replaces the many legacy connectors as the only applicable connector for USB4.
The Type-A and Type-B connectors came in Standard, Mini, and Micro sizes. The standard format was the largest and was mainly used for desktop and larger peripheral equipment. The Mini-USB connectors were introduced for mobile devices. Still, they were quickly replaced by the thinner Micro-USB connectors. The Type-C connector, also known as USB-C, is not exclusive to USB, is the only current standard for USB, is required for USB4, and is required by other standards, including modern DisplayPort and Thunderbolt. It is reversible and can support various functionalities and protocols, including USB; some are mandatory, and many are optional, depending on the type of hardware: host, peripheral device, or hub.
USB specifications provide backward compatibility, usually resulting in decreased signaling rates, maximal power offered, and other capabilities. The USB 1.1 specification replaces USB 1.0. The USB 2.0 specification is backward-compatible with USB 1.0/1.1. The USB 3.2 specification replaces USB 3.1 while including the USB 2.0 specification. USB4 "functionally replaces" USB 3.2 while retaining the USB 2.0 bus operating in parallel.
The USB 3.0 specification defined a new architecture and protocol named SuperSpeed, which included a new lane for a new signal coding scheme providing full-duplex data transfers that physically required five additional wires and pins, while preserving the USB 2.0 architecture and protocols and therefore keeping the original four pins/wires for the USB 2.0 backward-compatibility resulting in 9 wires in total.
The USB 3.1 specification introduced an Enhanced SuperSpeed System – while preserving the SuperSpeed architecture and protocol – with an additional SuperSpeedPlus architecture and protocol adding a new coding schema ; for some time marketed as SuperSpeed+.
The USB 3.2 specification added a second lane to the Enhanced SuperSpeed System besides other enhancements so that the SuperSpeedPlus USB system part implements the Gen 1×2, Gen 2×1, and Gen 2×2 operation modes. However, the SuperSpeed USB part of the system still implements the one-lane Gen 1×1 operation mode. Therefore, two-lane operations, namely USB 3.2 Gen 1×2 and Gen 2×2 , are only possible with Full-Featured USB-C. As of 2023, they are somewhat rarely implemented; Intel, however, started to include them in its 11th-generation SoC processor models, but Apple never provided them. On the other hand, USB 3.2 Gen 1 and Gen 2 have been quite common for some years.

Connector type quick reference

Each USB connection is made using two connectors: a receptacle and a plug. Pictures show only receptacles:

Objectives

The Universal Serial Bus was developed to simplify and improve the interface between personal computers and peripheral devices, such as cell phones, computer accessories, and monitors, when compared with previously existing standard or ad hoc proprietary interfaces.
From the computer user's perspective, the USB interface improves ease of use in several ways:
  • The USB interface is self-configuring, eliminating the need for the user to adjust the device's settings for speed or data format, or configure interrupts, input/output addresses, or direct memory access channels.
  • USB connectors are standardized at the host, so any peripheral can use most available receptacles.
  • USB takes full advantage of the additional processing power that can be economically put into peripheral devices so that they can manage themselves. As such, USB devices often do not have user-adjustable interface settings.
  • The USB interface is hot-swappable.
  • Small devices can be powered directly from the USB interface, eliminating the need for additional power supply cables.
  • Because the use of the USB logo is only permitted after compliance testing, the user can have confidence that a USB device will work as expected without extensive interaction with settings and configuration.
  • The USB interface defines protocols for recovery from common errors, improving reliability over previous interfaces.
  • Installing a device that relies on the USB standard requires minimal operator action. When a user plugs a device into a port on a running computer, it either entirely automatically configures using existing device drivers, or the system prompts the user to locate a driver, which it then installs and configures automatically.
The USB standard also provides multiple benefits for hardware manufacturers and software developers, specifically in the relative ease of implementation:
  • The USB standard eliminates the requirement to develop proprietary interfaces to new peripherals.
  • The wide range of transfer speeds available from a USB interface suits devices ranging from keyboards and mice up to streaming video interfaces.
  • A USB interface can be designed to provide the best available latency for time-critical functions or can be set up to do background transfers of bulk data with little impact on system resources.
  • The USB interface is generalized with no signal lines dedicated to only one function of one device.

    Limitations

As with all standards, USB possesses multiple limitations to its design:
  • USB cables are limited in length, as the standard was intended for peripherals on the same tabletop, not between rooms or buildings. However, a USB port can be connected to a gateway that accesses distant devices.
  • USB data transfer rates are slower than those of other interconnects released in the same timeframe.
  • USB has a strict tree network topology and master/slave protocol for addressing peripheral devices; slave devices cannot interact with one another except via the host, and two hosts cannot communicate over their USB ports directly. Some extension to this limitation is possible through USB On-The-Go, Dual-Role-Devices and protocol bridge.
  • A host cannot broadcast signals to all peripherals at once; each must be addressed individually.
  • While converters exist between certain legacy interfaces and USB, they might not provide a full implementation of the legacy hardware. For example, a USB-to-parallel-port converter might work well with a printer, but not with a scanner that requires bidirectional use of the data pins.
For a product developer, using USB requires the implementation of a complex protocol and implies an "intelligent" controller in the peripheral device. Developers of USB devices intended for public sale generally must obtain a USB ID, which requires that they pay a fee to the USB Implementers Forum. Developers of products that use the USB specification must sign an agreement with the USB-IF. Use of the USB logos on the product requires annual fees and membership in the organization.