What Is Universal Serial Bus?

Universal Serial Bus (USB) is a serial communication interface standard that specifies the protocols, connectors, and electrical characteristics for connecting peripheral devices to host computers and other host controllers. First released in 1996 by a consortium led by Intel, Compaq, Microsoft, and IBM, USB was designed to replace the proliferating and incompatible ports of the mid-1990s personal computer, including serial COM ports, parallel printer ports, and proprietary input device connectors, with a single, hot-pluggable interface that required no manual configuration.

The USB Implementers Forum (USB-IF), a consortium of more than 700 member companies, maintains the specification and certifies compliant products. The standard covers the physical connector geometry, the electrical signaling, the protocol stack, and the power management rules that allow host controllers to enumerate devices automatically when they are attached.

Interface Architecture and Protocol

USB employs a tiered star topology in which a host controller manages all traffic on a bus segment. Devices do not communicate with each other directly; all data passes through the host, which polls the connected devices according to their declared transfer type. Four transfer modes are defined: control transfers for command and status exchanges, bulk transfers for large data blocks such as file transfers, interrupt transfers for low-latency periodic data from input devices, and isochronous transfers for streams such as audio or video where timing regularity matters more than guaranteed delivery.

The protocol uses a structured packet format with a start-of-frame signal generated by the host at 1 ms intervals. Each device receives a 7-bit address assigned by the host during enumeration, allowing up to 127 devices per host controller. The USB-IF specifications library provides the full normative text for each specification generation.

USB Versions and Performance

USB 1.1, released in 1998, supported low-speed (1.5 Mbit/s) and full-speed (12 Mbit/s) modes, adequate for keyboards, mice, and low-data-rate peripherals. USB 2.0, introduced in 2001, added high-speed mode at 480 Mbit/s, making it practical for external storage and video capture. USB 3.x, beginning with USB 3.0 in 2008, introduced SuperSpeed signaling at 5 Gbit/s, later extended to 10 and 20 Gbit/s variants, and adopted a backward-compatible connector family.

USB4, standardized by USB-IF in 2019 and updated with USB4 Version 2.0 in 2022, operates at up to 80 Gbit/s symmetrically and incorporates Thunderbolt 3 tunneling, enabling PCIe and DisplayPort traffic over a single USB-C cable. USB4 is available exclusively via the USB-C connector and remains backward compatible with USB 3.x and USB 2.0 devices.

USB Power Delivery

USB initially supplied 5 V at modest current levels, sufficient for low-power devices such as keyboards and mice. USB Power Delivery (USB PD), standardized alongside the USB-C connector, introduced negotiated power levels allowing the source and sink to agree on voltage and current via a power data object (PDO) exchange. The USB Power Delivery specification at USB-IF defines a negotiation protocol in which the source advertises its capabilities, the sink requests a specific voltage-current pair, and the source adjusts its output before signaling readiness.

USB PD Revision 3.1, released in 2021, raised the maximum supported power to 240 W over a full-featured USB-C cable, enabling USB-C to replace barrel-connector chargers across a wide range of laptops, monitors, and industrial equipment. This power delivery capability, combined with data transfer and video output over the same cable, makes USB-C the dominant interface in current portable electronics design.

Applications

Universal Serial Bus has applications in a wide range of systems and products, including:

• Computer peripherals including keyboards, pointing devices, cameras, and external storage
• Consumer electronics charging and synchronization for smartphones and tablets
• Audio and video interfaces replacing legacy analog connectors in professional and consumer equipment
• Embedded systems programming and debugging via USB-to-UART and JTAG bridges
• Industrial instrumentation and test equipment requiring host-connected data acquisition