Usb
USB, short for Universal Serial Bus, is an industry standard that defines connectors, cables, electrical signaling and communication protocols for linking peripherals and chargers to a host computer, replacing the earlier mixture of serial, parallel, PS/2 and game ports with a single, hot-pluggable interface. A USB connection carries data and, optionally, electrical power over the same cable, and the host computer controls it, polling devices and assigning them addresses. The standard is maintained by the USB Implementers Forum, and it has grown from a way to attach a keyboard to a worldwide standard for charging phones and laptops.
USB was created in the mid-1990s by a group of companies including Compaq, DEC, IBM, Intel, Microsoft, NEC and Nortel, with Intel's Ajay Bhatt a leading architect, to make connecting devices simple. Before it, adding a modem or printer meant choosing the right port, setting jumpers or interrupt numbers and often restarting. USB 1.0 was released in January 1996, offering a low speed of 1.5 megabits per second for devices like mice and a full speed of 12 megabits per second; USB 1.1 fixed early problems and made the standard practical, and the 1998 iMac, which dropped older ports in favor of USB, did much to popularize it. USB 2.0, released in 2000, raised the top speed to 480 megabits per second. USB 3.0 followed in 2008 at 5 gigabits per second, later renamed within the USB 3.1 and 3.2 generations, which reach 10 and 20 gigabits per second, and USB4, announced in 2019, reaches 40 gigabits per second and is based on the Thunderbolt 3 protocol contributed by Intel.
A USB connection works through a process called enumeration, in which the host discovers and configures a new device without user involvement. When a device is plugged in, the host detects it by a change in voltage on the data lines, resets it, assigns it a unique address and reads descriptors, small data structures in which the device states its identity, power needs and what it can do. Descriptors place the device in a class, such as human interface devices like keyboards and mice, mass storage like flash drives, audio or video, and the operating system loads a matching generic driver, which is why many devices work immediately. Data moves in transfers of several kinds: control transfers for setup, bulk transfers for large data such as files, interrupt transfers for small low-latency reports like keystrokes, and isochronous transfers for steady streams like audio. The host, not the device, initiates every transfer, which is why two computers cannot simply be joined with a cable unless one end behaves as a device.
USB-C is a connector shape, not a speed, and this is the most misunderstood fact about the standard. The USB-C connector, whose specification was finalized in 2014, is small, symmetrical and reversible, and replaces the older USB-A and USB-B shapes with one that works either way up. What passes through it depends on the port and cable: a USB-C port may support only USB 2.0 speeds, or 10 or 20 gigabits per second, or USB4, and may or may not carry video, charging and Thunderbolt, so two identical-looking ports and cables can behave very differently. The USB Power Delivery specification negotiates voltage and current over the cable, allowing a charger to supply up to 100 watts in its original form and up to 240 watts in the extended version published in 2021, enough for laptops. The confusing naming of versions, in which the same speed has been renamed more than once, adds to the difficulty, and looking at the logo on a port or cable is more reliable than judging by shape.
USB matters well beyond keyboards, because it is the common channel through which data and power reach machines of every kind. Microcontroller boards, external drives and many cameras use it, and engineers working on embedded systems depend on USB to flash firmware onto boards. Machine learning practitioners use USB cameras, accelerators and external GPU enclosures, and data recorders and sensors for science are very often USB devices. Security is a genuine concern, because a USB device is trusted to tell the truth about what it is: a malicious one can pose as a keyboard and type commands, and charging at an unknown public port exposes the data lines unless a cable or adapter blocks them. A device that charges but will not transfer data is often just a cable without data wires, and it is a frequent cause of apparent hardware failure.
Read USB as one agreed way for a host to discover, power and talk to almost any device: the connector is only the visible part, while enumeration, descriptors, device classes and negotiated power make plugging in simple, and the version and cable, not the shape of the plug, decide how fast and how much it can do.