Universal Serial Bus (USB)

Universal Serial Bus (USB) was designed to replace the myriad of connectors found on Personal Computers (PCs) in the 1990s. The primary motivation was to allow peripherals to be automatically configured when they are plugged into a PC (often referred to as plug-and-play). Shortly after being introduced, USB ports quickly replaced the legacy connectors on most personal computers. Today USB has grown beyond PCs to become the common interface for many embedded industrial and consumer applications.

What Makes a USB System?

A USB system is a unidirectional communications hierarchy consisting of a single USB HOST and anywhere from 1 to 127 USB Devices. If more than one device is present, one or more USB Hubs must be used. When used, a Hub will be considered one of the 127 devices. All components of a USB system connect to each other using one of the USB specified cables and connectors.

Low, Full, and High-Speed USB signals are transmitted using differential signaling on a twisted-pair data cable ( D+ and D-). In addition to the twisted pair, USB cables also provide a 5 V reference from the Host (VBUS) and a ground reference.

Communications on High-Speed, Full-Speed, and Low-Speed USB systems are exclusively half-duplex with the host controlling the direction of data transmission. To achieve the bandwidth for SuperSpeed and SuperSpeed Plus, additional high-speed differential data lines are added to D+ and D-. The added lines provide for full-duplex communications to occur in SuperSpeed and SuperSpeed Plus systems.

USB Specifications

The USB Implementer's Forum is responsible for updating and approving the USB specifications. Specifications include the protocols, speeds, connectors, and power distribution used by USB systems.

USB Speeds

USB has five supported speeds, each with a slightly different protocol and frame format:
Speed Maximum
USB Specification Note
Low Speed 1.5 Mbit/s 1.0 Not typically used in contemporary designs
Full Speed 12 Mbit/s 1.0
High Speed 480 Mbit/s 2.0
SuperSpeed 5 Gbit/s 3.0 Specification is also referred to as USB 3.1 Gen 1
SuperSpeed+ 10 Gbit/s 3.1 USB 3.1 Gen 2

Power Distribution

From the onset, USB hosts have been able to supply a limited amount of power to an attached USB device, eliminating the need for some devices to have their own power supply. As USB began to be used in more industrial applications, the need for more robust power distribution became evident. Today, there are several specifications for USB power distribution from hosts and Hubs.
Mode of
USB 1.x/2.0 5 V 500 mA
USB 3.0/3.1 5 V 900 mA
USB Battery-Charging 1.2 5 V 1.5 A Legacy Charging
USB Type-C Current 5 V 3 A Uses new (CC) cable.
USB Power-Delivery 2 20 V 5 A Directional control and
power level management

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