USB signals are transmitted using differential signaling on twisted-pair data wires with 90 Ω ± 15% characteristic impedance. [93] USB 2.0 and earlier specifications define a single pair in half-duplex (HDx). USB 3.0 and later specifications define one dedicated pair for USB 2.0 compatibility and two or four pairs for data transfer: two pairs in full-duplex (FDx) for single lane variants require SuperSpeed connectors; four pairs in full-duplex for two lane (×2) variants require USB-C connectors. USB4 Gen 4 requires the use of all four pairs but allow for asymmetrical pairs configuration, [94] in this case one lane is used for the upstream data and the other three for the downstream data or vice-versa. USB4 Gen 4 use pulse amplitude modulation on 3 levels, providing a trit of information every baud transmitted, the transmission frequency of 12.8GHz translate to a transmission rate of 25.6 GBd [95] and the 11-bit–to–7-trit translation provides a theoretical maximum transmission speed just over 40.2 Gbit/s. [96] Operation mode Battery Charging Specification 1.2: [28] with increased current of 1.5A on charging ports for unconfigured devices, allowing High Speed communication while having a current up to 1.5A Revision 1.2 of the specification was released in 2010. It made several changes, and increased limits including allowing 1.5A on charging downstream ports for unconfigured devices—allowing High Speed communication while having a current up to 1.5A. Also, support was removed for charging port detection via resistive mechanisms. [52]

IEC 62680-1.1:2015 - Part 1-1: Common components - USB Battery Charging Specification, Revision 1.2

Please note:

Brant, Tom (4 March 2019). "Thunderbolt 3 Merges With USB to Become USB4". PC Magazine. Archived from the original on 5 March 2019 . Retrieved 4 March 2019. Parameter Values". Battery Charging Specification, Revision 1.2. USB Implementers Forum. 7 December 2010. p.45. Archived from the original on 28 March 2016 . Retrieved 29 March 2016. Joel Johnson (29 May 2019). "The unlikely origins of USB, the port that changed everything". Fast Company. Shuler, Kurt (31 March 2011). "Interchip Connectivity: HSIC, UniPro, HSI, C2C, LLI... oh my!". Arteris IP. Archived from the original on 19 June 2011 . Retrieved 24 June 2011.

Silvia (5 August 2015). "USB 3.1 Gen 1 & Gen 2 explained". www.msi.org. Archived from the original on 8 July 2018 . Retrieved 5 April 2018.USB cables exist with various combinations of plugs on each end of the cable, as displayed below in the USB cables matrix. Shaikh, Roshan Ashraf (3 November 2020). "USB-IF releases final specification of Media Agnostic USB". tweaktown.com. Archived from the original on 15 March 2021 . Retrieved 21 July 2021. The different A and B plugs prevent accidentally connecting two power sources. However, some of this directed topology is lost with the advent of multi-purpose USB connections (such as USB On-The-Go in smartphones, and USB-powered Wi-Fi routers), which require A-to-A, B-to-B, and sometimes Y/splitter cables. See the USB On-The-Go connectors section below for a more detailed summary description.

The intent is to permit uniformly charging laptops, tablets, USB-powered disks and similarly higher-power consumer electronics, as a natural extension of existing European and Chinese mobile telephone charging standards. This may also affect the way electric power used for small devices is transmitted and used in both residential and public buildings. [61] [62] The standard is designed to coexist with the previous USB Battery Charging specification. [63] A USB system consists of a host with one or more downstream facing ports (DFP), [52] and multiple peripherals, forming a tiered- star topology. Additional USB hubs may be included, allowing up to five tiers. A USB host may have multiple controllers, each with one or more ports. Up to 127 devices may be connected to a single host controller. [53] [30] :8–29 USB devices are linked in series through hubs. The hub built into the host controller is called the root hub. The first Power Delivery specification defined six fixed power profiles for the power sources. PD-aware devices implement a flexible power management scheme by interfacing with the power source through a bidirectional data channel and requesting a certain level of electrical power, variable up to 5A and 20V depending on supported profile. The power configuration protocol can use BMC coding over the CC wire if one is present, or a 24MHz BFSK-coded transmission channel on the V BUS line.

Hyde, John (February 2001). USB Design by Example: A Practical Guide to Building I/O Devices (2nded.). Intel Press. ISBN 978-0-970-28465-5.

Use class information in the interface descriptors. This base class is defined to use in device descriptors to indicate that class information should be determined from the Interface Descriptors in the device.

September 2022 naming scheme [ edit ] An overview of USB naming scheme that was put in place in September 2022.