The initial versions of the USB standard specified connectors that were easy to use and would have long life spans; revisions of the standard added smaller connectors useful for compact portable devices. Higher-speed development of the USB standard gave rise to another family of connectors to permit additional data links. All versions of USB specify cable properties. Version 3.x cables, marketed as SuperSpeed, added a data link; namely, in 2008, USB 3.0 added a full-duplex lane (two twisted pairs of wires for one differential signal of serial data per direction), and in 2014, the USB-C specification added a second full-duplex lane.
USB has always included some capability of providing power to peripheral devices, but the amount of power that can be given has increased over time. The modern specifications are called USB Power Delivery (USB-PD) and allow up to 240 watts. Initially USB 1.0/2.0 provided up to 2.5 W, USB 3.0 provided up to 4.5 W, and subsequent Battery Charging (BC) specifications provided power up to 7.5 W. The modern Power Delivery specifications began with USB PD 1.0 in 2012, providing for power delivery up to 60 watts; PD 2.0 version 1.2 in 2013, along with USB 3.1, up to 100 W; and USB PD 3.1 in 2021 raised the maximum to 240 W. USB has been selected as the charging format for many mobile phones and other peripheral devices and hubs, reducing the proliferation of proprietary chargers. Since USB 3.1 USB-PD is part of the USB standard. The latest PD versions can also provide power to lower power laptops.
A standard USB-C cable can carry at least 60 watts and at least of USB 2.0 (480Mbit/s half-duplex).
In 2019, USB4, which uses only the USB-C connector, added connection-oriented video and audio abilities (DisplayPort) and compatibility with Thunderbolt 3 and later.
Connectors
USB implements a tiered star-like network topology. Unlike other data buses (such as Ethernet), USB connections are directional. Ports are classified as either downstream-facing ports (DFP), or upstream-facing ports (UFP), though there are also those that can be dual-role. Ports on a host, and those on a hub into which devices are connected, are DFP, while ports on devices, and a port on a hub used to connect it to a host, are UFP. Only downstream-facing ports originally provided power by default. This topology was chosen to easily prevent electrical overload and damage to equipment.
Prior to the introduction of Type-C, USB receptacles and their plugs had physically distinct shapes for different roles (DFP, UFP, and dual-role), preventing the possibility of two hosts erroneously being connected together. DFP receptacles and their corresponding plugs were known as Type-A; UFP receptacles and their corresponding plugs were known as Type-B; while those that were dual role were known as Type-AB. Consequently, a standards-conforming USB cable with such legacy plugs on both ends would have had a Type-A plug on one end, and a Type-B plug on the other. USB extension cables with a Type-A plug on one end and a Type-A port on the other existed but these were not compliant with USB standards. A Type-AB receptacle accepted both Type-A and Type-B plugs, and a device with such a receptacle took either a DFP or UFP role according to the type of plug attached.
With the introduction of Type‑C came transitional cables: a Type‑C plug at one end and a Type-A or a Type-B plug at the other. These transitional cables are still directional, and in such a cable the Type‑C plug is electrically marked as either A or B as appropriate to complement the opposite connector. The modern standard is a cable with a Type-C plug on each end; these cables are non-directional, and connected devices instead negotiate their respective roles.
Legacy plugs and receptacles (types A, B and AB) came in three different sizes: The original Standard size; Mini, which was the first attempt to accommodate handheld mobile equipment (now mostly deprecated); and Micro. All of these were superseded in 2014 by Type‑C, which is required for operational modes with two lanes (USB 3.2 1×2 (10 Gbit/s), USB 3.2 2×2 (20 Gbit/s), or any USB4 modes) and allows power up to 240 watts in either direction.
Before USB4, there were five speeds for USB data transfer: Low-Speed; Full-Speed (both USB 1.0 and 1.1); High-Speed (USB 2.0); SuperSpeed (USB 3.0, later designated as USB 3.2 Gen 1×1); and SuperSpeed+ (designated as USB 3.1 Gen 2, later as USB 3.2 Gen 2×1).
Legacy connectors have differing hardware and cabling requirements for the first three generations of the standard (USB 1.x, USB 2.0, and USB 3.x). USB devices have some choice of implemented modes, and since USB 3.1 the USB revision alone does not sufficiently designate implemented modes. Which capabilities a device supports are defined by the device's chipset or included SoC and the OS's supported drivers (therefore one must check the full names of the supported USB operation modes in the device's specification; the printed icons usually do not specify all modes, or precisely enough). In the USB 3 specifications it is recommended that the insulators visible inside Standard‑A SuperSpeed plugs and receptacles be a specific blue color (Pantone 300 C). In Standard‑A receptacles with support for the 10 Gbit/s (Gen 2) signaling rate introduced in USB 3.1, some makers instead use a teal blue color, but the standards recommend the same blue for all SuperSpeed-capable Standard‑A receptacles, including those capable of the higher rate.
Properties
The connectors the USB committee specifies support a number of USB's underlying goals, and reflect lessons learned from the many connectors the computer industry has used. The connector mounted on the host or device is called the receptacle, and the connector attached to the cable is called the plug.
By design, it is difficult to insert a USB plug into its receptacle incorrectly. The USB specification requires that the cable plug and receptacle be marked so the user can recognize the proper orientation. The USB‑C plug, however, is reversible. USB cables and small USB devices are held in place by the gripping force from the receptacle, with no screws, clips, or thumb-turns as other connectors use.
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.
There are so-called cables with A plugs on both ends, which may be valid if the "cable" includes, for example, a USB host-to-host transfer device with two ports. This is, by definition, a device with two logical B ports, each with a captive cable, not a cable with two A ends.
Durability The standard connectors were designed to be more robust than many past connectors. This is because USB is hot-swappable, and the connectors would be used more frequently, and perhaps with less care, than previous connectors.
Standard USB connectors have a minimum rated lifetime of 1,500 cycles of insertion and removal, and this increased to 5,000 cycles for Mini-B connectors. The rating for all Micro connectors is 10,000 cycles, and the same applies to USB-C. To accomplish this, a locking device was added and a leaf spring was moved from the jack to the plug, so that the most-stressed part is on the cable side of the connection. This change was made so that the connector on the less expensive, and more replaceable, cable would bear the most wear.
In standard USB, the electrical contacts in a USB connector are protected by an adjacent plastic tongue, and the entire connecting assembly is usually protected by an enclosing metal shell.
The shell on the plug makes contact with the receptacle before any of the internal pins. The shell is typically grounded, to dissipate static electricity and to shield the wires within the connector.
Compatibility The USB standards specify dimensions and tolerances for connectors, to prevent physical incompatibilities, including maximum dimensions of plug bodies and minimum clear spaces around receptacles so that adjacent ports are not blocked.
Pin assignments
USB 1.0, 1.1, and 2.0 use two wires for power (VBUS and GND) and two wires for one differential signal carrying serial data. Mini and Micro connectors have five contacts each, rather than the four of Standard connectors, with the additional contact, designated ID, electrically differentiating A and B plugs when connecting to the AB receptacles of On-The-Go devices. The Type‑C plug of a Type‑C-to-legacy cable or adapter is similarly electronically marked as A or B: In a cable, it is marked as the complement of the connector on the opposite end because every legacy cable by definition has an A and a B end, and in an adapter the Type‑C plug is marked to match the plug the adapter accepts.
USB 3.0 added a (bi-directional) lane (two additional differential pairs with a total of four wires, SSTx+, SSTx−, SSRx+ and SSRx−), providing full-duplex data transfers at SuperSpeed, making it similar to Serial ATA or single-lane PCI Express.
+ Standard USB pin assignments Pin Name Wire color Description 1 VBUS Red or Orange +5 V 2 D− White or Gold Data− 3 D+ Green Data+ 4 GND Black or Blue Ground
+ Mini- and Micro-USB pin assignments Pin Name Wire color Description 1 VBUS Red +5 V 2 D− White Data− 3 D+ Green Data+ 4 ID When a cable is connected to a Mini- or Micro-AB receptacle, the ID pin indicates to the On-The-Go device whether the plug is the Type-A (host) or Type-B (peripheral device) end of its cable, causing the device to behave as a host or peripheral accordingly. Type-A plug (host end): connected to GND Type-B plug (peripheral device end): not connected 5 GND Black Signal ground
Colors
+ Usual USB color-coding Color Location Description White(required by USB standards) Receptacles and plugs USB 1.0/1.1: Low-Speed (1.5 Mbit/s) – Full-Speed (12 Mbit/s)Micro‑A, Mini‑A Black(required by USB standards) Receptacles and plugs USB 2.0: Hi-Speed USB (480 Mbit/s)Micro‑B, Mini‑B Gray(required by USB standards) Receptacles Micro‑AB, Mini‑AB Blue (Pantone )(recommended in USB standards) Receptacles and plugs Indicates a Standard‑A connector that supports USB 5Gbps (introduced as SuperSpeed in USB 3.0), and possibly USB 10Gbps (introduced in USB 3.1) Teal blue(not part of USB standards) Receptacles and plugs Indicates a Standard‑A or Standard‑B connector that supports USB 10Gbps (introduced as SuperSpeed+ in USB 3.1) Green(not part of USB standards) Receptacles and plugs Type‑A or Type‑B, Qualcomm Quick Charge (QC) Purple(not part of USB standards) Plugs only Type‑A or Type‑C, Huawei SuperCharge Yellow or red(not part of USB standards) Receptacles onlyRed: Usually Always On Yellow: Always On, Passive Power DeliveryHigh-current or sleep-and-charge Orange(not part of USB standards) Receptacles only Always On, Passive Power DeliveryHigh-retention connector, mostly used on industrial hardware
USB ports and connectors are often color-coded to distinguish their different capabilities and modes. Color coding is only required for the insulators visible inside Micro and Mini connectors: A connectors are white, B black, and AB receptacles, which accept both A and B plugs, gray. Pantone 300 C is recommended for USB 3.0 Standard‑A connectors, including those with signalling rate of 5 Gbit/s and 10 Gbit/s, though some manufacturers instead use nonstandard teal for receptacles with signalling rate of 10 Gbit/s.
Types