USB hardware
The initial versions of the USB standard specified connectors that were easy to use and that would have high 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, 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 provided has increased over time. The modern specifications are called USB Power Delivery 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 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 is specified for 60 watts and at least of USB 2.0 data capability.
In 2019, USB4, now exclusively based on USB-C, added connection-oriented video and audio interfacing abilities and compatibility to Thunderbolt 3+.
Connectors
Unlike other data buses, USB connections are directed; a host device has downstream-facing ports that connect to the upstream-facing port of hubs or peripheral devices. USB implements a tiered star-like network topology.Only downstream-facing ports originally provided power by default; this topology was chosen to easily prevent electrical overloads and damaged equipment.
Every legacy USB cable has two distinct ends with mechanically distinct plugs, one Type-A plug and one Type-B plug. Each format has a plug and receptacle defined for each of the A and B ends. A USB cable, must have one Type-A plug and one Type-B plug, the exception to this is Micro-A plug to Standard-A receptacle cables which are allowed, USB extension cables with one Type-A plug and one Type-A receptacle do exist but these are not compliant with the USB standards.
With the release 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, leaving it to the connected devices to negotiate their respective roles. All legacy receptacles are either Type-A or Type-B except the Micro‑AB and Mini‑AB receptacles. Such an Type-AB receptacle accepts both Type-A and Type-B plugs, and a device with such a receptacle takes the DFP or UFP role according to the type of plug attached.
There are three sizes of legacy USB connectors: The original Standard, the Mini connectors, which were the first attempt to accommodate handheld mobile equipment, and Micro, all of which were superseded in 2014 by Type‑C, which is required for operation modes with two lanes, USB 3.2 2×2 and allows power up to 240 watts in either direction.
Before USB4, there are five speeds for USB data transfer: Low-Speed, Full-Speed, High-Speed, SuperSpeed, and SuperSpeed+.
Legacy connectors have differing hardware and cabling requirements for the first three generations of the standard. USB devices have some choice of implemented modes, and since USB 3.1 the USB release 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. In the USB 3 specifications it is recommended that the insulators visible inside Standard‑A SuperSpeed plugs and receptacles be a specific blue color. In Standard‑A receptacles with support for the 10 Gbit/s 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. The USB specification documents also periodically define the term male to represent the plug, and female to represent the receptacle.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, 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 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 and two wires for one differential signal of serial data. Mini and Micro connectors 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 lane, providing full-duplex data transfers at SuperSpeed, making it similar to Serial ATA or single-lane PCI Express.
| 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 or Type-B end of its cable, causing the device to behave as a host or peripheral accordingly.
| |
| 5 | GND | Black | Signal ground |