# USB 3 Standard-A receptacle > a legacy USB receptacle that preceded USB-C (“A” indicating the host end) **Wikidata**: [Q133459081](https://www.wikidata.org/wiki/Q133459081) **Source**: https://4ort.xyz/entity/usb-3-standard-a-receptacle ## Summary The USB 3 Standard-A receptacle is a legacy USB interface located at the host end of a connection, designed to accept USB 3 Standard-A plugs. Officially referred to as the USB 3.1 Standard-A receptacle, it preceded the USB-C standard and is distinguished by its ability to support SuperSpeed data transfer while maintaining backward compatibility with USB 2.0 plugs. ## Key Facts * **Official Name:** USB 3.1 Standard-A receptacle. * **Classification:** Instance of a USB 3 Standard-A connector and USB Standard-A receptacle. * **Predecessor Status:** It is a legacy interface that preceded the USB-C receptacle. * **Function:** Serves as the host end ("A" designation) for connecting peripherals. * **Pin Configuration:** Features a 9-pin layout (expanding on the 4-pin USB 2.0 layout) to support SuperSpeed data transmission. * **Backward Compatibility:** Physically connects with USB 3 Standard-A plugs and USB 2 Standard-A plugs. * **Differentiation:** Distinct from the USB 3 Standard-A plug (the cable end), the USB 3 Standard-B receptacle, and the USB 2 Standard-A receptacle. * **Pinout Details:** * **Pin 1:** VBUS (Red, +5V Power) * **Pin 2:** D− (White, Data) * **Pin 3:** D+ (Green, Data) * **Pin 4:** GND (Black, Ground) * **Pin 5:** SSRX− (SuperSpeed Receive) * **Pin 6:** SSRX+ (SuperSpeed Receive) * **Pin 7:** GND_DRAIN (Ground Drain) * **Pin 8:** SSTX− (SuperSpeed Transmit) * **Pin 9:** SSTX+ (SuperSpeed Transmit) ## FAQs ### Q: Is the USB 3 Standard-A receptacle compatible with older USB cables? A: Yes, the receptacle is designed to connect with USB 2 Standard-A plugs, allowing users to insert legacy cables into the newer host port. ### Q: What is the difference between this receptacle and a USB-C port? A: The USB 3 Standard-A receptacle is a legacy host interface that is physically distinct from the newer, reversible USB-C receptacle, which has effectively replaced it in modern device designs. ### Q: What are the extra pins used for in this receptacle? A: Beyond the standard VBUS, D-, D+, and GND pins, the receptacle adds five pins (two receive, two transmit, and a drain) to facilitate SuperSpeed data transfer rates. ## Why It Matters The USB 3 Standard-A receptacle represents a critical evolution in the Universal Serial Bus standard, bridging the gap between the original USB 2.0 era and the modern USB-C era. By maintaining the physical form factor of the Standard-A host while introducing high-speed data lanes (SuperSpeed), it allowed for a gradual transition for manufacturers and consumers. Users could upgrade to faster transfer speeds without immediately abandoning their existing inventory of cables and devices. Its design exemplifies the USB standard's core philosophy of backward compatibility. The retention of the "A" shape ensured that the billions of existing USB peripherals could still interface with new computers, while the additional pins provided the bandwidth necessary for the storage and video demands of the 2010s. Although it is now considered a "legacy" connector replaced by the USB-C receptacle, the USB 3 Standard-A receptacle remains a ubiquitous fixture on desktop computers, servers, and industrial hardware, serving as the workhorse of high-speed connectivity for over a decade. ## Notable For * **Legacy Host Standard:** Being the final major iteration of the rectangular "Standard-A" host port before the industry transition to USB-C. * **Backward Compatibility:** Featuring a design that physically accepts USB 2.0 Standard-A plugs, a key advantage over some competing standards. * **Expanded Pinout:** Introducing 5 additional contacts (total of 9) to handle dual simplex signaling for SuperSpeed data, compared to the 4 pins of its predecessor. * **Ubiquity:** Serving as the standard high-speed host port on motherboards and laptops throughout the USB 3.0 and 3.1 generations. ## Body ### Definition and Classification The USB 3 Standard-A receptacle is a female connector class found on host devices such as computers and powered hubs. It is technically classified as an instance of the **USB 3 Standard-A connector** and the **USB Standard-A receptacle**. In official USB-IF documentation, it is named the **USB 3.1 Standard-A receptacle**. It is fundamentally defined as the host-side interface ("A") that preceded the **USB-C receptacle**. ### Physical and Technical Layout The receptacle is engineered to mate with a **USB 3 Standard-A plug**. While it maintains the external dimensions of previous Standard-A ports to ensure backward compatibility, the internal geometry is extended to accommodate additional contacts. The pinout is structured to support both USB 2.0 and USB 3.x functionality: * **Power and Ground:** Pin 1 (VBUS, Red) supplies power at 5 volts, while Pin 4 (GND, Black) serves as the ground. * **USB 2.0 Data:** Pins 2 (D-, White) and 3 (D+, Green) retain the differential pair signaling used for legacy data transfer. * **SuperSpeed Data:** Pins 5 through 9 are dedicated to high-speed transfers. * **Receive:** Pins 5 (SSRX-) and 6 (SSRX+) handle incoming SuperSpeed data. * **Drain:** Pin 7 (GND_DRAIN) manages signal shielding. * **Transmit:** Pins 8 (SSTX-) and 9 (SSTX+) handle outgoing SuperSpeed data. ### Relationships and Succession The USB 3 Standard-A receptacle connects physically with the **USB 3 Standard-A plug** and the **USB 2 Standard-A plug**. It is explicitly differentiated from the **USB 3 Standard-A plug** (its male counterpart), the **USB 3 Standard-B receptacle** (typically found on devices), and the **USB 2 Standard-A receptacle** (which lacks the SuperSpeed pins). As the standard progressed, this interface was eventually **replaced by** the **USB-C receptacle**, which supports the USB 3.2 and USB4 protocols among others. ## References 1. [Source](https://www.usb.org/document-library/usb-31-legacy-cable-and-connector-revision-10) 2. [Source](https://www.usb.org/document-library/universal-serial-bus-31-legacy-connectors-and-cable-assemblies-compliance-document) 3. [Source](https://www.usb.org/document-library/usb-32-revision-11-june-2022) 4. [Source](https://www.usb.org/document-library/usb-type-cr-connectors-and-cable-assemblies-compliance-document-rev-21b) 5. [Source](https://www.usb.org/document-library/usb-type-cr-cable-and-connector-specification-release-24) 6. [Source](https://www.usb.org/document-library/usb-20-specification)