USB Type‑A Connector
Definition
USB Standard‑A, commonly called USB Type‑A, is the rectangular connector historically used on the host side: computers, hubs, chargers, and host equipment. It is not reversible. USB 1.x/2.0 uses 4 contacts; the SuperSpeed version adds 5 contacts, for 9 contacts total, while remaining mechanically compatible with USB 2.0 plugs.
Revisions and capabilities
| Specification | Common marketing name | Maximum raw rate | Type‑A |
|---|---|---|---|
| USB 1.0 / 1.1 | Low‑Speed / Full‑Speed | 1.5 / 12 Mbit/s | 4 contacts |
| USB 2.0 | Hi‑Speed | 480 Mbit/s | 4 contacts |
| USB 3.0 / USB 3.1 Gen 1 | SuperSpeed USB 5Gbps | 5 Gbit/s | 9 contacts |
| USB 3.1 Gen 2 / USB 3.2 Gen 2x1 | SuperSpeed USB 10Gbps | 10 Gbit/s | 9 contacts |
USB 1.x / USB 2.0 pinout — 4 contacts
Pin numbers are electrically identical on the plug and receptacle. Their visual positions are mirrored when both parts are drawn from the front, ready to mate.
| Pin | Name | Common color | Function |
|---|---|---|---|
| 1 | VBUS | Red | Nominal +5 V supply |
| 2 | D− | White | Negative differential data |
| 3 | D+ | Green | Positive differential data |
| 4 | GND | Black | Ground and power return |
| Shell | Shield | Braid / drain | Cable shield and chassis |
SuperSpeed USB 3.x pinout — 9 contacts
The four USB 2.0 contacts remain present. Five additional contacts carry the SuperSpeed pairs and their reference ground.
| Pin | Name | Short function |
|---|---|---|
| 1 | VBUS | Nominal +5 V supply |
| 2 | D− | Negative USB 2.0 data |
| 3 | D+ | Positive USB 2.0 data |
| 4 | GND | Power return |
| 5 | SSRX− | Negative SuperSpeed receive |
| 6 | SSRX+ | Positive SuperSpeed receive |
| 7 | GND_DRAIN | SuperSpeed reference / drain |
| 8 | SSTX− | Negative SuperSpeed transmit |
| 9 | SSTX+ | Positive SuperSpeed transmit |
| Shell | Shield | Shield and chassis |
RX/TX names are given from the host port's perspective. Within a cable, transmit and receive pairs cross over toward the opposite device.
Voltage and current
| Power mode | Nominal voltage | Maximum advertised current |
|---|---|---|
| USB 2.0, before configuration | 5 V | 100 mA per unit load |
| USB 2.0, configured device | 5 V | 500 mA |
| USB 3.x, before configuration | 5 V | 150 mA per unit load |
| USB 3.x, configured device | 5 V | 900 mA |
| USB Battery Charging 1.2 | 5 V | up to 1.5 A on a compatible port |
Proprietary Type‑A chargers may advertise more current, but such modes must not be assumed without explicit identification or documentation. For an interoperable design, observe the limits of the USB or Battery Charging mode that was actually detected.
Cable selection and recommended length
Data conductors
- USB 1.1 / 2.0: use a shielded 90 Ω differential twisted pair, commonly 28 AWG, with a practical Hi‑Speed maximum of 5 m.
- USB 3.x 5 Gbit/s: use a controlled 90 Ω SuperSpeed cable with shielded pairs; the typical practical maximum is about 3 m.
- USB 3.x 10 Gbit/s: prefer a short certified cable, typically 1 m and up to roughly 2 m depending on construction. AWG alone does not guarantee signal integrity.
Power conductors
The following is an engineering recommendation, calculated for copper at 20 °C and a maximum voltage drop of approximately 5% (0.25 V) over the complete VBUS + GND loop. Final length must also remain within the data limit.
| Cable power wire | Typical resistance per conductor | 500 mA | 900 mA | 1.5 A |
|---|---|---|---|---|
| 28 AWG | 0.213 Ω/m | 1.2 m | 0.65 m | 0.39 m |
| 26 AWG | 0.134 Ω/m | 1.9 m | 1.0 m | 0.62 m |
| 24 AWG | 0.084 Ω/m | 3.0 m | 1.65 m | 0.99 m |
| 22 AWG | 0.053 Ω/m | 4.7 m | 2.6 m | 1.57 m |
| 20 AWG | 0.033 Ω/m | 7.5 m | 4.2 m | 2.50 m |
Practical advice: for a 3–5 m USB 2.0 cable that actually powers a device, use 22–24 AWG power conductors and a 28 AWG data pair. For 900 mA or 1.5 A, shorten the cable or use 20–22 AWG. Verify voltage at the device under load.