2.1.4 Universal Serial Bus Cables - Notes
USB (Universal Serial Bus) is the standard interface for connecting most peripheral devices to a computer. USB devices are grouped into classes such as human interface devices like keyboards and mice, mass storage devices like disk drives, printers, and audio devices.
USB is managed by a host controller, which supports multiple ports on the same bus. Each controller can theoretically support up to 127 devices, but to avoid bandwidth limitations, most PC motherboards include multiple USB controllers with roughly three to four ports each.
USB 2 Connector Types
| Type | Shape | Common Use |
|---|---|---|
| Type A | Flat rectangle; insert with USB symbol facing up | Host ports; keyboards, mice, flash drives |
| Type B | Square with beveled top | Large devices like printers |
| Type B Mini | Smaller trapezoidal shape | Early digital cameras; now rarely used |
| Type B Micro | Flat, elongated trapezoid | Smartphones and tablets |
USB cables can connect Type A to Type A, or convert between connector types such as Type A to Type B or Type A to Micro Type B.
USB 3 Connector Types
| Type | Notes |
|---|---|
| Type A | Backward compatible with USB 1.1 and 2.0; often identified by a blue tab. |
| Type B | Not compatible with USB 2 Type B ports. |
| Type B Micro | Dual-layer design; not compatible with USB 2 Micro ports. |
| Type C | Introduced in USB 3.1; compact, reversible, durable, and usable at both ends of a cable or with converter cables for Type A and B. |
Compatibility rule: USB 3 Type A ports accept USB 2 Type A cables, but USB 3 Type B cables do not fit USB 2 Type B ports.
Cable Length Limits
| Speed Class | Max Length |
|---|---|
| Low Speed | 3 meters |
| Full Speed / High Speed | 5 meters |
| SuperSpeed | 3 meters recommended (no official max) |
Performance may degrade beyond these lengths even if longer cables are available from vendors.
Power Delivery
USB carries both data and power to connected devices. Most Type A and Type C ports can charge device batteries.
| Port Type | Max Power |
|---|---|
| Basic USB port | Up to 4.5W (varies by version) |
| Power Delivery (PD) port | Up to 100W with compatible connectors and cables |