USB Cables

Introduction to USB and Serial Cables

  • USB (Universal Serial Bus) was designed to replace serial cables.

  • Serial cables were the primary method for connecting devices to computers before USB was introduced.

Serial Cable Overview

  • Connectors Used:

    • DB9 and DB25 connectors.

    • DB9 is most common and resembles a D-shaped subminiature pin.

  • Connection Method:

    • Typically secured with two thumbscrews connecting to the motherboard's serial port.

  • Data Transmission:

    • Data sent serially as bits (ones and zeros) in a straight line, meaning only one bit can be transmitted at a time.

    • Measured in bits per second (bps) or kilobits per second (kbps).

  • Speed Limitations:

    • Maximum speed is 115,000 bits per second (115 kbps).

    • Suitable mainly for older devices like mice, keyboards, and external modems.

    • Only one device can connect to one serial port, limiting connectivity.

Transition to USB

  • USB Advantages:

    • Allows connection of up to 127 devices in a daisy chain configuration.

    • Each port controlled by a host controller managing device bandwidth.

    • Speed varies by USB version:

      • USB 1.0: 1.5 Mbps (low speed).

      • USB 1.1: 12 Mbps (full speed).

      • USB 2.0: 480 Mbps (high speed).

      • USB 3.0: 5 Gbps (super speed).

USB Connection Types

USB Type A

  • Description:

    • Most common and standard USB connector.

    • Inserts in one direction only.

    • Found on desktops and some laptops.

USB Type C

  • Description:

    • Newer connector introduced with USB 3.0 and beyond.

    • Smaller oval shape; can be inserted in either direction.

    • Commonly used in modern laptops, tablets, and smartphones.

USB Type B

  • Description:

    • Primarily on larger devices like printers.

    • Square-shaped with rounded top corners.

    • Variants include:

      • Type B Mini: Trapezoid shape, used on early portable devices.

      • Type B Micro: Smaller and skinnier, used on wearables and portable devices.

    • USB 2 vs USB 3 Type B Connectors:

      • USB 2 Type B: Square with rounded top corners.

      • USB 3 Type B: Square with an additional rectangle on top; not interchangeable.

USB Type B Micro

  • Description:

    • USB 2 Type B Micro: Oval/trapezoid shape.

    • USB 3 Type B Micro: Resembles a figure 8; specific to USB 3.0.

    • Typically found on portable storage devices, capable of 5 Gbps speeds.

Exam Preparation Tips

  • Understand:

    • The history and evolution from serial cables to USB.

    • The various USB standards and their respective speeds.

    • Physical characteristics of different USB connectors.

    • Cable length limitations and power capacities of USB ports.