Ruigu Electronic

1. Definition and Development

The Apple Desktop Bus (ADB) is a proprietary serial communication protocol developed by Apple in 1987 for connecting peripherals to Macintosh computers. Designed to replace multiple dedicated ports (e.g., keyboard, mouse, joystick), ADB enabled daisy-chaining of up to 16 devices via a single connector, prioritizing simplicity and plug-and-play functionality.

2. Physical Connector and Cable

• ADB Port Design:
  • A small, round 4-pin connector (similar to a mini-DIN socket) found on classic Macs (e.g., Macintosh II, Power Mac G3) and some Apple keyboards/mice.
  • The connector is reversible, eliminating confusion about orientation—an innovative feature for its era.
• Cable Specifications:
  • Shielded twisted-pair cable with a maximum length of 1.8 meters (6 feet) per segment, though daisy-chaining allowed extended reach via multiple devices.

3. Technical Specifications

• Data Transfer Rate:
  • Transmits at 127 kbps, sufficient for low-bandwidth peripherals like keyboards, mice, trackballs, and game controllers.
• Power Delivery:
  • Provides 5V DC at up to 500mA via the ADB bus, enabling bus-powered devices without external adapters.
• Protocol Features:
  • Uses a polling mechanism where the computer periodically queries each device for input, ensuring real-time response for pointing devices and keyboards.

4. Daisy-Chaining and Device Management

• Bus Architecture:
  • Allows connecting devices in a chain (e.g., computer → keyboard → mouse → joystick), with each device acting as a pass-through for the bus signal.
• Auto-Configuration:
  • ADB devices are automatically recognized by the Mac, eliminating the need for drivers or manual setup—groundbreaking for 1980s computing.

5. Key Applications in Apple Ecosystem

• Input Devices:
  • Standard on Apple keyboards (e.g., Apple Extended Keyboard) and mice (e.g., Apple Desktop Bus Mouse), replacing the earlier Serial Mouse Interface (SMI).
• Game Controllers:
  • ADB-compatible joysticks and game pads were popular for classic Mac games (e.g., Myst, Doom).
• Specialized Peripherals:
  • Used in barcode scanners, drawing tablets, and other office equipment, streamlining connectivity for Mac users.

6. Advantages Over Contemporary Interfaces

• Simplicity and Usability:
  • Consolidated multiple ports into one, reducing cable clutter—a significant improvement over PCs of the 1980s/1990s, which required separate PS/2, serial, and parallel ports.
• Hot-Plugging:
  • Allowed devices to be connected or disconnected without rebooting, a rare feature in pre-USB era.
• Power Efficiency:
  • Bus-powered design eliminated the need for wall warts for most peripherals, enhancing desktop cleanliness.

7. Limitations and Decline

• Bandwidth Constraints:
  • 127 kbps was insufficient for high-bandwidth devices (e.g., printers, external storage), which still required separate SCSI or parallel ports.
• Proprietary Nature:
  • ADB was exclusive to Apple, limiting third-party compatibility and innovation compared to open standards like USB.
• Replacement by USB:
  • Apple phased out ADB in 1998 with the iMac G3, adopting USB 1.1 for its higher speed (12 Mbps) and broader industry support.

8. Modern Relevance and Legacy

• Vintage Mac Restoration:
  • ADB remains essential for maintaining classic Macs, requiring adapters (ADB to USB) to use modern keyboards/mice with old systems.
• Collectible Value:
  • Original ADB devices (e.g., Apple Design Keyboard, Mighty Mouse) are sought after by vintage computing enthusiasts.
• Technological Predecessor:
  • ADB’s focus on user-friendly connectivity influenced Apple’s later interface designs, including the universal USB and Thunderbolt standards.

9. ADB vs. Contemporary Interfaces (1980s–1990s)

Interface	Speed	Device Chaining	Power Delivery	Compatibility
ADB	127 kbps	Daisy-chaining (16 devices)	5V/500mA	Exclusive to Apple Macs
PS/2	~10 kbps	No	Minimal (5V/50mA)	IBM-compatible PCs
Serial (RS-232)	~115 kbps	No	None	General-purpose computing

Summary

Apple Desktop Bus (ADB) was a pioneering interface that simplified peripheral connectivity for Mac users in the pre-USB era, prioritizing ease of use and plug-and-play functionality. Though obsolete in modern systems, its legacy endures in vintage Mac setups and as a testament to Apple’s early focus on user-centric design. ADB’s role in consolidating ports and enabling daisy-chaining laid the groundwork for the universal interfaces that followed.