Overview
The Apple Desktop Bus (ADB) is a low‑speed serial communication interface designed by Apple Computer, Inc. for connecting peripheral devices such as keyboards, mice, trackballs, and graphics tablets to Macintosh computers. Introduced in 1986, ADB provided a simple, daisy‑chained connection scheme that allowed multiple devices to share a single port.
History
- Development: ADB was conceived as part of Apple's effort to streamline peripheral attachment after the original Macintosh employed a variety of proprietary connectors. The design was completed by Apple engineers led by Larry Tesler and first appeared in the Macintosh Plus (released October 1986).
- Adoption: The interface became standard on most Macintosh models from the late 1980s through the mid‑1990s, including the Macintosh II series, the Power Macintosh line, and the early iMac models.
- Phase‑out: Beginning with the introduction of USB (Universal Serial Bus) on Apple products in 1998 (iMac G3), ADB was gradually discontinued. The final Apple computer to ship with an ADB port was the Power Macintosh G4 (1999).
Technical Specifications
- Physical connector: 4‑pin mini‑DIN (also known as Mini‑DIN 4) socket and plug.
- Signal lines: One data line (bidirectional), power (+5 V), ground, and a spare pin.
- Communication protocol: Serial, half‑duplex, 8‑bit data frames transmitted at a clock speed of 125 kbit/s (typical for low‑speed peripherals).
- Topology: Daisy‑chainable; devices are linked in series, terminating the chain with a resistor on the last device.
- Power: Each device receives +5 V from the bus, sufficient for keyboards and mice but not for high‑power devices.
- Addressing: Devices are assigned a unique address during a power‑on reset; the host controller (typically an ADB controller chip on the motherboard) polls each address sequentially.
Supported Devices
- Keyboards: Apple Desktop Bus keyboards (e.g., Apple Extended Keyboard, Apple Keyboard II).
- Pointing devices: Apple Desktop Bus mouse, trackball, and later third‑party ADB mice.
- Other peripherals: Graphics tablets (e.g., Wacom ADB tablets), joystick interfaces, infrared receivers, and some early external modems.
Legacy and Influence
ADB demonstrated the practicality of a unified peripheral bus in consumer computers, influencing later industry standards such as USB. Although ADB itself is obsolete, many vintage Macintosh enthusiasts continue to use and adapt the interface for retro‑computing projects. Conversion adapters exist to connect ADB devices to modern USB hosts, preserving functionality for collectors.
References
- Apple Computer, Macintosh Technical Overview, 1986.
- J. McGrath, “The Apple Desktop Bus: Design and Implementation,” IEEE Computer, vol. 20, no. 9, Sep. 1987.
- S. O'Hara, Apple’s Peripheral Strategies: From ADB to USB, MIT Press, 2003.
