Apple Desktop Bus

Apple Desktop Bus. It was a low-speed serial bus protocol developed by Apple Inc. for connecting input devices like keyboards, mice, and tablets to its personal computers. Introduced in 1986 with the Apple IIGS, it became the standard peripheral connection for the Macintosh line for over a decade, simplifying cabling by allowing multiple devices to be daisy-chained. The technology was eventually phased out in favor of the USB standard by the end of the 1990s.

History and development

The system was conceived by Steve Wozniak as a solution to the clutter of multiple proprietary ports on earlier Apple II and early Macintosh computers. Its development was part of a broader initiative at Apple Inc. to create a more user-friendly and unified ecosystem, paralleling efforts in the design of the Macintosh SE and Macintosh II. The bus was first implemented in the Apple IIGS in 1986, marking a significant shift from the Apple IIe expansion card model. This innovation was quickly adopted across the next generation of Macintosh computers, becoming a defining feature of the Macintosh Plus and all subsequent models until the late 1990s, solidifying Apple Inc.'s reputation for integrated hardware design.

Technical specifications

The interface used a simple serial communication protocol operating at a fixed rate of 125 kbit/s, with a polled architecture where the host computer managed all data transactions. It employed a distinctive 4-pin mini-DIN connector and supported up to 16 addressable devices, though typical usage involved only a few peripherals like a Kensington mouse or an Wacom tablet. Power for connected devices was supplied directly from the computer at +5 volts, simplifying peripheral design. The protocol's command set allowed for device enumeration and the transmission of keypress or movement data, with timing governed by the host's VIA or MOS controller chips.

Implementation and usage

On the desktop, a typical setup involved connecting an Apple Keyboard directly to the computer, with an Apple Mouse then plugged into a port on the keyboard, creating a simple daisy chain. This design philosophy was central to the Macintosh LC and PowerBook series, reducing cable sprawl. Third-party manufacturers like Kensington and Logitech produced a wide range of compatible pointing devices and specialized input tools, including trackballs and graphics tablets. The system was also used for less common peripherals such as barcode readers and Midi interfaces, demonstrating its flexibility within the Mac OS ecosystem throughout the 1980s and early 1990s.

Legacy and influence

The bus is remembered as an elegant and influential precursor to modern peripheral interfaces, directly inspiring aspects of the PS/2 connector standard used by IBM and other clone manufacturers. Its philosophy of a unified, daisy-chainable bus for human interface devices clearly paved the way for the development of the USB standard in the mid-1990s. The transition away from the technology began with the iMac G3 in 1998, which boldly adopted USB as its sole peripheral bus, a move that accelerated the industry-wide shift. This decision marked the end of an era for Apple Inc. and is now a noted milestone in the history of personal computer connectivity.

Compatibility and adapters

With the demise of the native port, a market emerged for adapters to allow legacy input devices to function on modern computers. Companies like Griffin Technology and Belkin produced active USB to ADB converters, which contained the necessary protocol translation chips to bridge the systems. These adapters were particularly valuable for users of specialized old Wacom tablets or collectors maintaining vintage systems like the Macintosh Classic. Conversely, passive adapters also existed to connect newer PS/2 mice and keyboards to older Macintosh Quadra or Power Macintosh computers, providing stopgap solutions during the transitional period of the late 1990s.

Category:Apple Inc. hardware Category:Computer buses Category:Macintosh