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Apple desktop bus (ADB) is an outmoded, low-speed serial bus used to connect input devices such as a keyboard or mouse to a personal computer (PC). It was used mostly with Macintosh platforms like the Apple II models. It is capable of holding several bits of data in its register and can decode a single digit or address. With the introduction of Apple’s iMac in 1998, the ADB was replaced by the universal serial bus (USB). Although ADB equipment is still available, it has not been supported by most Apple hardware manufacturers since 1999.
The ADB is one of the most cost-effective buses in history. It was developed by Stephen Wozniak, the co-founder of Apple Computer Inc. The ADB was created in the mid-1980s after a month of research and was first implemented with the Apple IIGS in 1998.
After the development of the Apple IIGS, the ADB was used on all Apple machines starting with the Macintosh SE and Macintosh II. The ADB was also used on various systems that used a 680x0-base microcomputer such as NeXT, Hewlett Packard (HP) and Sun Microsystems.
While the ADB has not been used for input device connection since it was superseded by the USB, it has been used as an internal interface protocol for iBooks and PowerBooks since February 2005.
The ADB uses a four-pin mini-DIN connector with a maximum length of five meters, which can also be used for separate video (S-Video). One pin is used for data, two pins are used for the +5V power supply, and one pin is for the ground. In addition, a power switch or PSW pin is connected to the power supply of the host computer, which allows signals to be interpreted for start-up operations from a key on the keyboard instead of relying on ADB software.
The ADB supports up to 16 single devices with unique addresses. However, due to possible signal degradation it is recommended that only three devices be connected to each ADB.