The Apple Desktop Bus, or ADB, is an inspired system that was years ahead of its time. Pioneered at Apple by Michael R. Clark. It is characterized by its round 4-pin plugs that resemble S-Video connectors.

The inspired part comes from that fact that the ADB is a multidrop and multiconnected bus system. Meaning, basically, you can string devices together in series or use a hub for star topographies. It is very much like USB in that respect, and in fact the USB spec was based heavily on ADB design principals.

The downsides of ADB were few, but major. One, it was an Apple design, and therefore most peripherals were few and expensive. The second being that the ADB data rate max'ed out at 256kbps, or 25-kilobytes per second. Though that didn't stop every conceivable device under the sun to be made ADB-ized, from the regular mice and keyboards to scanners and ISDN TAs.

ADB was Apple's primary peripheral connection until the iMac in 1998 and, later, the Blue & White Power Macintosh G3 in 1999.

Actually, the Apple Desktop Bus first appeared on the Apple IIgs in 1986, one year before it appeared on any Macintosh. This is the reason that ADB (and USB) keyboards had two symbols on their Command keys: the Clover Leaf used with Mac software, and the Open Apple used with Apple II software. The Open Apple symbol wasn't removed until 2008, with the introduction of the Flat Aluminum Keyboard.

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