Apple Desktop Bus

Apple Desktop Bus (ADB) was a proprietary Mini-DIN-4 serial port used by many Macintosh computers to connect low-bandwidth devices, such as keyboards and mice. The connector is physically identical to S-Video.

History
ADB was originally designed by Apple Computer co-founder Steve Wozniak for the Apple II GS and was also in use by Power Macintosh computers. Third party manufacturers developed modems and radio tuners for the Apple Desktop Bus, but the specialized driver software for some of these devices do not work with PowerPC processors. With the exception of the "Wall Street" PowerBook G3 series, hot swapping of ADB devices is not recommended.

Discontinuation
ADB and several other legacy technologies were eventually phased out by Apple, starting with Steve Jobs' introduction of the iMac G3 in May 1998.