The Serial Input/Output system, universally known as SIO, was a proprietary peripheral bus and related software protocol stacks used on the Atari 8-bit family to provide most input/output duties for those computers. Unlike most systems of the era, such as RS-232, SIO included a lightweight protocol that allowed multiple devices to be attached to a single daisy-chained port that supported dozens of devices. SIO required additional logic in the devices which drove up costs. The designer, Joe Decuir, credits his work on Atari SIO as the basis of USB, which he also designed and on which he holds patents.
Devices that used this interface included printers, floppy disk drives, cassette decks, modems and expansion boxes. Some devices had ROM based drivers that were copied to the host computer when booted allowing new devices to be supported without native support built into the computer itself.