Yes, Atari had a serial bus that was quite nice in its day. Too bad they didn't promote it as a standard for other computer makers of the day to use. There are some significant parallels to USB, as well as many differences.
But what really sets it apart from USB is the lack of standardization, not technical differences. USB was part of a vision for all computers. Atari never considered pushing SIO for use with Apple or Commodore.