Bus categories  -  Four sub-types.
  Power - not always present (Ethernet).
    May have to provide different voltages for different devices or functions.
      ISA : +/-5 V. for electronics.  +/-12 for CD/hard-drive mechanics

  Address - multi-device.
    Direct address designs fairly simple - think memory address.

    But many devices support a secondary addressing system.
      Multiple hard drives - LBA->CHS on each hard drive.
      Optical discs - serial LBA

      Printer/scanner/fax - singe hardware device - functions independent.

      Secondary address often sent as data.      

  Data - Commonly bi-directional.
    Lines may be used to pass data in either direction. 
      USB - single pair of differential lines.

    Or have dedicated lanes for each direction.
      SATA, HyperTransport

    Or bus designed for specific function may be unidirectional.
      Original parallel printer (Centronics) port.
        Data only traveled from system to printer.

  Control - examples - not complete list.

    Clock - used to synchronize chips and actions.

    Interrupt handling - slave devices requesting attention.
      Mask-able and non-mask-able.
 
    Arbitration - competing master devices wanting control of bus.

    Data direction.

    Reset.

    If bus is multiplexed.
      Memory / I/O selection. (80x86)

      Address/data selection.

    Ready/wait - memory handling.  
 
    Status/Acknowledgment 
      Handshaking - start of message, sequence, end of message.
 
      Sync/Acknowledgment - negotiation of connection:w
 
      More common and complex for asynchronous devices.