Parallel
  Legacy parallel "Centronics" port. (SPP)
  4 control lines - PC to printer
  5 status lines  - printer to PC
  8 uni-directional data lines
  8 ground lines
  150 kilobytes/sec

Controller to single device.

  Printers.

  Later protocol tweaks allowed
    Bidirectional data transfer
    Daisy chained devices.

Most logic performed at software level on early implementation
  * allowing non-conforming logic if desired.

* Using "virtual address and control", companies such as Iomega were able
  to daisy-chain additional devices on single controller.

SPP improvements
  Reverse - has to be supported by software.
    Nibble mode - 4 bit reverse data transfer using status lines for data
       transfer. Supportable with "Centronics" ports at software level.

    Byte mode - Hardware manufacturers redesigned ports to allow software 
       to disable the data output driver circuits and reverse data direction. 
       Late model "Centronics" ports.
    
Bidirectional EPP (1991)
  EPP - Enhanced parallel port (Pre 1284)
    2.77 MBps - support devices other than printers.

    Four modes of data transfer
      Data write cycle

      Data read cycle

      Address write cycle

      Address read cycle
        Addresses
        Channel - distinguish function in a fax/modem/printer. 
        Commands
        Control.

    However, multiple vendor protocols - not always compatible. 
    Under IEEE 1284 - single protocol