Broadcast
  Multiple devices share a common bus.

  Devices must be able to recognize when they are target of communication.
    Pre-defined memory mapped locations and/or jumpers on interface cards.
    Plug-n-Play - software initialization on power up.

  Broad range of control signals/lines to support a flexible protocol
    for dealing with a variety of 'devices'.

  Bi-directional. May use separate unidirectional lines for each direction
    HyperTransport

  Or may allow lines to transmit in either directions.
      USB

  Signals strong enough to reach all interfaced devices.

  All devices usually synchronized.
    Have defined behavior based on shared bus timing.

  Some implemented with routing nodes.
    Synchronization implemented between endpoints of node hop.


    Industrial Standard Architecture (ISA) - PC bus (mid 1980s-90s.
    
      Most address, data, and control lines accessible.

      Basically extended the CPU pin-out to memory and interface cards.


    Peripheral Component Interconnect (PCI) - early 1990s - current.
    
       Controlled by a custom controller. 
         Which maps devices onto unused memory space.

         Controller then acts as memory decoder for commands addressing
           those devices/addresses.

       Pins/lines
         32-bit address/data lines multiplexed on same 'pins'.
 
         !FRAME - flags the transmission of a frame of data.
           A frame consists of 1 address transfer followed by 0 or more
             data transfers.

         4-bit command/byte enable (C/BE) 
           For address transfer, acts as a 4 bit command.
           For data transfers, acts as a byte mask across the 32-bit bus.

         4 shared interrupt lines.

         1 Request/Grant (arbitration) for each slot.

         Various other control lines.

       Used transaction (session) based but not packet based communication.

Networked/Virtual point to point 
  * Modern bus technology has hybridized the broadcast bus design.

  Implemented using packet based messaging.

  Devices interface with controller or router/bridge 
    using a point to point connection.

  Controller identifies each device and determines it functions and needs.

  Addressing, data, and control communication are packetized and routed
    by controller between CPU and devices.

  All devices may directly interface with controller,
    but can also be routed through other devices.

  Routers/bridges can serial link devices to extend bus reach.
    Allowing variety of physical topologies. Daisy chain, tree, etc.

  Routers route packets only to device or known path to device with 
    minimal disturbance of packets.

  Timing restraints apply to the 2 end points of a link.
    Although there may be a 'stop clock' on the response time of a target.

  HyperTransport - high performance router that interfaces a CPU with other
    CPUs and/or other bus controllers such has PCI controller.

    Uses separate paths (links) for data transmission in either direction 
      and performs both at same time.

    Each link is serialized.

    Links are arranged in multiples of 2 up to 32 links in each direction. 
      2,4,8,16,32 

    Transmission in opposite directions don't have to use same number of
      links. 

    Data is packetized as a set of 1 or more 32-bit words.


  USB - universal serial bus.
    Treats each device as an isolated point to point connection.

    All communication occurs between the root server/hub and an individual
      device even if moving data between 2 devices.

    USB considers all connections virtual point to point that can be
      implemented over chained hubs.

    Designed to allow packets to be routed through hubs that can be used to 
      physically extend connection length.

    Hubs can also be used to implement a physical tree topology.

    Protocol requires packet to be only to target or the routing hub the 
      target resides on.  The hub then forwards the packet only to the target
      or the next routing hub in chain.