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Computer Buses

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• A bus is a set of wires used to transfer data between different devices in a computer

  A bus is akin to a "pneumatic tube" used in drive-up banking locations to transport "item" from the client to the bank-representative and back.

• So in other words: a bus is used to make communication possible between different devices that are connected to the bus

• There are many different types of buses used inside a computer.

  Some of the more well-known buses used in a PC today are:

  • the ISA bus (developed by IBM)
  • the PCI bus (developed by Intel)

• The PCI bus:

  Currently by far the most popular I/O bus is the Peripheral Component Interconnect (PCI) bus was developed by Intel and introduced in 1993. PCI is a 32-bit bus that normally runs at a maximum of 33 MHz (this is the clock rate, so the PCI is an example of a synchronous bus protocol - see later).

  Here is the homepage of the Special Interest group for PCI: click here

• Every type of bus has its own bus protocol

  A bus protocol consists of:

  • A specification of the bus signals (wires) and how to use each signal
  • The steps that devices must follow to perform a transfer operation

  The steps include timing relationships that must be followed to guarantee that a data transfer operation is successful

• A bus protocol contains a sequence of clear and rigid steps that communicating devices must follow to complete a data transfer operation.

  The rigid steps is necessary to enable machines to communicate with each other... machines are not as flexible as humans and must follow a very rigid course to make a (complex) task (such as data transfer) successful

• So summary:

  Bus protocol contains:

  • Specification for each signal and its meaning
  • A timing diagram that specify the steps that communicating devices must follow

  See for an example the specification of the PCI bus: click here

  We will study a much more simple bus protocol that distills the essence of communication techniques between automata (machines).

• Bus protocol are often proprietary or open (international) standards. The reason to standardize a bus protocol is:
  • Economics

  In order to connect a device on, say a PCI bus, the device must implement the PCI bus protocol to the letter

  On the other hand, any device that implements the PCI bus protocol to the letter, can be connected to a PCI bus.

  Here is a little secret: most bus protocols that are economically successful are/were open standards, i.e., manufacturers do not need to pay royalty to the developer of the bus protocol in order to manufacture devices that implements that bus protocol

  PCI is such open standard.

  With an open standard, manufacturers of computer devices can compete fairly, for example, each manufacturer of network card can compete in the PC market by making sure that their network cards implement the PCI bus protocol (to the letter). They will be certain that their network card will work when it is inserted into any PC

• The are two type of bus protocols:
  • Synchronous bus protocols:
    • The bus signals include a "CLOCK" signal (a clock signal is used to pace events, see: click here
    • events on a synchronous bus are paced by the "CLOCK" signal of the bus

  • Asynchronous bus protocols:
    • The bus signals include a "Master Sync" and a "Slave Sync" signals
    • Events on a synchronous bus are paced by the master and slave signaling each other's readiness.

  To understand the operations of these two type of protocols, you have to consider the capabilities of the devices inside a computer... All that these devices (CPU, memory, IO device....) can do is sense.... They cannot use sight to make preparations to communicate (e.g., when you see the teacher walk to the black board, you may start picking up your pen and paper and prepare to write down any notes). So you need to take explicit steps to ensure that the devices are "ready" when the communication occurs. These "extra steps" are synchronization steps. An example of an "extra step" in communication is: before saying something to someone, tell that someone that you are gonna say something to him. (That may sound dumb in human communication, but that's what computers (dumb machines :-)) have to do first.)