PC BUSES

    The PC's bus is the circuitry that ties all the devices on your motherboard together. The bus concept is to: "extend and link the signals of your system"; however, you have to know what it means to understand it.

    In this section we will discuss:

    1. Bus Basics
    2. Bus Types
    3. Multiple Buses
    4. Bus Standards

    BUS BASICS:

    To understand a PC's bus, you must be aware that a computer has many components such as a Central Processing Unit (microprocessor), clocks, storage devices, input/output devices, etc. These devices must be linked together to perform a function. The 'bus' (or should I say 'BUSES'?) provides a highway for passing information between the devices on the system.

    Since this is not an electrical engineering level tutorial, let me say that the bus ties these devices together, so that a signal from your keyboard is displayed on your screen (CRT); a record from a file on a hard disk drive is read into memory and processed at the direction of your CPU; a file is sent to a printer for printing, etc. SO, electrical signals representating information flows along the bus from one device to another.

    Your computer may contain several types of buses, among them are:

    1. CPU bus or 'system' bus.
    2. An address bus
    3. Memory bus
    4. I/O or Expansion Bus

    and they are all located on the motherboard! There is a new bus called the "Universal Serial Bus" that connects to the motherboard, and some consider the wires between the connector and the devices attached to it to be the bus.

    When we speak of the 'bus', most of us are speaking of the 'expansion' bus. Most external devices and storage devices will be connected to the I/O bus through the a local bus. External devices are usually connected to "expansion" cards (like this illustration) or controllers which are placed in an "expansion" bus slot located on the motherboard. Electronic circuitry connects the expansion slots together and connect to the system bus. Each of the expansion slots have an address, used to route information to a particular expansion card installed in the expansion slot. Other devices also have addresses such as the keyboard, the memory, etc.

    Buses have many factors that affect their performance and that must be considered and solved for them to function:

    1. Data Sharing
    2. Addressing
    3. Power
    4. Timing
    5. Flow Control
    6. System Control

    BUS TYPES:

    There are several "bus" standards, among them are:

    1. Industry Standard Architecture (ISA).
    2. Micro Channel Architecture (MCA)
    3. Enhanced Industry Standard Architecture (EISA)
    4. Peripheral Component Interface (PCI)
    5. VESA Bus
    6. Universal Serial Bus

    The most popular in the newer models of PCs are PCI and combinations of PCI and others.

    Now, bus size (number of binary bits) greatly affects the performance of your computer. The first microprocessors in 1971, had a 4 bit bus, later models introduced 8, 16, 32, and 64 bit buses. Most of the early buses had a speed of 8.25MHz, where current buses operate at higher levels such as 66MHz (all multiples of 8.25Hz.

    Let me use an example:

    1. Relate a car on a roadway to a bit on a bus.
    2. A four lane road (one direction) is a 4 bit bus.
    3. Every car on the roadway must travel at exactly the same speed.
    4. Every car must be perfectly aligned with its companion bits.
    5. Now, assume you have millions of cars that must go down this roadway.
    6. Assume that you have this major highway with lots of cities having access to it at major intersections (enter and exit ramps or traffic lights).
    7. You have to allow "controlled" access to the major highway for the individual city's traffic.
    8. Traffic must flow in one direction at one time, then in the other direction at another time in all lanes. No two way traffic simulataneously.

    Ok, if you have only 4 lanes in each direction, it will take more time to pass 1,000,000 cars from point A to point B than if you had 8 lanes of traffic. Using this knowledge - if we make the roadway (bus) wider (more lanes) we can send more cars (bits) simultaneously along the road (bus).

    Now, there are other access roads, from which we have to use an interrupt (traffic lights) to allow other roadways (devices) to use the same major roadway (BUS). We can look at "bus" controllers as entrances from a slower speed road to a higher speed road. My analogy would be this:

    1. A city street with a 35mph speed limit is an ISA Bus.
    2. An interstate highway with a 70mph speed limit is a system bus.
    3. We use red light (lets say a VESA Local Bus controller) to allow the slower traffic from the city street to enter the higher speed roadway (system bus).
    4. So, we control access through a controller to a higher speed roadway. Since the ISA bus is a max of 8mbps, it cannot keep the system bus fully occupied. The controller then handles the interfaces of devices to the higher speed bus.

    BUS TYPENR.BITSTHROUGHPUTSPEEDNr.Connectors
    ISA84mb/s8.25MHz31 pairs
    ISA168mb/s ?8.25MHz49 pairs
    MCA3210MHz93
    EISA3297
    VESA Local Bus32107MB/s50MHz49+36
    PCI32132MB/s33MHz

BUS SIZES:

Storage Devices