The PC Bus
The bus is actually a set of circuits that run throughout the board and connect all the expansion slots, memory, and CPU, etc. together. 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 we say 'BUSES'?) provides a highway for passing information between the devices on the system.

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 we can say, electrical signals representating information flows along the bus from one device to another.

Your computer may contain several types of buses (all located on the motherboard). Some of the more common buses found are:

  • CPU bus or 'system' bus.
  • An address bus
  • Memory bus
  • I/O or Expansion Bus
    • ISA Bus
    • PCI Bus
    • Micro Channel Bus
    • EISA Bus
  • External Buses (Can have external cables connecting devices)
    • SCSI Bus
    • PC Card Bus
    • USB Bus

All buses are located on the motherboard with the exception of the External buses. External buses connect to standard ISA or PCI expansion slots via a controller/adapter card. This card acts as an interface between the ISA/PCI bus architecture. The cables connected to the adapter/controller card are actually considered as the bus. Thus the 50 pin cable connecting SCSI devices to the controller is the actual bus.

There is a new bus called the "Universal Serial Bus" that fits into this category. The difference is that this bus connects directly to the system bus on the motherboard.

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.

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:

  • Data Sharing
  • Addressing
  • Power
  • Timing
  • Flow Control
  • System Control
BUS TYPES:

Bus Type
 Speed
 Used For
 Size
Vesa Local Bus
(VL Local Bus )
 130MB
 AT Class Computers
 32 Bit
Industry Standard Architecture - (ISA)
 16 MB
 AT Class Computers
 8/16/32 Bit
Micro Channel Architecture (MCI)
 40 MB
 Network Servers
 32 Bit
PCI
 90 MB
 486/Pentium Computers
 32/64 Bit

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 simultaneously.

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
Type		 Number
Bits		 CPS Speed No.
Connectors
ISA 8 4mb/s 8.25MHz 31 pairs
ISA 16 8mb/s ? 8.25MHz 49 pairs
MCA 32 unk 10MHz 93
EISA 32 Unk Unk 97
VESA Local Bus 32 107MB/s 50MHz 49+36
PCI 32 132MB/s 33MHz Unk

    BUS SIZES: