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Expansion Cards & Buses

You can think of a bus as a data highway connecting different parts of your system.  Without these data highways nothing could be transferred from one part of a system to another.  There are various types of buses on a computer system; these notes look at expansion buses.

What is an expansion bus?

Bus Types

What is an Expansion Buses

Physically, a bus is a group of parallel wires on the motherboard which transfer data from one device to another.  A motherboard has many different buses.  We have a data bus and an address bus which are controlled by the CPU.  We also have an expansion bus leading from the line of expansion slots  Expansion cards can be inserted into the expansion slots so that you can effectively expand your system - adding devices such as sound cards, modems, network cards, etc.

Buses are digital and the flow of information moves along a bus in step with a clock.  Bus speed is measured in MHz.  A bus has a certain number of wires and this is known as it's width.  A bus with 8 lines would be an 8-bit bus. The faster the bus speed, the faster the rate of data transfer.  Common bus speeds are 66MHz and 100 MHz.   

Now, there of the many different types of expansion bus; three commonly used buses we will be looking at are, ISA, PCI and AGP.  

Bus Types


ISA stands for Industry Standard Architecture.  ISA buses were developed around 1993 and were used to connect expansion boards to the motherboard.  They are more or less out of date now.  Originally, ISA was a slow 8-bit bus used on IBM PCs.  Later, ISA was stretched to allow 16-bits of data to pass through each clock cycle.  This increased its speed to between 8 and 10 MHz.  ISA has a data transfer rate of 20MB/s.


PCI stands for Peripheral Component Interconnect.  This type of internal bus is much more commonly used in today's systems.  Many modern expansion boards are connected to PCI slots.  A PCI bus is either 32-bit running at 33 MHz or a 64-bit running at 66 MHz.  PCI can transfer data at a rate of 150 MB/s.


AGP stands for Accelerated Graphics Port.  It is meant for data-intensive graphics cards.    Your graphics card is responsible for telling the monitor what to draw on the screen.  With the increasingly high speed demands of 3D video games and video, AGP became necessary as a fast connection for getting video information from the computer's main memory to the graphics card. 

You don't have to use an AGP graphics card.  Many older computers do not have AGP and PCI graphics cards are used.

There are different speeds of AGP cards, all operatring on a 32-bit bus.

AGP PCI66 (bus speed 66MHz)
AGP 1x  (bus speed 66 MHz with extra capabilities) (256 MB/s)
AGP 2x  (bus speed 66 MHz but effective speed of 133MHz) (512 MB/s)
AGP 4x  (bus speed 66 MHz but effective speed of 266MHz) (1024 MB/s)

The AGP design allows for data transfer at rates of up to 1 GB/sec.

That's it!!


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