Buses Explained (ISA / VESA / EISA / PCI)

Have you ever been stuck on the expressway during rush hour and passed one of the signs that limits the speed from 45mph to 55mph? Your computer system is not much different than the expressways where speed and traffic is concerned.

Manufacturers list technical capabilities by the highest limit available. Manufacturers or resellers do not always discuss or divulge all of the specifications. The problem is, the traffic within the computer can't always perform as fast as the technical limits may indicate. Because of the traffic jam that can be created it is important to understand what options are available when purchasing a computer.

This article will address one area of computers, where traffic jams are often created and which most buyers of computers overlook. This article is directed to those of you who are not highly technical and attempts will be made to simplify many areas to ease the understanding. So with the introduction out of the way, here we go.


Transportation By BUS

Computer components send and receive data between different devices by the use of a bus. The design and type of the bus therefore has a crucial effect on how well a computer system will operate. If you have a high speed drive and a fast CPU, but a slow bus, data will be held up and the individual components will not operate at the speed they are capable.

Over the years, software programs and features have created more data traffic. One major culprit is the GUI (graphical user interface) type of program. Windows is an example of a GUI program. Graphical images, versus text or character based data, cause more traffic. Rush hour occurs more frequently.

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Industry Standards

Manufacturers have developed standard methods for connecting different devices and for the design of bus technology. This has benefited everyone by allowing the purchase of devices from different manufactures and not having to worry that the devices will talk to each other.

Bits Bus Type Description
16 Bit ISA Industry Standard Architecture
32 Bit EISA Extended Industry Standard
Micro-Channel IBM's proprietary architecture
VESA (VL-Bus) Video Electronics Standards Association
64 Bit PCI Peripheral Component Interconnect

Why so many standards and why do they keep changing? Read on!

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Faster and Faster

Today's computer peripherals are becoming faster and faster and applications are requiring more sophistication. ISA and EISA were adequate, but time marches on. Before discussing the pro and cons for each bus type, I think it is important to discuss a few buzzwords. Stick with it, I'll try to make it simple.

Buzz Words
  • MHz - Think of 33 MHz or 66 MHz as miles per hour or speed.
  • Mbps (Mega bytes processed per second, transfer rate) translates how many cars could travel the highway within a time period of one second. A higher Mbps value represents a faster device.
  • Bit specifications are the number of lanes on the highway. 16 bit = 16 lanes, 32= 32 lanes, etc. The more lanes, the more traffic that can be accommodated.
  • Voltage references, like 5V or 3.3V, is similar to miles per gallon with the exception that the lower the voltage, the better the gas mileage.

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Bus Comparisons

Now that you are armed with the buzzword definitions, lets compare the different bus types.

Bus Type
Property
ISA
EISA
VESA
PCI
MHz
8.3
8.3
33
33
Bits
16
32
32
32 or 64
Mbps
8.3
33
160
132 or 264
Voltage
5
5
5
3.3 or 5

You should be to able to quickly see that the PCI technology surpasses the others.

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Benefits of PCI

A PCI bus does provide improved performance for high speed devices such as graphic display adapters, network cards, and disk controllers. PCI is also auto configurable and is controlled by software and not jumpers or switches on a board. This is a feature referred to as "Plug-and-Play" or "PnP".

The clock speed (MHz) of a PCI bus works independently of the CPU speed. A VL-bus is tied to the CPU speed.

A PCI bus also works independent of the CPU. When a VL-bus is active, the CPU must wait. PCI peripherals can continue to place data on the bus, even when the CPU is active.

A PCI bus can operate at 3.3V which is important for battery life on portables and the new energy saving "Green PC" requirements.

A PCI bus will operate on several computer platforms, such as the PowerPC from Apple and IBM and DEC's Alpha chip.

Because a PCI bus requires less components, pins and boards, there is a higher reliability rate. There are less parts to manufacturer and fewer parts that will break.

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