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Unit Subtitle: Bus Structures Excerpted from 1.

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1 Unit Subtitle: Bus Structures Excerpted from 1

2 There are a lot of incredibly complex components in a computer. And all of these parts need to communicate with each other in a fast and efficient manner. Otherwise, the amazing speed and capabilities of each individual component is lost in the whole. That's where the bus comes in. Essentially, a bus is the channel or path between the components in a computer. Computer Maintenance - Bus Structures 2

3 3 Buses have grown and evolved over the years in an effort to match the performance of all the other computer components. Even so, the evolution of the bus has been surprisingly slow compared to other technologies. Most computers sold today still have an Industry Standard Architecture (ISA) bus that will accept computer cards developed for the original IBM PC in the early 1980s.

4 Computer Maintenance - Bus Structures 4 A typical computer has two key buses. The first one, known as the system bus or local bus, connects the microprocessor (central processing unit) and the system memory. Other buses, such as the ISA and PCI buses, connect to the system bus through a bridge, which is a part of the computer's chipset and acts as a traffic cop, integrating the data from the other buses to the system bus.

5 Computer Maintenance - Bus Structures 5 The illustration shows how the various buses connect to the CPU.

6 ©2004 Texas Trade and Industrial Education Computer Maintenance - Bus Structures 6 The other main bus, the shared bus, is for connecting additional components to the computer. It is called a shared bus because it lets multiple devices access the same path to the CPU and system memory. These devices includes such items such as: Modem Hard drive Sound card Graphics card Controller card Scanner

7 Computer Maintenance - Bus Structures 7 As technology advanced, other buses were developed. Key among these were Extended Industry Standard Architecture (EISA) -- which was 32 bits at 8 MHz-- and Vesa Local Bus (VL-Bus). The VL-Bus was 32 bits wide and operated at the speed of the local bus, which was normally the speed of the processor itself.

8 During the early 1990s, Intel introduced a new bus standard for consideration, the Peripheral Component Interconnect (PCI). PCI presents a hybrid of sorts between ISA and VL-Bus. It provides direct access to system memory for connected devices, but uses a bridge to connect to the front side bus and therefore to the CPU. Basically, this means that it is capable of even higher performance than VL-Bus while eliminating the potential for interference with the CPU. ©2004 Texas Trade and Industrial Education Computer Maintenance - Bus Structures 8

9 PCI can connect up to five external components. Also, you can have more than one PCI bus on the same computer. The PCI bridge chip regulates the speed of the PCI bus independently of the CPU's speed. This provides a higher degree of reliability and ensures that PCI-hardware manufacturers know exactly what to design for. ©2004 Texas Trade and Industrial Education Computer Maintenance - Bus Structures 9

10 Bus TypeBus WidthBus SpeedMB/sec ISA16 bits8 MHz16 MBps EISA32 bits8 MHz32 MBps VL-bus32 bits25 MHz100 MBps VL-bus32 bits33 MHz132 MBps PCI32 bits33 MHz132 MBps PCI64 bits33 MHz264 MBps PCI64 bits66 MHz512 MBps PCI64 bits133 MHz1 GBps Computer Maintenance - Bus Structures 10

11 Originally known as 3rd Generation I/O (3GIO), PCI Express, or PCIe, was approved as a standard on July 2002 and is a computer bus found in computers. PCI Express is designed to replace PCI and AGP and is available in several different formats: x1, x2, x4, x8, x12, x16 and x32. Below are some graphic illustrations of what the PCI Express would look like on the motherboard. Computer Maintenance - Bus Structures 11

12 When the computer starts up, PCIe determines which devices are plugged into the motherboard. It then identifies the links between the devices, creating a map of where traffic will go and negotiating the width of each link. This identification of devices and connections is the same protocol PCI uses, so PCIe does not require any changes to software or operating systems. Computer Maintenance - Bus Structures 12

13 Computer Maintenance - Bus Structures 13 Each lane of a PCI Express connection contains two pairs of wires -- one to send and one to receive. Packets of data move across the lane at a rate of one bit per cycle. A x1 connection, the smallest PCIe connection, has one lane made up of four wires. It carries one bit per cycle in each direction. A x2 link contains eight wires and transmits two bits at once, A x4 link transmits four bits, and so on. Other configurations are x12, x16 and x32.

14 Computer Maintenance - Bus Structures 14

15 Computer Maintenance - Bus Structures 15 The 64-bit PCI-X bus has twice the bus width of PCI. Different PCI-X specifications allow different rates of data transfer, anywhere from 512 MB to 1 GB of data per second. Devices using PCI share a common bus, but each device using PCI Express has its own dedicated connection to the switch.

16 Prioritization of data, which allows the system to move the most important data first and helps prevent bottlenecks Time-dependent (real-time) data transfers Improvements in the physical materials used to make the connections Computer Maintenance - Bus Structures 16

17 Degradation are common in parallel connections. Poor materials and crossover signal from nearby wires translate into noise, which slows the connection. The additional bandwidth of the PCI-X bus means it can carry more data that can generate even more noise. Using the Accelerated Graphics Port (AGP) slot for video cards removes a substantial amount of traffic. Computer Maintenance - Bus Structures 17

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