External Data Bus in Computer – Functions with Example

External Data Bus in Computer – Functions with Example

In computer organisation, the CPU or the Central Processing Unit uses an electrical path to exchange data from one component to another component. The path which is used for carrying data is known as the data bus. The external data bus is a special type of data bus. In this article, we are going to discuss what is an external data bus, the functions of the external data bus and also some examples of the external data bus in details.

Computer Bus :

The 3 major components of CPU, use this electrical pathway to communicate with each other which is known as the computer bus. There are mainly 3 types of system buses in computer organisation – Data Bus, Control Bus and Address Bus. They do their jobs as their name suggests. Data Bus is responsible for carrying data from one component to another, Control Bus carries control signal and Address Bus carries the address of memory locations. Here we are not going to discuss these all bus structures since our main topic of discussion is external data bus. But before heading on to the external data bus, you should know what is data bus and what is an external bus.

What is a Data Bus?

Data Bus is the most common bus in the computer bus organisation. Data bus carries data from one component to other components in the CPU. It is an electrical path that connects the CPU, Memory, Input/Output devices and secondary storage devices. It provides a bi-directional path to transfer data from system components to another.

The data bus contains some parallel group of lines. The number of lines in bus affects the speed at which the data travels between different components. So, the width of the data bus is a key factor in determining the overall performance of a computer system.

What is an External Bus?

In today’s generation of computer, there are two types of buses available, internal bus and the external bus. The internal bus also called as local bus are those which provides a path for communication between the internal components of a CPU like video cards, memory etc.

On the other hand, the external bus or the expansion bus are those types of communication pathway which provides a medium between external components and CPU.

Now, I think you got a basic understanding of what is a data bus and what is an external bus! So, let’s head on to the external data bus.

External Data Bus:

We have already discussed data bus and the external bus. By combining both these two terms, the external data bus can be defined. So, the external data bus is a type of data bus which carries data between various external components and the CPU. It performs a data communication between external devices like Monitor, Keyboard, Mouse, External Hard Disk etc.

An external data bus can be a serial or parallel. As compared to the internal bus, the external data bus is much slower in data communication. The reason behind this is it is external and do not lie within the circuitry of the CPU. The two most common examples of the external data bus are – Universal Serial Bus (USB) and Peripheral Component Interconnect (PCI).

Examples of External Data Bus:

Universal Serial Bus (USB) :

The Universal Serial Bus (USB) was introduced in 1996. This is an industry standard developed through a collaborative effort of several computer and communications companies, including Compaq, HP, Intel, Lucent, Microsoft, Nortel Networks and Philips.  It is an external data bus which uses asynchronous serial communication mode to transfer data. In today’s generation of computers, it is found to be the most widely used data port. The popularity of USB has already replaced the old BS2 port, parallel port, serial port etc.

The USB has been designed to meet several key objectives:

  • Provide a simple, low-cost, and easy to use interconnection system that overcomes the difficulties due to the limited number of input/output ports available on a computer.
  • Accommodate a wide range of data transfer characteristics for input/output devices, including telephone and internet connections.
  • Enhance user convenience through a ‘plug-and-play’ mode of operation.

There are several versions of USB are introduced. The version 1 or USB 1.0 was the very first version of USB introduced in 1996. The theoretical maximum bandwidth of USB 1.0 was 1.5 Mbps (Megabit per second). The next version of USB was USB 1.1, introduced in 1998 with a maximum bandwidth of 12.5 Mbps. In the year 2000, the USB 2.0 was released with a maximum bandwidth of 480 Mbps and it still is the most popular and most common USB version found in today’s devices. After USB 2.0, in the year 2008 USB 3.0 was introduced which supports a maximum bandwidth of 5Gbps. In 2013 USB community released the USB 3.1 with a supportive bandwidth of 10Gbps and it is the latest and fastest version of USB that can be seen in some modern devices.

Peripheral Component Interconnect (PCI):

PCI was introduced in 1992 by Intel. The PCI was developed as a low-cost external data bus that is truly processor independent. From 1992 to 2004 it was very popular since the PCI express was not introduced. PCI slots are used to expand the capability of a computer system. PCI has 2 different versions. One of them can be found mostly in the desktop motherboard which is a 32-bit version and another one is a 64-bit version. Both of the versions have another sub-version of PCI like 3.3v, 5v and universal.

At first, PCI bus had a frequency of 33.33 MHz but later it was developed to clock at 66.66 MHz. The 32-bit version of PCI with a clock speed of 33.33 MHz had a maximum theoretical data transfer speed of 133.32 MBps. The 64-bit version with frequency 33.33 MHz had the maximum data transfer rate of 266.64 MBps. The last version of PCI i.e. the 64-bit with a frequency of 66.66 MHz had the maximum bandwidth of 533.28 MBps.

In today’s date, the major disadvantage of the PCI bus is that it transfers data parallelly. When there is a parallel data transmission, if we increase the frequency for a greater speed the chances of latency and data corruption also increases. That is why PCI bus can’t provide a greater data transmission speed. Another mechanical disadvantage of PCI is the lack of locking mechanism. We can’t lock our cards on PCI slots. To overcome these major disadvantages, PCI express is introduced. The high-end motherboards of today’s come with only PCI express slots which provides a much greater data transmission rate than conventional PCI.

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Also Read: Cache Memory Mapping Techniques

Debarshi Das

Debarshi Das is a passionate blogger & web developer from Guwahati, Assam. He has a deep interest in robotics too. He holds a BSc degree in Information Technology & currently pursuing Masters of Computer Application (MCA) from a premier govt. engineering college. He is also certified as a chip-level computer hardware expert from an ISO certified institute.

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