Sat 24 October 2020

Buses :System Unit

Buses are electrical paths or lines inside computer. These buses are used to carry electrical signals between different components of computer. One line of wire can carry one bit at a time. 

The capacity of computer bus depends on the number of data lines it contains. Thus an 8—bit bus can carry 8 bits of data from one component to the other simultaneously. The amount of data that a bus can be carry at one time is called bus width. CPU communicates with other components of computer through buses.

Typically, computer bus consists of 40 to 150 electric wire lines running parallel to each other different types of buses are as follows:

Data Bus 

The electrical path through which data is transferred between components of computer is called data bus. The data bus typically consists of 8, 16, 32 or 64 separate lines.
The data bus lines are bi-directional. It meals that the CPU can:

  1. Read data from memory using these lines.
  2. Write data to memory locations using these lines.

Different devices such as keyboard, memory and mouse work with data bus.

Address Bus

Many components are connected to one another through buses. Each component is assigned a unique ID. This ID is called the address of that component. If a component wants to communicate with another component, it uses address bus to specify the address of that component.

The address bus is also used to designate the source or destination of data on the data bus. The address bus typically consists of 8, 16, or 32 separate lines. The number of address lines determines the number of memory locations that CPU can address.


Control Bus 

Control bus is used to transmit different commands from one component to another component. For example, if CPU wants to read data from main memory, it uses control bus to send the memory read command to the main memory of the computer.

The control bus is also used to transmit control signals like. ACKS (Acknowledgement signals). Suppose that CPU gives a command to the main memory to write data. The memory sends acknowledgement signal to CPU after writing the data successfully. CPU receives the signal and then moves to perform some other action.

Some common commands are as follows:

Command                                           Purpose

MEMORY WRITE                               Write data to a given location in main memory

MEMORY READ                                 Read data from a given location in main memory

l/O WRITE                                          Write data to a given output device

I/O READ                                            Read data from a given input device

BUS REQUEST                                   Request for a control on the bus for transmitting data

BUS GRANT                                      Indicate the grant to the bus to a device


Expansion Bus

Some devices connect to a port on a card which is inserted into an expansion slot. This expansion slot connects to the expansion bus. The expansion bus allows the processor to communicate with the peripheral device attached to the card. Different types of expansion buses are as follows:



ISA stands for Industry Standard Architecture. It is the most common and slowest expansion bus. Mouse, modem card, sound card and low—speed network interface card are connected to ISA bus directly or through ISA bus expansion slot.


 Local Bus

It is a high-speed expansion bus. It connects higher speed devices like hard disk. VESA local bus is the first standard local bus. It was used for video cards. The current local bus standard is PCI bus. PCI stands for Peripheral Component Interconnect. It is more versatile than VESA. Video cards, sound cards, SCSI cards-and high-speed interface cards are inserted into PCI bus expansion slot.



AGP stands for Accelerated Graphics Port. It is designed by Intel to improve the speed of 3-D graphics and video transmit. It provides faster dedicated interface between the video card and memory.



USB stands for Universal Serial Bus. It eliminates the need to install cards into expansion slots, Many USB devices can be attached to computer with single USB port.


PC Card Bus

It is designed for PC Card. It transmits data between PC Card bus and PCI bus.