COA-Computer Bus

What Is Bus

Bus is a subsystem that is used to transfer data and other information between devices.Means various devices in computer like(Memory, CPU, I/O and Other) are communicate with each other through buses.In general, a bus is said to be as the communication pathway connecting two or more devices.
A key characteristics of a bus is that it is a shared transmission medium,as multiple devices are attached to a bus.

Typically, a  bus consists of multiple communication Pathways or lines which are either in the form of wires or metal lines etched in a card or board(Printed Circuit Board).Each line is capable of transmitting binary 1 and binary 0.

Computer System Contains a number of different buses that provide pathways between components at various levels of computer system hierarchy.
But before discussing them  i.e., types of buses , i will first describe here one of the most important aspect of buses which is described below-

Any Bus consists,typically of form about 50 to 100 of separate lines.And on any bus, the lines may generally be classified into three functional groups, as depicted in figure below:

 Now,we will discussing about each in brief one by one.

• Data Lines: 

          Data Lines provide a path for moving data between system modules.It is bidirectional which                     means data lines are used to transfer data in both directions.As an example, CPU can read data             on these lines drom memory as well as send data out of these lines to a memory location or to a             port.And in any bus the no. of lines in data lines are either 8,16,32 or more depending on size                 of bus.These lines , collectively are called as data bus.

• Address Lines: 

          Address Lines are collectively called as address bus.In any bus, the no. of lines in address are                 usually 16,20,24, or more depending on type and architecture of bus.On these lines, CPU                       sends out the address of memory location on I/O Port that is to be written on or read from.In                     short, it is an internal channel from CPU to Memory across which the address of data(not data)               are transmitted.Here the communication is one way that is, the address is send from CPU to                   Memory and I/O Port but not Memory and I/O port send address to CPU on that line and hence               these lines are unidirectional.

• Control Lines:  

          Control Lines are collectively called as Control Bus.Control Lines are gateway used to transmit                 and receives control signals between the microprocessor and various devices attached to it.In                 other words, Control Lines are used by CPUs for Communicating with other devices within the                 computer.As an example-CPU sends signals on the Control bus to enable the outputs of address             memory devices and port devices.Typical Control Lines signals are:

           -Memory Read                                
           -Memory Write                      
           -I/O Read

           -I/O Write                                        
           -Bus Request                      
           -Bus Grant, etc.

Operation of Bus

The Operation of Bus is as follows:

• If One module wishes to send data to another, it must do two things:

             1.Obtain the use of module Bus
             2.Transfer for data to the Bus

• If one module wishes to request data from another module, it must:

             1.Obtain the use of Bus
             2.Transfer a request to other module over the appropriate control and address lines.It must then                 wait for that second module to send the data.                                      
             

Types Of Bus

There are variety of Buses, but here i will describe only those that are widely used.

• System Bus:   

           A Bus that connects major computer components (Processor, Memory, I/O) is called a                             System Bus.It is a single computer bus among all Buses that connects all these                                       components of a computer system.And it is the only Bus, in which data lines, address,                             control lines all are present.It is also Known as "front side " Bus.It is faster than peripheral                         Bus(PCI, ISA, etc) but slower than backside Bus.

• Peripheral Bus(I/O Bus /External Bus):

          Peripheral Bus also known as "I/O Bus".It is data pathway that connects peripheral devices to                 the CPU.In other words , in computing, a peripheral bus is a computer bus designed to support               computer peripheral like printers,hard drives.The PCI and USB buses are commonly used                       Peripheral Buses, and are today used in commonly many PCs.Now we will discuss both of                       them in brief:

           

              PCI(Peripheral Component Interconnect):   PCI Bus connects the CPU and expansion                         boards such as modem cards ,network cards and sound cards.These expansion boards are                     normally plugged into expansion slots on the motherboard.That's why PCI bus is also                               known as expansion bus or external Bus.

   

              USB(Universal Serial Bus): Universal Serial Bus is used to attach USB devices like Pen                         Drive, etc to CPU.

• Local Bus:

           Local Bus are the traditional I/O(Peripheral) buses such as ISA,MCA, or EISA Buses.Now we                  will discuss about each in brief one by one.

               ISA(Industry Standard Architecture Bus): The ISA Bus permit bus mastering i.e., it                              enabled peripheral connected directly to the bus to communicate directly with other                                  peripherals without going through the processor.One of the consequences of bus mastering                    is Direct Memory Access.Up to end of 1990s almost all PCs computers were equipped with                      ISA Bus, but it was progressively replaced by the PCI Bus,which offer a better performance.

               MCA(Micro Channel Architecture):  It is an improved proprietary bus designed by IBM                          in 1987 to be used in their PS/2 lines of computers.

               EISA(Extended Industry Standard Architecture):  The EISA Bus use connectors that                            were same size as the ISA connectors but with 4 rows of contacts instead of 2 for 32 bit                            addressing. 

• High Speed Bus:

          High Speed Bus are specifically designed to support high capacity I/O devices.High Speed Bus               brings high demand devices into closer integration with the processor.This Bus supports                           connection to high speed LANs, such as Fast Ethernet at 100 Mbps, video and graphic                             workstation, firewire etc.