Computer_Interface _ Lec 2 _ Buses

Buses

Dr. Amr Elsayed

Simple Computer SystemSimple Computer SystemSimple Computer SystemSimple Computer System

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Simple Computer SystemSimple Computer SystemSimple Computer SystemSimple Computer System

�The main elements of a basic computer system The main elements of a basic computer system The main elements of a basic computer system The main elements of a basic computer system are: are: are: are: � CPU (Central Processing Unit or Microprocessor)CPU (Central Processing Unit or Microprocessor)CPU (Central Processing Unit or Microprocessor)CPU (Central Processing Unit or Microprocessor)� MemoryMemoryMemoryMemory� I/O interfacing circuitryI/O interfacing circuitryI/O interfacing circuitryI/O interfacing circuitry

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� I/O interfacing circuitryI/O interfacing circuitryI/O interfacing circuitryI/O interfacing circuitry� BusesBusesBusesBuses

� A bus is a collection of common electricalconnections grouped by a single name. Externaldevices such as a keyboard, display, disk drivescan connect directly onto the data, address andcontrol buses or through the I/O interface circuitry

Bus ComponentsBus ComponentsBus ComponentsBus Components

�The bus can either be anThe bus can either be anThe bus can either be anThe bus can either be an� Internal bus Internal bus Internal bus Internal bus (such as the IDE) � External bus External bus External bus External bus (such as the USB)

�Busses typically have a number of basic Busses typically have a number of basic Busses typically have a number of basic Busses typically have a number of basic components:components:components:components:

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components:components:components:components:� Data bus� Optional

� Address Bus� Control Lines� Handshaking Lines


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Bus SpecificationBus SpecificationBus SpecificationBus Specification

1. Data rate (in bytes per second or bits per second).2. Maximum number of devices which connect to the bus.3. Bus Reliability (A good bus should be able to detect if it

has received data which has been corrupted by noise)4. Data Robustness (Busses such as the CAN bus can

isolate incorrectly operating devices)

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isolate incorrectly operating devices)5. Electrical/physical robustness (This is the ability of the

bus to cope with electrical faults, especially due to openor short-circuits and power surges)

6. Electrical characteristics (Such as the range of voltagelevels used and electrical current ranges)


7. Ease-of-connection (USB)8. Communications overhead (This is a measure of the amount

of data that is added to the original data, so that it can be sent in areliable way)

9. Bus controller topology (This relates to the method thatis used to control the flow of data around the bus)

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is used to control the flow of data around the bus)7. SCSI, require a dedicated bus controller which is involved in all

of the data transfers.8. PCI bus can operate with one or more bus controller devices

taking control of the bus.9. Ethernet have a distributed topology where any device can

take control of the bus.


10.Software interfacing (This defines how easy it is tointerface to the bus with software)

11.Cable and Connectors (Coaxial cables use BNCconnectors while UTP cables use either the RJ-11 orthe RJ-45)

12.Standardisation of the bus (Most busses must comply

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12.Standardisation of the bus (Most busses must complywith a given international standard, which allowshardware and software to interconnect in a standardform)

13.Power supply modes (Some busses allow power savingmodes)

Data BusData BusData BusData Bus

� The data bus is responsible for passing data from onedevice and another. This data is either passed in� ParallelParallelParallelParallel (8 bits, 16 bits, 32 bits, or 64 bits wide)� SerialSerialSerialSerial (one bit at a time)

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Data BusData BusData BusData Bus

� ParallelParallelParallelParallel� Faster� Require many more lines� Requires handshaking lines to synchronise the flow of data

between devices.� Typically used for local busses

� SerialSerialSerialSerial

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� SerialSerialSerialSerial� Uses a start and end bits to define the start and end of

transmission.� External buses (Serial Communication)� can operate at very high transmission rates; the main limiting

factor is the transmission channel and thetransmitter/receiver electronics.

� For example, Gigabit Ethernet uses a transmission rate of1Gbps (125MB/s) over high-quality twisted-pair coppercables, or over fibre optic cables


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Data Transfer RatesData Transfer RatesData Transfer RatesData Transfer Rates

�TheTheTheThe datadatadatadata transfertransfertransfertransfer raterateraterate (in(in(in(in bits/second)bits/second)bits/second)bits/second) isisisis defineddefineddefineddefinedasasasas::::

� IfIfIfIf operatedoperatedoperatedoperated withwithwithwith aaaa fixedfixedfixedfixed clockclockclockclock frequencyfrequencyfrequencyfrequency forforforfor eacheacheacheach

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� IfIfIfIf operatedoperatedoperatedoperated withwithwithwith aaaa fixedfixedfixedfixed clockclockclockclock frequencyfrequencyfrequencyfrequency forforforfor eacheacheacheachoperationoperationoperationoperation thenthenthenthen thethethethe datadatadatadata transfertransfertransfertransfer raterateraterate (in(in(in(inbits/second)bits/second)bits/second)bits/second) willwillwillwill bebebebe

Data Transfer RatesData Transfer RatesData Transfer RatesData Transfer Rates

� For example, the ISA bus uses an 8MHz (8x106 Hz)clocking frequency and has a 16-bit data bus. Thus themaximum data transfer rate (in bps) will be:

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� For serial communication, if the time to transmit a singlebit is 104.167ms then the maximum data rate will be

Computer Bus ConnectionsComputer Bus ConnectionsComputer Bus ConnectionsComputer Bus Connections

�Normally there are several different types ofbusses which connect to the system, thesedifferent busses are interfaced to with a bridge,which provides for the conversion between onetype of bus and another.

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�Sometimes devices connect directly onto theprocessor’s bus; this is called a local bus, and isused to provide a fast interface with directaccess without any conversions.

Computer Bus ConnectionsComputer Bus ConnectionsComputer Bus ConnectionsComputer Bus Connections

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Data HandshakingData HandshakingData HandshakingData Handshaking

�Handshaking lines are also required to allow the orderly flow of data.

�The most basic type of handshaking has two lines:

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lines:� Sending identification line Sending identification line Sending identification line Sending identification line – this identifies that a

device is ready to send data.� Receiving identification line Receiving identification line Receiving identification line Receiving identification line – this identifies that

device is a device is ready to receive data, or not.

Simple Handshaking of DataSimple Handshaking of DataSimple Handshaking of DataSimple Handshaking of Data

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Data HandshakingData HandshakingData HandshakingData Handshaking

�The main types of communication are:� Simplex communication. Simplex communication. Simplex communication. Simplex communication. � HalfHalfHalfHalf----duplex communication. duplex communication. duplex communication. duplex communication. � FullFullFullFull----duplex communicationsduplex communicationsduplex communicationsduplex communications

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CablesCablesCablesCables

� TheTheTheThe cablecablecablecable typetypetypetype usedusedusedused totototo transmittransmittransmittransmit thethethethe datadatadatadata overoveroverover thethethethe busbusbusbusdependsdependsdependsdepends onononon severalseveralseveralseveral parameters,parameters,parameters,parameters, includingincludingincludingincluding::::� The signal bandwidth.� The reliability of the cable.� The maximum length between nodes.

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� The possibility of electrical hazards.� Power loss in the cables.� Tolerance to harsh conditions.� Expense and general availability of the cable.� Ease of connection and maintenance.� Ease of running cables, and so on.

CablesCablesCablesCables

� The main types of cablesThe main types of cablesThe main types of cablesThe main types of cables� Standard copper cable

� Unshielded Twisted-Pair copper (UTP) [Maximum lengths of 100 m]

� Shielded Twisted-Pair cable (STP)� Coaxial Cables [Maximum lengths of 1 km]

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� Coaxial Cables [Maximum lengths of 1 km]� Fiber Optic

Cable characteristicsCable characteristicsCable characteristicsCable characteristics

�The main characteristics of cables are� AttenuationAttenuationAttenuationAttenuation

� CrossCrossCrossCross----talk talk talk talk (defines the amount of signal that crosses from one signal path to another)

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� CharacteristicCharacteristicCharacteristicCharacteristic impedanceimpedanceimpedanceimpedance� Accurate characteristic impedance reduces

reflections gives a maximum power at the output.

Computer_Interface _ Lec 2 _ Buses

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