Unit 2.1
Hardware

INPUT DEVICES

An INPUT device is a peripheral which accepts data and sends it to the central processing unit.

Input devices considered:   

• Keyboard,     

• Mouse,          

• Joystick,        

• Light Pen                                            

• Bar codes,    

• Magnetic stripes

• Touch sensitive screens

• Document Readers : OCR, OMR, MICR

• Graphics Tablets

• Scanners

• Sound input

KEYBOARD

The most common method of input, because of its flexibility. Keyboards may be designed for a specific purpose (a customised keyboard) or may just be the standard ‘QWERTY’ keyboard.

There are a number of ‘keys’ supplied on the keyboard,

• Alphabetic keys A B C

• Other text characters > = <

• Numerical Keys + - * /

• Control Keys CTRL, Alt

• Function Keys F1, F2 etc

• And Cursor Keys

The process of transferring data from the keyboard straight to disc is called KEY – TO – DISC

In the case of data being transferred onto tape it is KEY – TO – TAPE

MOUSE

A mouse is an input device designed to fit snugly under the hand while it is moved over a table. Moving the mouse produces movements of a pointer on the screen. A mouse is also provided with one, two or three buttons

INVESTIGATE :   What is a TRACKER BALL?

JOYSTICK

A JOYSTICK is a device which enables the user to control movement on the screen by manoeuvring a small lever.

BAR CODES

A BARCODE is a set of parallel printed lines of differing thickness, which represent a number. Often the number represented by the barcode is also printed above or below the barcode.

When a barcode appears on shop goods, the number coded identifies the product, usually giving the code for: country of origin, manufacturer, item number and size of the object.

PRICE IS NEVER INCLUDED ON A BARCODE !!!!

A number of devices are available to read a barcode. These are commonly :

• a light pen

• a hand-held scanner gun

• a flat scanner

MAGNETIC STRIPES

A MAGNETIC STRIPE is a short length of magnetic coating printed onto the surface of a ticket or card.

To SWIPE a card with a magnetic stripe means to move the card through a reader so that it can be read.

Card which use a magnetic stripe may be hard plastic permanent cards : bank card, membership cards etc, or disposable cards such as train tickets, clothing tags etc.

INVESTIGATE : What is a SMART CARD ?

‘POINT AND TOUCH METHODS’

THE LIGHT PENA LIGHT PEN is a pen-shaped device, held in the hand, which can detect the presence or absence of light. The pen is used to select a point on the screen, the screen is ‘refreshed every fiftieth of a second and therefore the computer can work out where the pen is on the screen at a given instant.

(A light pen used to read a bar code works in a similar principle only that a LED detects the black and white bars)

A common use for light pens is in CAD (Computer Aided Design) – see later case study.

TOUCH SENSITIVE SCREENS

A TOUCH SCREEN is a screen through which data can be entered into a computer just by touching the screen with a finger.

The screen contains a fine pressure sensitive mesh which can detect the presence of pressure and the location of the pressure.

 ADVANTAGES :

• No other peripherals necessary

• System is very effective in situations where a keyboard or mouse would become wet or dirty.

DISADVANTAGES

• Can be quite slow – not suitable for inputting large quantities of data

GRAPHICS TABLET

To DIGITISE data means to convert it from analogue form to digital form

A GRAPHICS TABLET or DIGITISING TABLE is a board which can detect the position of a pointing device on its surface. 

The drawing is placed on top of the digitising table. The outline of the shape is then traced using the stylus (pen). The computer can pick up the position of the moving pen and therefore forms a ‘tracing’ of the drawing on the screen.

A series of further menus are normally available of the graphics tables similar to those found in a graphics package – enlarge, fill, select, cut, rotate etc.

SCANNERS

A SCANNER is a device used to examine and pictures, text or other information and represent them as computer data.

Scanners are of two main types :-

HAND – HELD SCANNER

Which moves across the picture being scanned:

FLAT – BED SCANNER

The picture is laid flat on a glass surface and the scanner remains stationary while the picture is being scanned (similar to a photocopier only the image comes out on the computer screen rather than on paper)

As well as digitising pictures, scanners can also be used to capture text from a document (see later notes on OCR). Many scanners can also recognise barcodes.

One disadvantage of scanners is that scanned images tend to take up a lot of memory.

VOICE INPUT (Voice recognition)

This is one of the areas which is developing fast because of its obvious benefits. Using a microphone, human speech is coded into a sequence of electrical signals. When voice input is detected, the computer searches its memory for pre-stored patterns for the sound which has been input.

The current problems with voice recognition are

• the memory space required to store the vast array of patterns

• the speed at which the processor has to ‘process’ the signals

• the fact that everyone’s voice is so different – accents and dialects as well as other factors which affect your voice – a cold, your mood, the time of day etc..

HANDWRITTEN INPUT

Again, this would be an extremely useful tool to have – problems are that everyone’s handwriting differs so much and your own handwriting can itself vary.

The process involves scanning the handwriting, separating individual letters and then deciphering what letter it is most likely to be.

Direct Data Capture

Data is collected using Documents and forms. Forms often take the form of questionnaires, which are filled in and the data is read from these SOURCE DOCUMENTS and transcribed into the machine by data preparation staff. TRANSCRIPTION ERRORS can be caused by the data being typed incorrectly. Direct data capture avoids these errors.

Methods of direct data capture.

Optical Mark Recognition  (OMR)

Optical Mark Readers can detect marks made on specially prepared documents. The school roll sheets are an example of these.

Advantages

• Data is quickly and usually accurately captured

• The data preparation stage is cut out

• No machine is needed to complete the source document

Disadvantages

• Only allows for a lmited number of alternatives

• Marks must be clear or they will be misread

• Source documents must be kept clean

• Cost of designing forms and purchasing equipment is high

Optical Character Recognition (OCR)

These can read letters or numbers, rather than just marks. As with OMR a light is shone on the document and marks on the document will not reflect light back, thus the OC reader can detect writing.

TURNAROUND DOCUMENTS are a useful application of OCR. These are documents which are produced by the computer, sent out to be filled in, and are then used for data input.

Advantages

• Data is quickly and accurately captured

• Data preparation is minimal

• OCR characters can be read both by eye and machine

Disadvantages

• High quality printing is necessary

• Documents must not be damaged

• Staff still needed to re-input data that has been read incorrectly

• Expensive for small volume applications

Magnetic Ink Character Recognition

Magnetic ink documents are printed with a special magnetised ink, these are read by Magnetic ink readers.

Advantages

• MICR characters can be read by botgh eye and machine

• MICR characters can be read even when dirty

• MICR offers high security

Disadvantages

• High quality printing is required

• Equipment is expensive

Bar Code Readers

Bar codes are very common in modern society, everything from groceries to books have bar codes attached. Data is stored in a series of thick and thin black and white lines.

Advantages

• Easy data capture

• No need for printing since the bar code is normally built into package design

Disadvantages

• Can only be used for simple codes

• Cannot be used to collect additional data like OCR and OMR

OUTPUT DEVICES

Once data has been input and processed, some form of output will have to be produced. The output may be in the form of :

Output as DATA

This is output in the form which is understood by the computer

• output onto a disc

• output as a communication signal

• output to control a device

Output as INFORMATION

This is output which is understood by humans

• output on a computer screen

• printed output

• computer output on microfilm

• voice output.

OUTPUT TO SCREEN

• In computer jargon, a computer screen is referred to as a MONITOR or a VISUAL DISPLAY UNIT (VDU)

• The screen is made up of a number of tiny dots (PIXELS – ‘picture element’).

• The RESOLUTION of the screen is a measure of how fine the detail is on it. The more pixels the clearer the image and the higher the resolution.

• The resolution of the screen is normally given as a measure of the number of ‘dots per square inch’ (dpi)

• A SCREEN DUMP is a printed copy of the screen output (i.e. what is displayed on the screen will be printed on paper exactly as it appears)

• A GRAPHICS DISPLAY UNIT (GDU) is a screen with an extremely high resolution and is used for applications where the clarity of the image is very important.

PRINTED OUTPUT

Printers provide a HARD COPY of the output.

Printers are broadly classified into four groups:

CHARACTER PRINTERS that print a character at a time

Examples

• Dot Matrix Printers

• Daisywheel Printers

• Inkjet printers

LINE PRINTERS that print one line at a time

Examples

• Drum Printer

• Chain Printer

PAGE PRINTERS that print one page at a time

Example

• Laser Printers

GRAPH PLOTTERS that are used to print out specialised graphics

Examples

• Flat Bed Plotter

• Drum Plotter

PRINTER STATIONARY

Paper can be fed through a printer in a number of ways:

 TRACTOR FEED

The paper used will have a set of holes punched along the left and right hand side (line-feed paper). The paper is pulled through the machine by a set of wheels with sprockets. Usually tractor feed paper is of poor quality but can be fed through the printer at high speed. 

FRICTION FEED

This is where single pages are fed through the printer one by one. Different sizes of paper can be used including pre-printed paper and plain paper.

COMPUTER OUTPUT ON MICROFILM (COM)

A page of output is photographed, miniaturised, and then reproduced on microfilm.

Typically, about 200 A4 pages can be miniaturised and copied onto a post card sized acetate sheet called a MICROFICHE or copied onto a roll of MICROFILM.

The microfilm is read using a microfilm reader that enlarges the sections of the sheet to a readable size.

VOICE OUTPUT (Voice synthesis)

Voice output from computers is generally easier and more successful than voice input (voice recognition).

Voices can be successfully synthesised (made artificially) by storing word patterns as a binary code.

Getting the computer to say "HELLO" uses three sound HHHH - L - OH

The sound "HHHHH" which is stored as 0001110110

The sound "L" which is stored as 1010000101

The sound "OH" which is stored as 0110001111

The binary pattern 1000111011010100001010110001111 would sound like HELLO

BACKING STORAGE DEVICES

There are two types of storage used by a computer :

IMMEDIATE ACCESS STORE (IAS)

This is memory inside the CPU of the computer and is normally found in the form of memory chips. IAS is extremely fast memory and is where data is stored while it is being used (processed). The size of the IAS memory depends on the computer and although its size is increasing it is still very small in comparison to the storage space required by a computer.

This type of memory is discussed later.

BACKING STORAGE (AUXILIARY STORAGE)

This is storage space for data and programs that are not currently in use. Common examples are MAGNETIC DISC, MAGNETIC TAPE, and OPTICAL DISC. These are called backing storage MEDIA (singular MEDIUM)

Backing storage can be of two types:

• FIXED where the medium is a fixed part of the machine (Hard Drive)
• EXCHANGEABLE where the medium can be removed from the machine and stored or transferred to another machine

HOW IS DATA STORED?

A REMINDER OF A FEW FACTS

• The computer changes a CHARACTER (letter, number, symbol) into a BINARY (two state) code before it can understand it.
• The most common method is the ASCII Code (American Standard Code for Information Interchange)
• A BIT is a binary digit (1 or 0) and a BYTE is a group of 8 bits.

Data is stored electronically, the character is first converted into a binary pattern.

In the case of a magnetic medium (magnetic disc or magnetic tape) the binary pattern is stored as magnetic patches with different polarities. (1 is North, 0 is South)

In the case of optical media small ‘pits’ can be formed on the surface (1 is pit, 0 is no pit)

MAGNETIC DISCS

A magnetic disc is a circular disc coated with a magnetic surface (Iron Oxide).

When a disc is used for the first time it is FORMATTED which means that the surface of the disc is prepared so that it is ready to store data. This preparation involves the computer mapping out the surface of the disc.

The disc is divided into BLOCKS, SECTORS and TRACKS with each sector being separated by an INTER-BLOCK GAP.

DIAGRAM TO SHOW THE LAYOUT OF THE MAGNETIC DISC

 Data is converted into binary (using the ASCII code)

The data is stored as magnetic ‘patches’ along the circular tracks.

QUESTION

How is data read off a disc?

What is a READ/WRITE HEAD?

Why does the disc spin?

HARD DISCS AND FLOPPY DISCS

A HARD DISC is a rigid magnetic disc (normally a metal disc covered in a magnetic coating) whereas a FLOPPY DISC is a thin plastic disc covered in a magnetic coating.

CHARACTERISTICS OF HARD DISCS

• The discs are normally fixed into the computer (the hard drive) each is built into a sealed unit to prevent contamination by dust and moisture
• Access to a hard disc is much faster than a floppy disc
• Hard discs store have a much higher storage capacity than floppy discs
• Hard discs are more reliable than floppys.

Some computers use DISC PACKS which are a series of hard discs held on a protective case which can be removed, stored and re-inserted at a later date or into another computer.

CHARACTERISTICS OF FLOPPY DISCS

• This is a smaller disc (commonly 3 � inches) and they are protected by a stiff plastic cover. There is a hole for the read-write head which is protected by a sprung cover.
• The discs are exchangeable and easy to transport

C D – R O M

Types of Compact discs : Music CD - standard music discs

Photo CD - discs containing photographic images

CD-ROM - compact discs designed for use as computer storage.

CD-ROM stands for Compact Disc Read Only Memory

CHARACTERISTICS OF CD-ROM

• Normally about 12cm in diameter
• Data is written on to the disc using a powerful laser beam to burn binary patterns in the surface. Data is read back using a relatively low-power laser to detect the pattern on the surface
• Discs are exchangeable and easy to transport
• USUALLY data is written onto the disc before it is sold. After that data can be read from the disc but not written onto it (hence ROM)

MULTI-MEDIA means the presentation on a computer of information combining animated and still graphics, sound and text.

MAGNETIC TAPES

Magnetic tape is the main type of serial access store.

QUESTION

What is the difference between SERIAL ACCESS and RANDOM (DIRECT) ACCESS?

 Originally computer tape was in separate reels. However, increasingly tape is being used in the form of a CARTRIDGE. This is better protected from dirt, is much more compact and is easier to use.

APPLICATION OF MAGNETIC TAPE:

THE TAPE STREAMER

A school has a network of PCs controlled by a fileserver with a hard drive. Attached to the fileserver is a magnetic tape unit called a tape-streamer. This holds a tape cartridge, which will store, on average, 160 Mb of data.

Every week the technician (or someone in charge of making back-ups) runs the ‘archive’ program which copies all of the important files onto the tape which is then removed and stored separately from the fileserver.

If a file is corrupted or deleted accidentally, then the tape can be re-loaded and the previous data restored.

THE CENTRAL PROCESSING UNIT

The CENTRAL PROCESSING UNIT (CPU) is the brain of the computer – it is where the processing takes place.

Functions -

• Store data and instructions which are currently in use

• To control the sequence of operations

• To carry out the processing (of DATA using a PROGRAM)

• To control all parts of the computer system.

A PERIPHERAL is any device connected to the CPU of the computer system

An INTERFACE is some hardware, and possibly some software, that is used to connect two devices or systems to enable them to communicate.

Because different parts of a computer system operate at different speeds :

• everything inside the CPU is extremely fast

• printers are relatively slow

• the hard drive is quite fast

• a floppy disc is quite slow etc,

the computer uses BUFFERS which can be thought of as ‘temporary waiting rooms’. When data is to be printed, it can be loaded into the output buffer thus freeing the CPU.

Data can be loaded into the input buffer slight before it is needed by the CPU so it can be used when ready and the CPU does not have to wait.

The CPU contains three components :-

• The IMMEDIATE ACCESS STORE (IAS)

• The CONTROL UNIT

• The ARITMETIC LOGIC UNIT (ALU)

each has a specific function. We will look at each in detail

THE ARITHMETIC LOGIC UNIT (ALU)

• This is where the program instructions are carried out.

• Calculations are done or actions carried out by the circuits in the ALU

• The ACCUMULATOR is the storage register where the results of a calculation or data are stored.

(A REGISTER is just a temporary storage location)

• Some logical operations of the ALU give the computer its decision making ability.

THE IMMEDIATE ACCESS STORE (IAS)

• Also known as the INTERNAL MEMORY or the MAIN STORAGE (RAM and cache memory in hardware specifications)

• It holds the programs and data which are currently in use (the backing store holds the programs and data which are not currently in use

• Data is ALWAYS transferred to the IAS before it can be processed or output

• Data is INSTANTLY accessible from the IAS – it is EXTREMELY FAST memory

HOW IS MEMORY ORGANISED?

The internal memory is divided into a number of storage compartments called LOCATIONS or CELLS. Each location is identified by means of an ADDRESS.

As mentioned before, everything inside the computer is in binary......

• So consider the case where Cell 6 is to hold the number 27

• Cell 7 is to hold the letter ‘D’

• Cell 8 is to hold the instruction 100 01001

• Cell 6 ( in binary 6 = 0 1 1 0) is to hold 27 (in binary 27 = 00011011)

• Cell 7 ( in binary 7 = 0 1 1 1) is to hold ‘D’ ( D according to ASCII is 01000100 )

• Cell 8 (in binary 8 = 1 0 0 0 ) is to hold the instruction " 10001001"

The following terms commonly encountered when discussing memory.

Define them

• VOLATILE memory

• CACHE memory

INTERNAL MEMORY CHIPS

A computer has a number of ‘types’ of memory chips :

• RAM RANDOM ACCESS MEMORY

• ROM READ ONLY MEMORY

• PROM PROGRAMMABLE ROM

• EPROM ERASABLE PROGRAMMABLE ROM

THE CONTROL UNIT

The CONTROL UNIT supervises the execution of the program instructions.

It controls:

• the ORDER in which instructions are carried out

• the timing of each operation

It contains a number of components :-

CONNECTING THE UNITS

When showing the connection between the main functional components (control unit, ALU, IAS)

We normally include :

The CONTROL BUS - the connections along which the contrrol signals travel

The DATA BUS - the connections along which the data can travel.

THE FETCH – EXECUTE CYCLE

This is the accurately timed sequence by which the control unit ‘fetches’ an instruction from the immediate access store, decodes it before fetching the next instruction

At the start of the program the PC is given the address of the first instruction

Fetch Cycle

• The PC passes the address to the MAR

• The MAR passes the address to the IAS

• The address in the IAS is accessed and the instruction ins passed to the MDR

• The MDR passes the instruction to the CIR

The instruction is decoded

Execute Cycle
The execute cycle will change depending on the instruction that is being executed. The example given is for a typical mathematical instruction like ADD

• The ASR passes the address of the data to the MAR

• The MAR passes the address to the IAS

• The address in the IAS is accessed and the data is passed to the MDR

• The MDR passes the data to the ALU where the calculation is performed and the result stored in the ACC

The PC is incremented (increased by 1)