information systems course

Plotters A plotter is an output device for producing hard-copy of a drawing or design done on a computer. It consists of a flat surface on which a piece of paper can be fixed, together with a moving arm to which a pen is attached. The pen reproduces on the paper the design that has been built up on the computer screen. By automatically replacing the pen by another from a bank of pens at the side of the surface the plotter is able to produce multicoloured diagrams. The price varies from a few hundred to several thousand pounds, the more expensive plotters permitting larger sheets of paper and working at higher speeds. Communications devices and connectivity For long-distance data communications over the telephone line, devices such as modems and fax machines are used. For computer communications within a building, network devices and cabling are required. All these are covered in Chapter 8. Connectivity is the term applied to computer communi­cations and networking. In many of

Ink-jet printers Like dot matrix printers, these create the shape of characters from a pattern of dots on the paper, but in this case the dots are created by squirting particles of ink from a fine jet. The quality is higher than that of dot matrix printers, almost as good as laser printers (see below), and being non-impact they are much quieter. Colour ink-jets are also available. Ink-jets have recently come down in price, and they now compete directly with better quality dot matrix models. They are therefore likely to increase their share of the market in the future. Daisy wheel printers The print head in this type of printer is a daisy wheel, a circular device with a diameter of 3 inches which resembles a daisy flower. The print characters are embossed on the tip of each 'stalk'. When printing, the wheel rotates to bring the required character uppermost, and a hammer strikes it against an ink or carbon ribbon and so produces the printed impression on the paper. The

Output devices- printers Computer printers are used to produce hard-copy of com­puter output, normally data or text, but also, in the case of certain printers, graphics (i.e. image, such as drawings or charts). Printers vary in their capabilities so far as text enhancements (such as underlining or emboldening) are concerned, and also in their ability to print graphics. Most printers are designed to receive data 'in parallel' from the computer, 8 bits at a time. These have to be connected to the parallel port (socket) on the computer, also called the Centronics port, and sometimes labelled LPTI (short for Line Printer 1). The connecting cable will contain a number of parallel wires, and may take the form of a ribbon cable, so-called because it is flat and wide, like a ribbon. Some printers are designed to receive data 'in serial' from the computer, i.e. one bit at a time. These have to be connected to the serial port (socket) on the computer, also called the RS-232

Cathode ray tubes Most computer monitors are based on cathode ray tubes (CRTs), similar to those used in TV sets. They consist of one or more 'guns' which fire streams of electrons at a special chemical which backs the surface of the monitor. These electron streams repeatedly scan the screen from top to bottom, dot by dot and line by line, each scan taking only a fraction of a second. In colour screens, there are three guns, one for each of the colours red, green and blue. These cause each dot on the screen to generate red, green, or blue light, the combination of these three giving the full colour spectrum. CRTs give a bright picture, with good colours, but they are bulky and consume a relatively large amount of power. Monochrome models display text and graphics as either green on a black background, orange on a black back­ ground, or white on a black background. (The foreground and background colours can be reversed by software.) Monochrome monitors are quite cheap, and t

Optical (compact) discs Compact discs are the same size as standard 5.25 magnetic disks, but use laser light technology to store data. Because a laser light beam can be focused with a high degree of precision, the tracks on these discs can be much closer together than is the case with magnetic disks. As a result they have very high capacities, measured in hundreds of Mbytes (typically, 600 Mbytes). The same technology used for recording and playing music COs is used in computer compact discs. Another advantage is the fact that data is more secure on compact discs, as it can't be corrupted by magnetic fields. Data is encoded on an optical disc by burning tiny pits in its surface with laser light. A pit represents a binary 1, the absence of a pit represents 0. Laser light is also used to read data from the disc, the pits and non-pits setting up different light-interference patterns that can be detected by the reading head. Retrieval times are faster than floppy drives, though not

Floppy disks There are two types of magnetic disk: hard disks (also called Winchester disks), and the smaller floppy disks (also called diskettes). Figure 3.2 shows the standard 5.25-inch diameter floppy disk, and as you can see it is enclosed in a protective flexible casing with an opening cut out to give the read/write head access to the magnetic surface. This type of disk is extremely light, and the disk-drive is able to bring it up to the required speed of rotation almost instantly. The disk­ drive's motor only needs to be switched on by the computer when files are actually being accessed; the rest of the time the disk remains stationary in the drive. A disk must be formatted, i.e. magnetically configured to run on the particular computer system, before it can be used. This process marks out sectors on the surface within which the data is stored. In the case of PCs of the IBM­ compatible variety, each track on the disk is divided into nine sectors, and each sector is able

Video cameras and scanners Video cameras are versatile devices, being able to capture images of any type, including solid objects. Scanners are limited to images on paper, but they are able to scan each spot on the paper with much greater accuracy than cameras, and so are more widely used for this type of input. Scanners provide a low-cost way of inputting material that's been typed or printed on paper into a computer system. (I say 'low cost' because rekeying an A4 page costs between £4 and £5.) More was said on this earlier in this chapter on page 58. Most scanners incorporate a special sort of camera made up of charged-coupled devices (CCDs). Each CCD receives light from the image, and, provided the light is strong enough, will generate an electrical charge. This means that light areas or 'dots' of the image are represented by charged cells, and dark areas by uncharged cells. As the paper containing the image moves past the camera during the scanning process,

Introduction A peripheral is a device which is outside the central process­ing unit of a computer but controlled by it. There are four main types of peripheral, namely input devices such as keyboards, storage devices such as magnetic disks, output devices such as monitors, and communication devices such as modems. The last chapter dealt with computers in general and covered the work of the CPU. This chapter covers the peripheral devices that might be connected to it. Modems and other telecommunications devices will be covered later in the chapter that deals with telecommunications (Chapter 8). Input devices A variety of input devices exist, able to convert information in any form (data, text, speech, or image) into the binary pulses recognized by computers. Some of these devices are described below. The keyboard The keyboard is still the primary device for inputting information to a computer, though it may ultimately be superseded by voice input devices. It operates by

Portable computers At the moment, the largest hardware growth area within personal computing is the personal computer. As in other areas, PC-compatibles dominate the portable scene, with offerings from most manufacturers. Portable computers fall into four categories: • Mains-only portables, which in effect are small desktop PCs which can be easily carried around but which can't be used 'on the move'. Typically, these boast most of the facilities of desktop PCs - including hard disks - but weigh about 15 lbs. • 'Laptop' computers which also offer most of the facilities of desktop PCs but which include rechargeable batteries and so can be used on the move. These may weigh as much as mains-only portables, and the batteries may only run for about 3 hours between charges. • 'Notebook' computers which are much lighter - they weigh 6 lb or less - but which may lack some of the facilities of desktop PCs, such as hard disks. Some notebook computers, such as

Character-recognition devices One type of character-recognition device enables the user to input text and numerical data by handwriting it in capital letters on a pressure-sensitive surface using a special pen. An invisible grid of the fine wires below the surface detects the shape of the letters, converting them to electrical signals which the computer interprets using special software. Although this device is a genuine replacement for the keyboard, it has never really caught on, being overtaken by other developments, in particular the advances being made in speech recognition devices described later. Much more useful are the optical character readers (OCRs) which scan text which has been typed or printed on paper, converting the characters to the binary code that the computer understands. These provide a way of passing information between machines which cannot communicate electronically. For example, they enable output from a typewriter to be passed to a word processor for editin

The clones and the add-ons Whatever the defects of IBM's personal computer, it had two uniquely redeeming features that have greatly helped the IT revolution: • It was an open architecture machine, meaning that it was designed to be expandable by adding on additional circuitry. • Its design can easily be copied. The large growing PC market encouraged other manufactur­ers to exploit these features, to the advantage of everyone (except possibly IBM). • There are many companies offering a variety of expan­sion cards, i.e. circuit boards containing chips which can be fixed in the expansion slots inside the PC's casing. These cards offer a range of facilities and enhancements, including improved screen displays, speech recognition, greater processing power, and so on. • There are now many companies making and assembling personal computers which are virtually identical to the IBM PC and its successors. These are called PC-compat­ibles, or clones. They are able to run the

Computer generations Another way of classifying computers is by generation. • The first generation of computers were in operation in the 1950s, and their CPUs were built out of thermionic valves. As explained earlier, these computers were very large, expensive, and unreliable, and their performance was feeble by today's standards. (Today's computers are about a million times faster!) Also, they consumed a great deal of electricity and generated a lot of heat, so cooling systems had to be built in. Internal memory was by means of magnetic drums (similar in principle to today's magnetic disks), so memory access times were slow. • The second generation were in operation in the early 1960s, the thermionic valves being replaced by transis­ tors (which at that time had to be soldered together rather than incorporated into a chip). These were smaller, cheaper, and more reliable than valves, and they consumed less electricity and produced less heat. They were about a thousand

Read only memory Read only memory, or ROM, is a permanent store on microchip, normally used for holding programs. In this type of memory the tiny transistor switches cannot be turned ON or OFF, but are permanently encoded at the time of the chip's manufacture to produce the required program. These chips are called read only because it is not possible to write new programs or data to them. The advantage of using ROM chips instead of storing data on disk and reading it into RAM as required is: • It is more convenient to have frequently-used software immediately available inside the computer instead of on disk. • When the computer is running, all of the RAM is left free for data (though note that for the PC the 640K limit applies to both ROM and RAM). If the computer has a hard disk (see next chapter), the first of these is of little account, as the software can be almost as quickly loaded from the hard disk. However, ROM-based software is useful (sometimes essential)

Digital signal processors Digital signal processors (DSPs) are used in voice recog­nition systems (page 59), computer video applications such as interactive compact disc (page 169), complex mathemat­ical calculations, music synthesis, as well as in more standard bits of equipment such as disk controllers and modems. They allow the high speed processing of digital signals from audio, video, and other sources. DSPs are microchips optimized to carry out, at high speeds and with a high degree of accuracy, complex numer ical calculations. They incorporate a number of enhance­ments to increase the processing speed. These may include dual arithmetic logic units, separate program and data memories, and high-speed memory access. This makes them suitable for numerically-intensive processing applications such as those listed above. The first high-speed DSP was produced by AT&T in 1978. Since then Motorola, Texas Instruments, and others have produced DSPs, and these chips are now incor

The OR operator The second logical operator is OR. Using the party example from the previous sections, The electrical circuit shown in Figure 2.4 reproduces the effect of this table. In this circuit, the switches A and B are in parallel, and current X will flow if either A orB (or both) are closed. The NOT operator The third logical operator is NOT. In the example we have used already, NOT A means Ann is not going (to the party) and NOT B means Barbara is not going. So if Ann is going is true, then NOT A is false. The truth table representing this is: In electronics, a NOT gate consists of a special sort of switch which closes and so makes a circuit when no current is applied to it, and opens and so breaks the circuit when there is a current. Combining logical operators We can combine these logical operators (and the circuits which represent them) in any number of ways. Staying with the party example, Colin may decide to go to the party if: (A OR B) AND