OUTPUT DEVICES. PRINTERS

Printers provide information in a permanent, human-read­able form. They are the most commonly used output devices and are components of almost all computer systems. Printers vary greatly in performance and design. Vfe will classify printers as character printers, line printers and page printers in order to identify three different approaches to printing, each with a dif­ferent speed range. In addition, printers can be described as ei­ther impact or nonimpact. Printers that use electromechanical mechanisms that cause hammers to strike against a ribbon and the paper are called impact printers. Nonimpact printers do not hit or impact a ribbon to print.

Character printers print only one character at a time. A type­writer is an example of a character printer. Character printers are the type used with literally all microcomputers as well as on computers of all sizes whenever the printing requirements are not large. Character printers may be of several types. A letter-quality printer is a character printer which produces output of typewriter quality. Letter-quality printers typically have speeds ranging from 10 to 50 characters per second. Dot-matrix print­ers form each character as a pattern of dots. These printers have a lower quality of type but are generally faster printers than the letter-quality printers — in the range of 50 to 200 characters per second. One of the newest types of character printer is the ink-jet printer. It sprays small drops of ink onto paper to form print­ed characters. The ink has a high iron content, which is affect­ed by magnetic fields of the printer. These magnetic fields cause the ink to take the shape of a character as the ink approaches the paper.

Line printers are electromechanical machines used for high-volume paper output on most computer systems. Their print­ing speeds are such that to an observer they appear to be print­ing a line at a time. They are impact printers. Trie speeds of line printers vary from 100 to 2500 lines per minute. Line printers have been designed to use many different types of printing mechanisms. Two of the most common print mechanisms are

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the drum and the chain. Drum printers use a solid, cylindrical drum, rotating at a rapid speed. Speeds of dram printers vary from 200 to over 2000 lines per minute. Chain printers have their character set on a rapidly rotating chain called a print chain. Speeds of chain printers range from 400 to 2400 lines per minute.

Page printers are high-speed nonimpact printers. Their print­ing rates are so high that output appears to emerge from the printer a page at a time. A variety of techniques are used in the design of page printers. These techniques, called electrophoto­graphic techniques, have developed from the paper copier tech­nology. Laser-beam printers use a combination of laser beam and electrophotographic techniques to create printer output at a rate equal to 18000 lines per minute.

MAGNETIC MEDIA DEVICES

Some of the devices mentioned above can perform both the input and output functions. Magnetic disc, magnetic diskette, and magnetic tape are examples of such devices. Magnetic discs, diskettes, and tapes can record data as output from primary stor­age and can also serve as input devices returning the data to primary storage.

Data are recorded on magnetic discs and magnetic tapes ei­ther by outputting the data from primary storage or by using a data recorder. Data recorders are not input devices, and they are not connected to the computer system. Instead they are offline recorders. The magnetic media recording devices are key-to-disk, key-to-diskette, and key-to-tape machines.

Key-to-disk devices are used as data recording stations in multistation shared-processor systems. They are able to correct data before storing it on a magnetic disk and before its entry into the main computer system.

Key-to-diskette systems store data on flexible discs, called diskettes. Diskettes are inexpensive and reusable.

Key-to-tape devices can record data on reels, on cassettes, and on tape cartridges. The magnetic tape reels produced by key-to-tape systems are in a computer-compatible format for subsequent direct data input into a computer. However, data on cartridges and cassettes often are transferred to higher-speed media, such as a full-sized reel of magnetic tape or magnetic disc, for data transfer to the computer.

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вариантам.

KEYBOARD DEVICES

1. There is a wide variety of keyboard devices, or terminals, available for use in entering data directly into a computer.

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The visual display terminal (VDT) is the most popular type of I/O device in use today. It consists of a typewriterlike key­board for inputting and a cathode ray tube (CRT) for display­ing output data. Each character entered through the keyboard is also displayed on the CRT. When keyed the data are held in a small memory, called a buffer, within the terminal itself. The data are not sent on to the computer until the operator presses an enter key on the keyboard. This allows the operator the op­portunity to proofread or verify the data being entered by read­ing the data displayed on the screen. There are three major uses of VDT's: alphanumeric displays, graphic displays, and input through a light pen.

Alphanumeric displays. The most common use of the visual display terminal is to display alphanumeric data, that is, char­acter data. Because of their relatively fast output rates and their ability to provide a viewer with an "instant" output, video dis­plays have replaced printers for many applications.

Graphic displays. Visual display terminals with a graphic dis­play capability provide a very powerful and versatile tool for many users. Graphic-display devices provide not only a means of displaying high-resolution drawings but also the capability of manipulating and modifying the graphic display. The busi-nessperson can use the graphic display to present data in the form of line charts, bar charts, or pie charts. Graphic displays can be very effective in information systems for business man­ager.

2. Different types of keyboard devices, such as visual display terminals, teleprinter terminals, and point-of-sale devices are among the keyboard devices.

A light pen is a photosensitive penlike instrument which can sense a position on the cathode ray tube (CRT) when the end of the pen is held against the screen. The light pen is an input device. By sensing the position on the screen when you touch it by the light pen, you are inputting data to the main storage. The light pen is commonly used by engineers to modify designs.

Teleprinter terminals. There are situations where it is desir­able to have a printed copy of data outputted to a terminal. If a user finds a printed copy to be required, the solution could be a teleprinter terminal. A teleprinter terminal has a keyboard for input and a typewriterlike printer for output. These printers are

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character printers and are therefore slower output devices than CRT displays.

A point-of-sale (POS) device is the electronic equivalent of a cash register, however it is capable of capturing more data than a cash register. Most point-of-sale devices are online terminals attached to a computer for processing the transaction while the customer is making the purchase. The significant features of most of the current electronic POS devices include: the capa­bility of entering extensive information about the sale, the guid­ing of the operator through the possible transactions by a series of lighted indicators or messages, a provision for transmission of the data to a central computer, and the provision for a local computational capability such as price extensions and tax cal­culations.

SCANNERS

Scanners provide a capability for direct data entry into the computer system. The major advantage of this direct data entry is that humans do not have to key the data. This leads to faster and more accurate data entry. The two major types of scanners

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are optical scanners and magnetic-ink character recognition devices.

Optical scanners are input devices that can "read" data re­corded on paper. The scanning techniques used involve a light source and light sensors; thus, they are called optical devices. The data to be scanned may be typed or handwritten charac­ters, data-coded as pencil marks, or data-coded as bars. The common optical scanner devices are called optical character readers, optical mark readers, and bar-code readers.

An optical character reader (OCR) inputs data by using opti­cal scanning mechanisms that can detect or scan alphabetic and numeric characters printed on paper. If the data are typewrit­ten, they must be typed using a special type font, called an OCR font. Examples of the use of OCR devices include the scanners used by the Postal Service to aid in sorting bulk mail, and as first-draft input for word processing system.

Optical mark readers (OMR) are able to detect pencil marks, made on special paper forms. The actual inputting of data through an OMR device involves shining a light on the page being scanned and detecting the reflections from the pencil marks. Pencil marks made with a soft lead pencil (high graph­ite content) will reflect the light. It is this reflection that the OMR device detects.

Optical bar-code readers detect combinations of marks or printed bars that represent the data. Bar codes have been used for a number of years for some types of credit card processing and by the post office for mail sorting. It is very common to use bar-code readers in conjunction with point-of-sale devices. The most widely known bar code is the universal product code (UPC), which now appears on almost all retail packages.

Magnetic-ink character recognition (MICR) devices were de­veloped to assist the banking industry. MICR devices speed up data input for the banking industry by reading characters im­printed on paper documents using a magnetic ink (an ink that contains iron oxide particles). Check and deposit form process­ing is the largest application of MICR.

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PERSONAL COMPUTERS

Personal computers are supposed to appear in the late 1970s. One of the first and most popular personal computers was the

133___________________________ Unit 10. Personal Computers

Apple II, introduced in 1977 by Apple Computer. During the late 1970s and early 1980s, new models and competitive oper­ating systems seemed to appear daily. Then, in 1981, IBM en­tered the fray with its first personal computer, known as the IBM PC. The IBM PC quickly became the personal computer of choice, and most other personal computer manufacturers fell by the way-side. One of the few companies to survive IBM's on­slaught was Apple Computer, which is sure to remain a major player in the personal computer marketplace. In less than a de­cade the microcomputer has been transformed from a calcula­tor and hobbyist's toy into a personal computer for almost ev­eryone.

What is a personal computer? How can this device be char­acterized?

— First, a personal computer being microprocessor-based,
its central processing unit, called a microprocessor unit,
or MPU, is concentrated on a single silicon chip.

— Second, a PC has a memory and word size that are small­
er than those of minicomputers and large computers.
Typical word sizes are 8 or 16 bits, and main memories
range in size from 16 К to 512 K.

— Third, a personal computer uses smaller, less expensive,
and less powerful input, output and storage components
than do large computer systems. Most often, input is by
means of a keyboard, soft-copy output being displayed on
a cathode-ray tube screen. Hard-copy output is produced
on a low-speed character printer.

— A PC employs floppy disks as the principal online and
offline storage devices and also as input and output me­
dia.

— Finally, a PC is a general-purpose, stand-alone system
that can begin to work when plugged in and be moved
from place to place.

Probably the most distinguishing feature of a personal com­puter is that it is used by an individual, usually in an interactive mode. Regardless of the purpose for which it is used, either for leisure activities in the home or for business applications in the office, we can consider it to be a personal computer.

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APPLICATION OF PERSONAL COMPUTERS

Personal computers have a lot of applications, however, there are some major categories of applications: home and hobby, word processing, professional, educational, small business and engineering and scientific.

Home and hobby. Personal computers enjoy great popularity among experimenters and hobbyists. They are an exciting hob­by. All hobbyists need not be engineers or programmers. There are many games that use the full capabilities of a computer to provide many hours of exciting leisure-time adventure.

The list of other home and hobby applications of PCs is al­most endless, including: checking account management, bud­geting, personal finance, planning, investment analyses, tele­phone answering and dialing, home security, home environment and climate control, appliance control, calendar management, maintenance of address and mailing lists and what not.

Word processing. At home or at work, applications software, called a word processing program, enables you to correct or modify any document in any manner you wish before printing it. Using the CRT monitor as a display screen, you are able to view what you have typed to correct mistakes in spelling or grammar, add or delete sentences, move paragraphs around, and replace words. The letter or document can be stored on a dis­kette for future use.

Professional. The category of professional includes persons making extensive use of word processing, whose occupations are particularly suited to the desk-top use of PCs. Examples of other occupations are accountants, financial advisors, stock brokers,

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tax consultants, lawyers, architects, engineers, educators and all levels of managers. Applications programs that are popular with persons in these occupations include accounting, income tax preparation, statistical analysis, graphics, stock market forecast­ing and computer modeling. The electronic worksheet is, by far, the computer modeling program most widely used by profes­sionals. It can be used for scheduling, planning, and the exam­ination of "what if situations.

Educational. Personal computers are having and will contin­ue to have a profound influence upon the classroom, affecting both the learner and the teacher. Microcomputers are making their way into classrooms to an ever-increasing extent, giving impetus to the design of programmed learning materials that can meet the demands of student and teacher.

Two important types of uses for personal computers in edu­cation are computer-managed instruction (CMI), and comput­er-assisted instruction (CAI). CMI software is used to assist the instructor in the management of all classroom-related activities, such as record keeping, work assignments, testing, and grading. Applications of CAI include mathematics, reading, typing, com­puter literacy, programming languages, and simulations of real-world situations