In order to use a computer,it is first necessary to insert the program and the data via the input devices.The Input devices to a Personal computer consist of keys on a keyboard .Numeric and alphabetic data are introduced via these keys.
When the program and data have been read and the necessary operations completed,it is necessary for the computer to deliver the results .A Great Variety of output devices are available . Business applications generally required that the accounting operation.Scientific results are more likely to consist of numeric data which must be clearly printed with little chance of error or graph showing the results of the calculations .For any of these applications ,one type of output devices ,many be more desirable than another. However , a great many applications require printers ,ranging from electromechanical typewriters,which print one letter or digit at a time,to high speed printer and an oscilloscope display, the two output devices used most . There are several important input-output devices in addition to keyboards ,printers and oscilloscope ,however and these are introduced in this
This is organized as follows, First input devices are discussed , followed by output devices .Then digital to analog and analog to digital converters are described
When the program and data have been read and the necessary operations completed,it is necessary for the computer to deliver the results .A Great Variety of output devices are available . Business applications generally required that the accounting operation.Scientific results are more likely to consist of numeric data which must be clearly printed with little chance of error or graph showing the results of the calculations .For any of these applications ,one type of output devices ,many be more desirable than another. However , a great many applications require printers ,ranging from electromechanical typewriters,which print one letter or digit at a time,to high speed printer and an oscilloscope display, the two output devices used most . There are several important input-output devices in addition to keyboards ,printers and oscilloscope ,however and these are introduced in this
This is organized as follows, First input devices are discussed , followed by output devices .Then digital to analog and analog to digital converters are described
Objectives
1. Input -output devices provide for communication between computers and humans ,computers and computers , and computer and external sensors and effectors .The Various devices such as tape ,cards, keyboards, etc
2. Alphanumeric codes provide for machine interfaces with humans where key boards and printers can be used to input and output information . Error- detecting codes provide for protection against errors .Both thee subjects are discussed , and examples are given
3. Digital-to-analog and analog-to-digital converters provide important interfaces between the outside world and computers.The Principles of operation of these devices , examples and general characteristics are presented along with a discussion of data collection from analog inputs
Card Readers
Most card readers are electromechanical devices which read the information punched into a card , converting the presence or absence of a hole to an electric signal representing a binary 0 or 1 . The Punched cards are placed in a hopper , and when the command to read is given , a lever pushes a card from the bottom of the stack.Generally , the card is moved lengthwise over a row of 80 read brushes.These brushes read the information Punched along the bottom row of the card. If a hole is punched in a particular row , a brush makes electric contact through the hole on the card , providing a signal which many be used by the computer . Then the next row up is read , and this process continues until all rows have been read , after which the next card is moved into position on the brushes.
Faster card readers are constructed by using photoelectric cells under the 12 punches positions along a column and an illuminating source above the card . As each column on the card is passed over the 12 photoelectric cells , whether a given position is punched is determined by the presence or absence of light on the responding cell.Card readers operate at speed of 12 to 1000 cards per minute.
Printers
A most continent and useful method by which the computer can deliver information is by means of printed characters . For the sake of convenience, the printer should have the ability to print alphabetic character ,decimal digit , and common punctuation marks.
The process in printing is the inverse of the encoding procedure in which a key corresponding to an alphanumeric character is depressed , causing a coded binary character to be punched into a tape or card. In this case,coded groups of binary bits are delivered to the printer , which decodes them and then prints the correct characters . The Basic binary-code groups may contain 7 through 9 bits depending on the coding for alphanumeric character that the printer provides.
The information delivered to a printer operated offline will be in the form of electronic signals directly from the computer . If the printer is operated offline,the reading and decoding of data stored on punched tape , Punched cards , or magnetic tape may be a part of the printing operation . Since the electronic circuitry of a devices, it is desirable that a printer operated online be capable of printing at a very high speed. Even if the printer is operated offline , Speed is highly desirable , since the volume of material to be printed may be quite large.
An example of a fast printer in which the raised characters are distributed around a print wheel that revolves constantly is shown . In this case the print wheel does not contain moving parts , but consist of a motor - driven drum with a number of bands equal to the number of characters printed per line .
Keyboards
The important data entry is the keyboard.In some cases keyboards are used to entry data into punched cards or punched tape , for example in other cases keyboards entry data directly into a computer . The most familiar keyboards are on terminals which include either a printer or an oscilloscope display , Sometime small keyboards with only a few keys . These small assemblies of keys are generally called keypads
When an operator depresses a key , electric signal must be generated which will enable the computer to determine which key was depressed. This is called encoding. The encoding process is dependent on the mechanism used to make the individual keys in the keyboards.
The most direct method for encoding is based on the use of keyboard switches, which contain a switch similar to the push button switch used in many electric devices . When the plunger is depressed , the contacts of the switch in the housing are closed , and the two terminals at the output
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