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Cards, readers and keyboards





[1] An essential requirement for making good use of computers is the 1

ability to put information into the machine. Until the early 1960s, one
of the most frequently used devices for providing input data to a
computer was the punched card,a major storage medium for computer
programs and data. Most people are very surprised to find that punched 5
cards were used as long ago as 1780 on textile machinery. However, the
first application of punched cards for the representation of large
quantities of data was made by Dr Herman Hollerith in 1890. Working
for the US Census Bureau, he realized that unless some means of
speeding up the analysis of census data were found, it would take more 10
than ten years to complete the job. He recognized the value of the
punched cards for this purpose, devised a code for representing data on
the cards, and invented the necessary machines to meet his needs.
Dr Hollerith went on to found a company to produce these machines,
which in 1924 became International Business Machines, or IBM for 15

short. Nowadays punched cards are rarely used.

[2] The use of punched cards actually required two separate pieces of
equipment. The first was a keypunch,which looked like a large
typewriter and was not physically connected to the computer; hence, it
was an off-linedevice. The second was a card reader,which, as its name 20

implies, read information from the cards. Unlike the keypunch, it was connected to the computer and was, therefore, said to be on-line.

P] Inorder to speed up input, some installations used magnetic tape or
disk as an intermediate input medium. Information was punched on
cards and then transferred from cards to magnetic tape or disk. It was 25

then transferred to the computer and stored in memory, one word at a time, by mounting the tape (or disk) on a tape drive (or disk drive) connected to the computer. This process is known as spooling.

[4] Today, most programming and data entry is done directly onto

magnetic tape or disk, eliminating cards and card readers — although 30

cards are still sometimes used as a backup systemin case of loss of data.
The instructions, or data, are typed on a keyboard, which records the
characters magnetically, and a screen shows what has been typed. When
a data set or program is complete, the disk or tape can then be read into
the computer at high speed. 35


(5] The most common cards used rectangular holes to store data, although
circular holes were also used (see Figure 13.1). The reason for one
corner of a card being cut off was so that the user had a reference point
when placing the cards in the card reader. Each time a keypunch
operator
pressed a key, the machine punched a number of holes in one 40
column of a card. There would be 1, 2 or 3 holes in any of 12 rows. For
this reason, each character was changed into a 12-bit word, which was
represented by writing a hole as a 1 and an unpunched area as a 0. The
cards usually had 80 columns; therefore 80 characters could be punched
on one card. 45

[6] Each of the letters, digits and characters was represented by the

particular pattern of punches in a column. Computer users could learn

to read the patterns of the holes, but this was unnecessary since the

characters were usually printed at the top of the cards at the same time

as the holes were punched. Because of this feature, both computer and 50

user could easily read a punched card.

m Once the information had been converted into holes in the cards, it was
ready to be fed into a card reader.This peripheral device was actually
attached to the computer by wires; hence it was on-line. The reader
examined a deckof cards one at a time by means of a light source with 55

photosensitive elements, which sensed the presence or absence of a hole. The printed characters were not read; these were only there to help the users interpret the cards. A modern card reader can read about 2,000 cards per minute.

[8] Obviously, cards could be used for storing binary information, with a eo

hole representing 1, and no hole, 0. If a card was used in this way, it was said to be a 'straight binary' card. More often, information was


entered in the Hollerith code or some other code. Each column had to

be read and interpreted individually, with a combination of punches in

that column representing a specific character. 65

[9] Modern key todisk machines use similar principles for storing data, but
in this case the data is stored as tiny magnets, one direction of
magnetisation representing 1 and the other 0. A common code uses 8
bits to store the code for each character; this is known as the American
Standard Code for Information Interchange (ASCII). 70




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