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Printers



[1]Printed output, which can be read by humans rather than by a 1

computer, is the most common output. It is two-dimensional, consisting
of characters within a line, and lines on a page. One of the most difficult
parts of printing output is the reaction of the user to the printed page,
because somehow the printed output produces a response from the 5

reader much like that of people to a roomful of new furniture. They have definite ideas where each piece should go, but after seeing it there, they are apt to change their minds. The same occurs when the reader first sees the printed output of a report, for instance.

[2] No matter how simple or complex the data-processing operation is, the 10 final results must be made available in a form usable by humans, and usually in the form of a permanent record. This is the purpose of the printers used with computing equipment.

[3] Printers vary in speed, size, and cost, and are designed to meet printing

requirements ranging from roughly the speed and volume of a 15

typewriter up to thousands of lines per minute. There are two ways of printing: by impact(striking a character through a carbon, like a typewriter) or non-impact(photocopying, like an office copier). Impact printers are generally a drum,or a chainor train.The drum has a spinning cylinder for each character position in a line, and each cylinder 20 contains all the characters in a character set. As each cylinder spins into the appropriate character position, it is hammered onto the paper through a carbon ribbon. Drum printers generally give the lowest quality printing, because if the hammer hits a little early or late, the character will appear slightly above or below the line. This is noticeable 25 on a printed line, giving it a wavy appearance.

[4] In train or chain printers, the characters move laterally across the page,
and as the proper character moves into position, it is hammered onto
the paper through a carbon ribbon. If the hammer hits a little early or
late, the character will appear slightly to the right or left of its proper 30

position, but variations in horizontal spacing are not so noticeable to humans.

[5] Other types of impact printers are the daisy wheel printer,the matrix
printer,
and the line printer.The daisy wheel printer uses a wheel with
up to 100 characters, each being on an individual arm, so that only 35


rotation is needed. The hammer hits a single letter, guaranteeing accurate positioning and uniform striking. This allows a fast, quiet, and reliable operation, with superb print quality.

[6] A matrix printer, on the other hand, uses pins to print a pattern of dots

on paper. The characters are generated by selecting the appropriate 40

combinations of pins in a rectangle of 5 rows by 7 columns or 9 columns. Each pin is equipped with a coil, which, when energized, throws the pin against the ribbon, printing a dot on the paper. For each character, seven or nine lines of dots have to be printed; however, the inertia is minimal, so fast operation is possible. The general advantages 45 of using matrix printers are relative low cost, high speed, and quiet operation. The main disadvantage is the relatively poor quality of printing, which is sufficiently good for the eye, but not for reproduction or for business letters.

[7] Finally, the printing mechanism in line printers is completely different 50
from matrix printers. In the line printer, the character set to be printed
is on a cylinder where each segment has the full set of characters in
raised form around its edge. All segments are aligned with one another;
that is, all the 'As', for example, are in line. Whenever the printer is in
print mode, the cylinder is rotating and each character in the set passes 55
by the line to be printed as one complete revolution of the cylinder is
made. The printer times the movement of a print hammer so that the
character on the cylinder segment is brought into contact with the
ribbon and paper. One complete line of characters is printed during one
revolution of the cylinder. It is interesting to note that all the 'As' used 60
in the line are printed first, then all the 'Bs', and so on until finally all
the letters that make up the words have been printed and the line is
complete. Whereas 200 to 600 lines per minute maximum are produced
by the other printers, a line printer will operate up to 3,000 lines per
minute. 65

[8] There are several types of non-impact printers called thermaland

electrosensitive printers (electrostatic printers).These printers use a special chemically treated paper and expose the characters onto it by some means such as a laser. The characters are then fixed onto the paper by heating it. Because the printing element is simple and has no 70 moving parts, these printers are inexpensive and silent. These newer devices are also much faster and allow any character set to be used. In addition to drawing lines and shading, light can be flashed through a translucent slide to expose a drawing onto the paper. Very fast non-impact printers are capable of rates up to 13,360 lines per minute. 75

[9] In the largest computer installations, where cost is no longer a

consideration, non-impact printers are used for very high speed. An ink jetprinter operates by projecting small ink droplets and deflecting them electrostatically. Speeds of 40,000 lines per minute may be achieved. In


the laser writer,the paper is charged electrostatically and attracts dry so

ink powder, as in a Xerox machine. The pattern is then baked in the paper. Many lines are printed simultaneously, and speeds of 20,000 lines per minute can be attained.

[ioi Multiple copies of output are often required, and can be produced

either by printing the report several times or by using multipart paper, ss which consists of layers of paper separated by carbon sheets. However, non-impact printers cannot use such multipart paper.

[11] Microfilm is often used as an alternative to the printer. The output is 'printed' on microfilm rather than paper, which, in addition to being faster, also condenses large stacks of paper down into small amounts of 90 microfilm with no special programming. The drawback of computer-output microfilm (COM) is that it takes a special device to print the microfilm and a special viewer to read it.


T F

□ □ 3. Drum printers move sideways across the page.

□ □ 4. The matrix printer is excellent for business letters because the print is very clear.

□ □ 5. Line printers are much faster than other impact printers.

□ □ 6. Line printers print one complete word at a time.

□ □ 7. Thermal and electrostatic printers are capable of shading, whereas impact printers are not.

□ □ 8. Where speed is required, line printers are used.

□ □ 9. Laser writers are capable of printing more than one line at a time.

□ □ 10. Computer microfilm is printed using the regular printer.


 


Exercises




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