Printers are output devices that print the result of an operation on paper. A printer is capable of producing output that contains printed words. It is also capable of printing straight lines and simple figures like squares, rectangles and circles. Printers are classified based on a number of parameters, like the mechanism used for printing, the speed of printing, the quality of output, the direction of printing, and the kind of interface they have with the computer. The following sections will elaborate on these parameters.


Depending upon the technique used for printing, printers can be classified into two broad categories - impact and non-impact printers.

In impact printers, characters are printed by pressing a typeface against an inked ribbon, which makes a mark on the paper. The most commonly-used impact printer is the Dot Matrix Printer (DMP).

In a DMP, an arrangement of tiny hammers or pins strike the ribbon to produce the desired characters. These tiny pins typically print in a matrix of 7 dots across and 9 dots down. The impact of the appropriate pins on paper through the inked ribbon forms letters made up of dots. There are printers which use up to 24 vertical pins to print characters with a higher resolution.

Dot Matrix Printer

Dot Matrix Printer

DMPs are inexpensive but noisy. They can print both text and graphics. They can print in any language without additional hardware. They can also be made to print in color by changing ribbons. These printers are used to produce internal reports and memos needed by organizations. The other types of impact printers are drum printers, daisy wheel printers and golf ball printers. However, these printers are not as popular as DMPs.

Examples of DMPs are: CENTRONICS-702, HP-2635A, OLIVETTI TC480 and Epson LX-80.

In non-impact printers, there is no contact between the typeface and the paper while printing. An example of a non-impact printer is the laser printer.

Laser printers use a light beam to form images on the paper using toner ink as the medium. The light beam strikes parts of a drum surface to form an image. Those parts of the drum surface, which are exposed to the light beam, become electrically charged. Only these electrically charged areas attract the toner ink particles. These toner particles are then deposited on, and permanently fixed to the paper using heat or pressure.

The other types of non-impact printers are thermal printers that use heat to print characters on paper and ink-jet printers that use jets of ink to print characters on paper.

Laser Printer

Laser Printer

Laser printers generate a very high quality output, both text and graphics, and are typically used for publishing. Most organizations use them for business correspondence, newsletters, brochures and presentations. Laser printers are expensive.

Examples of laser printers are: Xerox 4010 from Rank Xerox UK, LASERJet IlISi from Hewlett Packard USA and Lexmark Optra from Lexmark USA.


The speed of a printer is measured in terms of characters per second (cps), lines per second (Ips) or pages per minute (ppm). The speed of a dot matrix printer is measured in cps. The speed can vary from 200 cps to 540 cps. A line printer prints a line at a time. Its speed can vary from 5 to 50 Ips. A laser printer prints within the range of 4 to 20 ppm.

Quality of Output

Printers can operate in three different modes—draft, Near Letter Quality (NLQ), letter quality.

In the draft-quality mode, a DMP forms a character by arranging dots to assemble it. Although the characters can be distinguished, the output is not as good as that of near letter quality printouts. A laser printer prints in the draft mode by using less toner ink to form the characters.

For printing in letter quality, the DMP prints a character twice, thereby making its appearance darker. A laser printer prints in the letter quality by using more toner ink than it uses while printing in the draft mode.


Dot matrix printers and ink-jet printers can be unidirectional, bidirectional, or reverse. In a unidirectional printer, printing takes place in one direction only. After printing a line from left to right, the printer head (the component that carries the pins or characters) returns to the left without printing.

A bidirectional printer prints both ways, with logic-seeking capability, i.e. it selects the characters in the reverse direction while printing in reverse. A reverse printer has no such capability, and expects the computer to supply the characters in reverse order.


Depending upon the number of characters received at a time, printers can be serial or parallel. Serial printers are slower than parallel printers. A serial printer receives one character at a time for printing. A parallel printer receives more number of characters and is faster.