Hardcopy Devices

All the output devices can be categorised into two categories

Ø  Hard Copy Devices

Ø  Soft Copy Devices

Hard copy devices are those that give the output in the tangible form. Printers and Plotters are two common hard copy devices.

Soft copy devices give output in the intangible form or the virtual form, e.g. something displayed on a screen. All the computer monitors are covered under this category.

Printers

All the printers irrespective of the technology used can be categorised as

Ø  Impact Printers

Ø  Non Impact Printers

Impact printers are those printers in which there is a direct contact between the printing head and the paper on which the print is produced.

·         They work by striking a head or a needle against an inked ribbon which leaves a mark on the paper.

·         These printers produce a lot of noise when printing, because of the head striking the paper.

·         Examples are Dot Matrix, Daisy Wheel and Line printers.

In the case of non-impact printers the printing head never comes in direct contact with the paper.

·         These printers work by spraying ink on the paper.

·         Electrostatic or electromagnetic charge is used in these printers.

·         Examples are Ink-Jet and Laser printers.

Dot-Matrix Printers :

Ø  Dot Matrix is an impact printer.

Ø  These printer forms characters from individual dots.

Ø  These printers have a print head which runs back and forth on a paper.

Ø  The print head has a two-dimensional array of pins called dot matrix. There may be 9 to 24 pins in the dot matrix.

Ø  From this array of pins some pins are drawn out (or driven forward) to form the shape of a character.

Ø  The drawn out pins strike an ink soaked cloth ribbon against a paper. This forms that particular character on the paper.

Ø  Thus dot matrix printers can be used to print different fonts of characters.

Ø  Since mechanical force is used, carbon copies of documents can be taken.

Ø  40 to 250 characters can be printed per second.

Daisy Wheel Printers :

Ø  This is an impact printer.

Ø  Only preformed fonts of characters can be printed.

Ø  This printer contains a daisy wheel. Daisy wheel is made of plastic or metal. This holds an entire character set as raised characters moulded on each "petal".

Ø  A motor rotates the daisy wheel to position the required character between the hammer and the ribbon.

Ø  A small hammer then strikes the petal, which in turn strikes the inked ribbon to leave the character mark on the paper.

Ø  The daisy wheel and hammer are mounted on a sliding carriage similar to that used by dot matrix printers.

Ø  Different fonts cannot be printed using this technology.

Line Printers :

Ø  The line printer is a high speed impact printer in which one line is printed at a time.

Ø  600-1200 lines can be printed per minute.

Ø  Drum printer is an example of line printers.

Ø  These printers are very expensive.

Ø  These kind of printers were popular in the early days of computers, but the technology is still in use.

Drum Printers

·         In a drum printer, a fixed font character set is engraved onto a number of print wheels.

·         There are as many print wheels as the number of columns (letters in a line) the printer could print.

·         The print wheels are joined to form a large drum (cylinder),

·         This drum spins at high speed and paper and an inked ribbon is moved past the print position.

·         As the desired character for each column passes the print position, a hammer strikes the paper from the rear and presses the paper against the ribbon and the drum, causing the desired character to be printed on the paper.

Ink-Jet Printers :

Ø  Inkjet printer is a non impact printer, Core of an inkjet printer is the print head.

Ø  The print head contains an ink cartridge which has a series of nozzles that are used to spray tiny drops of ink on to the paper.

Ø  Ink cartridges come in various combinations, such as separate black and colour cartridges, colour and black in a single cartridge or even a cartridge for each ink colour.

Ø  A motor moves the print head back and forth across the paper.

Ø  Different types of inkjet printers form their droplets of ink in different ways. There are two main inkjet technologies currently used by printer manufacturers

o    Thermal bubble - This method is commonly referred to as bubble jet. In a thermal inkjet printer, tiny resistors create heat, and this heat vaporizes ink to create a bubble. As the bubble expands, some of the ink is pushed out of a nozzle onto the paper. When the bubble "pops" (collapses), a vacuum is created. This pulls more ink into the print head from the cartridge. A typical bubble jet print head has 300 or 600 tiny nozzles, and all of them can fire a droplet simultaneously.

o    Piezoelectric - This technology uses piezo crystals. A crystal is located at the back of the ink reservoir of each nozzle. The crystal receives a tiny electric charge that causes it to vibrate. When the crystal vibrates inward, it forces a tiny amount of ink out of the nozzle. When it vibrates out, it pulls some more ink into the reservoir to replace the ink sprayed out.

Ø  The ink droplets are subjected to an electrostatic field created by a charging electrode as they form. Charged droplets are separated by one or more uncharged “guard droplets” to minimize electrostatic repulsion between neighbouring droplets. The charged droplets pass through an electrostatic field and are directed (deflected) by electrostatic deflection plates to print on the Paper.

Laser Printers :

Ø  A laser printer is a non impact printer, which produces a page of text at a time.

Ø  Laser printer uses the principle of Static Electricity to print.

Ø  This printer has revolving cylinder called Drum.

Ø  Drum is given a positive charge.

Ø  A Laser beam is used to draw the image to be printed, on the drum with negative charge. This discharges some portion of the charge on the drum. This creates electrostatic image of the print on the drum with no charge, and the background is left positively charged.

Ø  The drum is then exposed to toner from which positively charged toner particles mixed with carbon black are released. Since positive charge repels positive charge, the toner particles settles on the discharged areas of the drum, this is exactly the image to be printed.

Ø  The paper is then pressed against the drum, this transfers the toner particles on to the paper.

Ø  Paper is then passed through a fuser, which is a set of heated rollers, this melts the carbon black on the paper to form the desired print. 

Plotters :

Another hard copy output device is plotter. Plotter is a printing device which can draw continuous lines. This is useful to print vector graphics rather than raster graphics unlike normal printers. Plotters are widely used in applications like CAD.

Ø  Plotters print by moving one or more pen across the surface of a piece of paper. This means that plotters are restricted to line art, rather than raster graphics as with other printers.

Ø  Pen plotters can draw complex line art, including text, but do so slowly because of the mechanical movement of the pens. They are often incapable of efficiently creating a solid region of colour, but can draw an area by drawing a number of close, regular lines.

Ø  Plotters offered the fastest way to efficiently produce very large drawings or colour high-resolution vector-based artwork when computer memory was very expensive and processor power was very limited.

Ø  There are a number of different types of plotters:

·         A drum plotter draws on paper wrapped around a drum which turns to produce one direction of the plot, while the pens move to provide the other direction.

·         A flatbed plotter draws on paper placed on a flat surface; and an electrostatic plotter draws on negatively charged paper with positively charged toner.

Ø  Pen plotters have essentially become obsolete, and have been replaced by large-format inkjet printers and toner based printers.

Ø   They are most frequently used for CAE (computer-aided engineering) applications, such as CAD (computer-aided design) and CAM (computer-aided manufacturing).