1. How printers operate

The peripheral capacity of transforming text and graphics from the digital form to its print form is called Printer, and it is an essential device for any PC user. In fact, long before the PCs, printers were used with the ancient computers as a main result presentation. And also, it seems to be that printers will not be abandoned in the future.

To get the printing process, all printers have 3 basic subsystems: hardware control, paper transport system and a paper printing mechanism. Hardware control governs the functioning of the printer´s components. The print mechanism makes the character and graphics well "drawn" on the paper. Finally, the transport system moves the paper vertically, making the ink go to the right place on the paper. (ie, to the correct line).

The origin of the information to print usually adopts three basic formats: text (ASCII code sequences), vector defined objects (mathematical) or bitmaps (defined as a group of dots)

In general, and as it used to be with screens, printers form images and text from dots called pixels. Printers are usually using RAM memory, which stores the bitmap of each character, and even a RAM Memory that allows the pc to send other fonts to the printer.

Basic Features
First, the printer speed is measured in pages per minute (ppm) or in character per second (cps). Nowadays, we always use ppm, and cps is used only for dot matrix printers. (not very extended in comparison with the rest of the types).

When interpreting the specified speed of the manufacturer, we must be really cautious and pay attention to the details: ¿how this speed is measured? Normally the manufacturers indicate that their printers reach 6 ppm, but they do not specify that this is on 5% of information, no graphics and low quality. They don't even count the calculus time used by the printer.

Printer's Resolution
A printers resolution is a parameter related directly to the printing quality. Indicates the number of dots (pixels) a printer can create on the paper by a surface unit. It can be measured in dots per inch (dpi), vertically and horizontally.

For example, a printer with a resolution of 600 x 300 dpi is capable of printing 600 dots on each 2,54 horizontal cm (inch), and 300 dots on each vertical inch. If only the number is indicated, the resolution is the same in both directions (for example, 600 dpi is a 600 x 600 dpi). We should not forget that resolution can not be converted to quality. If the printer presents a great resolution, but wont place the dots with correctly on the paper or the dots are too thick, the result won't be high quality.


The memory buffer
The memory buffer size it's another important data, because determines the performance on the communication between the PC and the Printer. The PC Works at a very higher speed that the printer. Therefore, without buffer the PC should wait constantly for the printer between deliveries. Thanks to the buffer, the PC sends the data to the printer, and makes other tasks while the printers process the information.
The bigger the buffer, the faster the printing. The normal size is 256 kB, although the newest printers have even MB.

Connection Interface
Finally, the last interesting parameter is the connection interface. Until recently the most common was the standard parallel PC port, using the Centronic connector of 36 terminals (watch post of this series on PC World nº 188, in june 2002).

There are also printers that work through the serial port RS-232, which minimizes the number of wires to use and allows deploying longer wires. However, the serial printing is much slower, so is not the most common connection interface. Today, the usb connection is the most common because of the speed against the parallel port.

Other common connections, normally shared with some of the mentioned, are the infrared ports, network or even wireless Bluetooth link.

Basic printing technologies
There are two basic printing technologies: the ones with impact (dot matrix and characters) and without impact.

Color Formation
On this series dedicated to the monitors we said that color is formed from the combination of three primary colors(red, green and blue).

CYMK System
In the case of printers, primary colors are complementary to the RGB system: Cyan (red complement), Yellow (blue complement) y Magenta (green complement). If these colors are mixed together, the result should be a black color. However, since this colors are note pure, you what you get is a brown color. This is why there is also a black color, making the system called CYMK (C for cyan, Y for yellow, M for magenta and K for black). In conclusion, a printer only has 4 different inks.
When printing a dot, the printer forms a "super dot", made by a group of dots from the different colors (CYMK). The eye (because of the low space resolution and for the distance on paper) it realizes a combination of colors, making us to see the super dot with the desired color.



Inkjet printers
Since the introduction in the second half of the 80s, inkjet printers have a great acceptance with great growing rate. Its price hasn't stop decreasing and its benefits haven't stopped growing. But what is an inkjet printer?

Inkjet printer
In this type of printers, a print head shoots tiny ink drops (with a 50micras diameter, smaller tan a single hair) on paper, forming text and images. The print head (which transports the ink cartridges) moves horizontally, while the sheet of paper moves line to line vertically, this is because of the paper transfer mechanism. The ink drops are positioned on the paper with extreme accuracy, reaching resolutions of 4800dpi. Since every dot can have a different color, it can generate images of high quality.
That every dot has a different color, images can be generated
In this kind of printers there are 2 basic technologies: bubble jet (by Canon) and desk-jet (by Hewlett-Packard), the difference between them is, basically in how the ink drops are created.



Bubble jet technology
In the bubble jet or de thermal injection, heat is applied over the ink, which is located into the deposit of the cartridge, which has several micro-conduits for the ink. This is achieved passing an electric current impulse through the resistances. The heat makes the ink go into boiling state, generating a bubble that grows in volume, and pushes the ink to the outside, through the conduits. This process lasts for approximately a millisecond, and unloads a default ink volume. The bubbles pressure produces a canon effect, which shoots a drop on the paper. Every time the electricity on the resistance ceases, the bubble disappears, therefore a suction effect happens that takes ink from the deposit and fills the conduits.

Deskjet technology
In the case of DeskJet printers, they use piezoelectric crystals as the main element, instead of resistances. Takes advantage of the basic features of a piezoelectric crystal: when electric tension is applied, there is a deformation of the crystal. Therefore, electric impulses are sent to the crystals, and the deformation produces ink pumping from the deposit to the micro conduits, shooting ink towards the paper. This operation philosophy is like an ink drop.

The most common cartridges usually have liquid ink, so it doesn't need any previous treatment before printing. However, there are cartridges in which, ink is solid. In this case, there are resistances to make the ink go from solid to liquid before is shot to the paper. During the path to the paper, ink gets solid, and stays onto the paper without being absorbed. This prevents a typical problem of common ink: the image remains "dry" and it is not necessary using special paper to avoid this effect.

Inks corresponding to DeskJet printers are usually cheaper, because they only have the cartridge, and no print head (this doesn't happen on bubble jet).

Laser printer
To understand the electrostatic print, you only need to know that electric charges either positive or negative, and that the charges with opposite charge attract, while different charges repel.



In the first place, the photosensitive drum surface gets charge with negative charge, of the size of a sheet of paper. Then, the drum starts moving line by line and a laser moves horizontally in every line, using a spinning mirror (in other words, a scan process is produced). The lasers will fall upon the points where ink is supposed to be fixed, reversing the charge (that will be positive). The laser shuts down on the places where there shouldn't be any ink (remaining with negative charge). Therefore, after going over the whole drum, there will be only positive charge in the spots were there must be ink, while the rest (which will be the white part of the paper) remains with negative charge. In other words, an electrostatic image of the page to print has been created, using positive charges over a negative charged background.

Positive charged dots
Positive charged points in the drum attract toner particles (electronegative material mixed with pigment that gives color to it). Therefore, the final image is drawn on the drum using black dots of toner.

The printing paper is charge with positive charges. Therefore, when making it pass through the drum, it will attract the toner particles (with negative charge), and the image will be finally formed on the paper. Finally, the toner adhered to the paper melts using heat, making it to be totally fixed to the paper. This is how you get to print a single page in one pass, contrary to the inkjet printers, where pages are printed line by line. Before printing a new page, an electrostatic sweep on the drum is made, leaving it ready for a new cycle.



Light emitting diodes printers
There is another variant of laser printers or sweep printers. Instead of a laser and a mirror system these have a tier of light emitting diodes (Laser-LED). For example, in a 300 dpi, there will be a LED tier covering a complete line of a paper covering a line of paper, at a reason of 300 LED per inch. They only turn on, for each line, those diodes that match where toner must be applied. This process is repeated line by line until processing the complete drum. Is the same effect as the laser scan, only faster.

Color toner
Liquid crystal Diode technolgy
Another variant uses liquid crystal diodes (LCD) instead of LED. These make a transparent material or opaque depending on the electric tension level that is applied. They will be forced to the transparent state those crystals corresponding to the spots where toner is supposed to be applied, keeping the rest of the diodes opaque. In the other hand, a halogen lamp is applied that illuminates all crystals, and light only goes through transparent diodes, reversing the charge on the drum.



Advantages and disadvantages of laser printers
Laser printers are faster than inkjet printers. Besides, they have more accuracy when placing the dots on the paper. They also save ink, because they deposit the right amount of toner, without exceeding that limit. Toner is not expensive in comparison to the ink cartridges and, besides, lasts more, which is profitable in an office environment, where large quantities of documents are printed daily. The main disadvantage is the price of these printers which is very high in comparison to the inkjet printers.