Technical Computer Peripherals Printer

The Printer

Printers attach to your personal computer system through a parallel port, commonly called the printer port. A parallel port transfers data 8 bits at a time in a parallel design to the printer. The printer or parallel port is sometimes referred to as the centronics port, after an early printer manufacturer called Centronics, which was selected by IBM for the original PC.

Most PCs come with a single parallel port as standard and it is sometimes integrated on the system motherboard instead of on an adapter board installed in one of the expansion bus slots.
Today's pentium PC's save on expansion slots and integrate the printer port function on a multi I/O port adapter.

Three basic types of printers are common on the personal computer desktop. Impact printers or dot matrix printers, laser printers and inkjet printers. There are more technologies on the market but they have not had the same impact.

Dot matrix was the dominant print technology in the home computing market in the days before the inkjet. Dot matrix printers produce characters by striking pins against an ink ribbon to print closely spaced dots in the appropriate shape. Print speeds, specified in characters per second (cps), varies from about 50 to over 500cps. Most dot-matrix printers offer different quality of print and this effects the speed of printing. Print quality is determined by the number of pins (the mechanisms that print the dots). Typically, this varies from between 9 to 24. The best dot-matrix printers are capable of near letter-quality type.

Laser printers became popular due to the high quality of their print and their relatively low running costs. As the market for lasers has developed, prices have gone down and down as manufacturers have found new ways of cutting costs. Output quality has improved, with 600dpi resolution becoming more standard, and build has become smaller, making them more suited to home use. Laser printers have a number of advantages over other inkjet technology. Laser printers produce much better quality black text documents, and they tend to be be more economic over the years.
If necessary, the printer first converts the print instructions into a bitmap. This is usually done by the printer's internal processor, and the result is an image (in memory) of which every dot will be placed on the paper. Windows printers use the host PC to create the bitmap, which then writes it directly to the printer's memory. At the heart of the laser printer is a small rotating drum, the organic photo-conducting cartridge (OPC), which has a coating that allows it to hold an electrostatic charge. A laser beam scans across the surface of the drum, applying positive charges onto the drum's surface in a way that will finally represent the printed image. The area of the drum is the same as that of the paper onto which the image will appear. Each point on the drum corresponds to a point on the piece of paper. Whilst the paper is fed in to the drum it passes through an electrically charged wire which deposits a negative charge onto it. The drum rotates to build one horizontal line at a time. As the drum rotates to present the next area to the laser, the written-on area moves into the toner. Toner is negatively charged therefore causing it to be attracted to the points of positive charges on the drum surface. After a full rotation, the drum's surface contains the whole of the required black image. As the sheet of paper completes it's rotation it lifts the toner from the drum which transfers the image to the paper. Negatively charged areas of the drum don't attract toner and result in white areas on the paper. The toner melts very quickly and a fusing system applies heat and pressure to the imaged paper.

Inkjets have the ability to produce colour, and that is what makes them so popular with home users. Although inkjets are generally cheaper to buy than lasers, they are more expensive to maintain. Cartridges need to be changed more frequently and the special coated paper required to produce high-quality output is very expensive. When it comes to comparing the cost per page, inkjets work out about ten times more expensive than laser printers over the year.
Inkjet printing emits ink from nozzles as they pass over a variety of possible media. Liquid ink in various colours squirts at the paper to build up an image. A print head scans the page in horizontal strips, using a motor to move it from left to right and back. Another motor rolls the paper in vertical steps. A strip of the image is printed, then the paper is moved up, for the next strip. The print head prints a vertical row of pixels at a time.
There are several types of inkjet technology but the most common is drop on demand (DOD), which works by squirting small droplets of ink onto the paper through tiny nozzles. The amount of ink propelled onto the page is determined by the driver software that dictates which nozzles shoot droplets, and when. Many of the problems earlier inkjets suffered has been mainly overcome, such as clogging jets and smudged paper.

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