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April 4th, 2009
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Next to the monitor the printer is the most common output device. Until everyone has access to a computer we shall have to rely on printing on paper to communicate. Another factor to consider is the environment. When the cost of paper is high enough we are sure to consider our alternatives but up to the time of writing it seems that the many trees we destroy daily is not all that important to paper manufacteres yet.
This lecture on printers focuses on:
INTRODUCTION
There are two main classifications of printers. There are impact printers and non-impact printers.
IMPACT PRINTERS
Impact printers form characters on paper by striking a print head mechanism against an inked ribbon which produces characters on paper. These printers are slow cheap and noisy. The noise they produce can actualy contribute to stress in an environment where there is much printing.
The speed of impact printers is measured in characters per second or CPS.
The two main types of impact printers are Dot-Matrix and Daisy-Wheel.
Dot Matrix Printers
Dot-Matrix printers are so called because the print head mechanism is made up of pins which strike the inked ribbon to form characters. Dot matrix printers are suitable for draft copies and home use, where quality of the finished type is not critical. They are usually used by businesses to print cheques as it is believed they are less of a security risk than non impact printers because of the impact that is made on the paper each time a character is printed.
Near Letter Quality
Older printers used nine pins to form a single character but the more modern printers use 24 pins to create a character providing a high quality print called Near Letter Quality print. NLQ is as the name implies very close to the quality of print one gets from a typewriter. The individual dots which make up each character is not recognizable.
Daisy-Wheel Printers
Daisy wheel printers use a spoked wheel with characters placed at the end of each spoke. A print hammer is used to strike the desired character onto the ink ribbon and then the paper .
Daisy-Wheel printers are now out of date.
NON-IMPACT PRINTERS
The print head of non-impact printers does not make contact with the paper, and no inked ribbon is necessary. Non-Impact printers print more characters per second. They print on most paper types but better quality is obtained when better quality paper is used.
Types of Non-Impact Printers
There are three main types of non-impact printers :
Laser Printers
Laserjet printers are very common today. The speed of non-impact printers is measured in pages per minute (ppm), while the quality of print is expressed as dots per inch (dpi).
The print job is sent from the computer to the printer which converts the print information into a series of off and on commands to the laser, creating corresponding pulses of light a spinning mirror deflects the laser beam creating a horizontal line across the surface of a light sensitive drum after creating one horizontal line, the drum rotates the next horizontal line is drawn by the laser paper is drawn into the printer and electrically charged.
Each point where the laser strikes the surface of the drum, it creates a dot of positive charge. Each positive charge on the surface of the drum represents a black dot which will be printed onto the paper the drum rotates past a container bin which contains toner.
The toner is negatively charged, and thus is attracted to the positive areas on the drums surface which was struck by the laser beam.
Laser printers that are fast and produce the highest quality print are expensive.
Ink-Jet Printers
Ink-Jet printers have become the standard for low cost printing. They offer good quality at an affordable price and are very popular in homes.
Ink is forced through a small nozzle producing a small droplet of ink, which is propelled towards the screen surface.
Each drop of ink, after leaving the nozzle, is charged as it passes an electrode
Thermal Printers
Thermal printers are generally used in low cost printers and fax machines.
The print head contains high temperature heat elements arranged in a matrix when the print head is pressed againt the paper, the heat elements burn small holes in the paper, forming the character the paper is white and develops color (black or blue) when heated above 110 degrees C printing is generally slow the fumes generated are unpleasant a messy residue is left over by the heating process and can clog up the printer mechanism print quality is poor and fades with time. Special paper is required.
Draft Mode
It is common to find a printer being able to print in a low quality draft mode as well as a high quality mode using a larger number of dots per inch. The Draft mode saves ink and time and is used when higher quality is not required.
Printer Buffer - Memory
Printers carry there own memory called printer buffer. When print jobs are sent to a printer by the CPU they line up and wait to be printed. This allows the fast CPU to continue with its work while the slow output device tries to complete its job. Without adequate printer buffer print information can be lost when large print jobs are sent to the printer..
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