Printers and scanners are available in different types and forms, with varying capa- bilities and pricing, depending on the technology they are built upon. In this section, we will take a look at the printing and scanning technologies, their components, and how they are connected to computers.
Laser printers
Laser printers use a sharp beam of light, thelaser beam, to produce the text or image on paper. This is called the ElectroPhotographic (EP) process, originally developed by Xerox, and it is based on use of static electricity to transfer the ink, which is in the form of toner, to the paper according to electrical signals received from the computer. They are used for high-quality text and image printing, have faster speed than other printers, and have low cost of printing per page. Laser printers are known as page printers and their printing speed is represented as
pages per minute (ppm).
Components of a laser printer. An internal view of the laser printer is shown in Figure 2-14. Different parts of a laser printer are as follows:
Main power supply
The main power supply converts the AC voltage into DC voltage used in various circuit boards and other parts of the printer. These DC voltages include +24 volts, +5 volts, and –5 volts.
High-voltage power supply
The high-voltage power supply produces very high voltages used in the printing process. These high voltages are used to charge the EP drum, the toner, paper, and the corona wires.
EP drum
The EP drum is a revolving cylinder coated with a highly photoconductive material (also called aphotoreceptor). This drum is charged with high negative voltage (–600 volts) by the primary corona wire. As the drum rotates, a highly focused laser beam strips the charge from certain points on the drum.
Main motor assembly
This assembly is used for movement of different rollers in the laser printer.
Scanning motor assembly
Parts in this assembly contain motors and mirrors to move the laser beam across the EP drum.
Primary corona wire
The primary corona wire is used to charge the EP drum with a high negative voltage (–600 volts).
Transfer corona wire
The transfer corona wire is used to charge the paper surface with a high posi- tive voltage so that it can attract the negatively charged toner.
Writing mechanism
The writing mechanism guides the laser beam across the EP drum according to the image stored in the printer memory.
Toner cartridge
The toner cartridge is made up of several subassemblies that include toner, developer, and a cleaner blade.
Figure 2-14. Components of a laser printer
Fusing section Mirror, lenses, etc. Laser Toner Drum 4 3 2 1
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Fuser assembly
The fuser assembly contains heat and pressure rollers to properly bond the toner particles to the paper.
Erasing mechanism
The erasing mechanism is used to remove the image from the EP drum after the image has been transferred to the paper.
Cleaning assembly
This assembly is used to remove the residual toner particles from the surface of the EP drum.
Paper movement assembly
This assembly is used to move the paper through different parts of the printer.
Electronic control package
This unit (also known as the logic assembly) contains the main circuit board of the printer that communicates with computers to receive print jobs. This assembly contains the raster image processor (RIP) that converts the incoming signals into signals used by other sections of the printer.
Control panel
This is the main user interface located on the top of the printer and contains different controls/buttons for the user. High-end laser printers might include a touch screen as well as buttons to get user input.
Connection interface
This connects the printer to the computer or to the network. It can be a parallel, serial, USB, IEEE 1394, or a networkport (for networkprinters), depending on the type of printer.
Some laser printers use corona rollers instead ofcorona wires in order to transfer the high voltage. Corona rollers are considered environment-friendly because they help reduce the emission of ozone gas during the high-voltage transfer process.
The Laser printing process. The following is the printing process used in most laser printers:
Cleaning
The EP drum must be clean of toner particles leftover from the previous image before it can take a new image on its surface. A rubber bladein the cleaning assembly removes the particles of toner residing on the drum surface. The removed toner is collected in the debris cavity or waste reservoir located on one side of the cleaning unit. Adischarge lamphaving a specific wavelength then removes the remaining charge from the drum. After the charge is removed, the drum becomes electrically neutral.
Conditioning
The electrically neutral drum is insensitive to light and cannot take any image. A very thin wire, called theprimary corona wire, is used to distribute a high negative charge (–600 to –1000 volts) evenly on the surface of the drum. This negative charge again makes the drum photosensitive or photoconductive.
Writing
At this stage of the process, the printer’s laser beam writing unit and a series of mirrors are used to draw tiny dots on the EP drum, which represent the final image to be produced. The area of the drum that the laser beam comes in contact with loses some of its negative charge (by approximately –100V) and becomes relatively more positive (the charge is still considered negative, just not as negative as the areas not hit by the laser beam). When the laser beam has finished creating the image on the relatively positive EP drum, the printer’s controller starts the paper sheet-feed process by pulling a sheet of paper into the printer. The paper stands ready at the printer’s registration rollers until the controller directs it further into the printer.
Developing
The drum is rolled in a toner reservoir containing fine dry plastic particles mixed with carbon blackor coloring agents. These toner particles are charged with –200 to –500 volts. The charged toner particles are electrostatically attracted to the drum’s surface where the laser light left the image. The surface of the drum now holds the image pattern in the form of toner particles.
Transferring
The drum is pressed over rolled paper, which is positively charged. A different type of corona wire known as the transfer corona wire is used to charge the paper with very high positive charge. The positively charged paper attracts the toner particles from the drum leaving the image on the paper.
Fusing
The paper is passed through the fuser assembly containing pressure rollers
andheating rollers. The rollers in the fuser assembly apply heat and pressure to the paper to firmly bond the toner particles to the paper surface. The heating rollers provide up to 200 degrees Celsius of temperature.
Once the printing process is complete, the printer paper is rolled out in an output tray. At the same time, the EP drum is cleaned of the residual toner particles and stripped of the negative charge to make it electrostatically neutral.
For the A+ exams, you must remember the steps in the laser print- ing process. Memorize all the voltages given in these steps. Note that the primary corona wire is used to transfer a high negative charge to the EP drum while the transfer corona wireis used to transfer a high positive charge to paper.
Inkjet printers
An inkjet printer creates text or image on the surface of paper by spraying small droplets of ink. The droplets are very small in size and are positioned very precisely. Inkjet printers are very popular because they are inexpensive, easy to use, and can produce high quality text and graphics. The speed of these printers is expressed as pages per minute (ppm) and resolution as dots per inch (dpi). Inkjet printers fall into the following categories:
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Thermal Inkjet
These printers use water-, pigment-, or dye-based inks. The print cartridge contains small, electrically heated chambers. The printer runs a pulse of current through these chambers to produce steam that forms an inkbubble. This ink bubble is dropped on to the paper to form an image.
Piezoelectric Inkjet
These printers use a piezoelectric crystal in each nozzle instead of heating elements. The piezoelectric process uses crystals that react to electric charge. When charged, a crystal draws or pulls inkfrom an inkstorage unit held above the crystal. In simple terms, the piezoelectric process can cut or refine the exact amount of inkneeded to refine the dot placed on paper. This reduces the smudging effect of traditional inkjet technology and provides better printer resolution. In fact, this process allows resolutions greater than 1440 dpi.
Continuous Inkjet
These printers are mainly used for marking and coding of products. A high- pressure pump directs liquid inkfrom the inkreservoir into a small nozzle, thereby creating a continuous stream of tiny inkdrops. A piezoelectric crystal creates inkdroplets from the stream of ink. These tiny inkdrops are electri- cally charged, which are further directed to printing paper.
Components of an inkjet printer. Inkjet printers consist of the following components:
Printhead assembly
The printhead assembly consists of a printhead, inkcartridge, printhead stepper motor, and belt. The printheadconsists of a series of nozzles that spray the inkon to the paper. Theink cartridgecontains the ink, which can be just blackinkor a combination of different colors. Thestepper motor is used to move the printhead assembly backand forth across the paper surface. The belt is used to attach the printhead assembly to the stepper motor. A
stabilizer bar ensures that the movement of the printhead is precise.
Paper movement assembly
The paper movement assembly consists of a paper feed tray or feeder, a paper feed motor, paper sensors, and rollers. Thepaper feed trayorfeederis where the paper is loaded. The paper feed motor moves the paper through the printer with the help of rubberrollers. The stepper motor precisely moves the paper from the paper feed tray through the printer and drops it in the output tray after printing. Inputpaper sensorscheckthe availability of paper in the paper feed tray.
Power supply
The power supply unit of the inkjet printer converts the mains AC voltage into the DC voltages used by various motors and circuit boards.
Controller assembly
The controller assembly consists of electronic circuitry that converts the incoming signals and controls the movement of different mechanical parts.
Connection interface
The connection interface connects the printer to the computer or to the network. Depending on the type of printer, the interface can be a parallel port, a serial port, a USB, or an IEEE 1394 port.
Inkjet printing process. The inkjet printing process involves the following steps:
1. The printing process starts with the cleaning of the printhead.
2. The controller assembly initiates the paper feed assembly. The stepper motor engages a number of rollers to picka piece of paper from the paper tray and guides it into the printer. A sensor checks whether the tray is empty and gives an “Out of Paper” error.
3. The printhead stepper motor uses a belt to move the printhead assembly across the paper. This assembly stops at each point for a fraction of a second to spray multiple dots of inkon the paper surface and then moves again to the next position.
4. The paper feed motor moves the paper to the next line. This process continues until the printing process is complete.
5. Once the printing process is complete, the paper feed assembly pushes the paper onto the paper tray. The printhead is thenparkedin its home position.
Impact printers
Impact printers use a head or a needle that is hit against an inkribbon to place a markon paper. The paper is held firmly on a solid roller called aplaten. These printers produce significant noise when they are operating but are considered very efficient for printing multipart forms such as invoices. Some of the commonly used impact printers include the following:
• Dot matrix printers • Daisy wheel printers • Line printers
The dot matrix printer is the most commonly used printer for personal and small business applications. The line printer is used where speed and volume of printing is a main requirement.
The speed of line printers is expressed aslines per minute (lpm), and the speed of dot matrix printers is expressed ascharacters per sec- ond (cps). Similarly, the printing speed of laser printers and inkjet printers is expressed as pages per minute (ppm) because these printers are also calledpage printers. The printing speed varies by the quality of printing. The higher the resolution, the lower the printing speed.
Daisy wheel printers. As the name indicates, these printers use a wheel on the print- head that contains raised characters on its petals. During printing, the printhead moves the wheel to select the appropriate character on the petal in front of the electromechanical hammer (calledsolenoid). When the hammer strikes, it pushes the daisy wheel petal against an inkribbon, which leaves a markon the paper.
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The movement of the paper, printhead, and daisy wheel is precisely bound by the control circuitry on the logic board (motherboard).
Dot matrix printers. Dot matrixrefers to the way a printer creates text characters or images on paper. Dot matrix printers use a printhead containing a number of pins held vertically in one or two columns. Low-resolution printers have a printhead with 1 column of 9 pins while high-resolution printers have a printhead with 2 columns containing 24 pins. The pins strike on an ink ribbon that makes impres- sions on paper as small dots when the printhead moves backand forth. As the printhead moves in a horizontal direction, the printhead controller sends elec- trical signals and forces pins to strike against the ink ribbon. The timing of the electrical signals is programmed in the printer for every character it is able to print. The impressions appear as small dots and form appropriate characters on paper. It is possible to change the character style by electronically controlling how and when the pins strike the paper.
Thermal printers
Thermal printers use heated printhead pins that are pushed against heat-sensitive paper called thermochromic paperorthermal paper. Some fax machines (except plain paper faxes) and calculators with printing capability use the thermal printing process. These printers are inexpensive, but the cost of thermal paper is one of the considerations when calculating recurring cost of consumables. Thermal printers also use a combination of dots to create the text or image impression on paper. The following are two main types of thermal printers:
Direct thermal printers
These create images by burning a matrix of dots on heat-sensitive paper when the paper is passed over a thermal printhead. The area of paper where it is stroked by the heated printhead turns black, while other areas remain white.
Thermal wax transfer
These use wax-based ink, which is melted from the ribbon and transferred to the paper surface to create text or graphic images. The main disadvantage of this type of printer is that the same amount of inkis used, whether the output is full of text/images or only a part of the paper is printed.
Solid ink printers
Solid ink printers use sticks of solid ink instead of inkjet or toner cartridges. Once the stickis installed in a printer, the inkis melted and used to produce images on paper. These printers are mainly used in offices for printing high-quality graphics images. The printing process in solid inkprinters is similar to that used in offset printing. The solid inksticks are installed in printers. The inkis melted and fed into printheads that contain piezoelectric crystals. The printhead sprays the ink onto a rotating drum that is coated with oil. The paper is then passed over the drum that transfers the image onto the paper.
Solid inkprinters can produce images on a variety of paper and even transparen- cies. The image quality is superb and better than many other printing technologies. Solid inkis helpful in protecting the environment due to reduced
waste output. The disadvantages of solid inkprinters include high power consumption and long warm up times.
Printer interfaces
Printers are connected to computers using standard interfaces such as parallel, serial, SCSI, USB, or IEEE 1394. Networkcapable printers have a built-in network interface and are connected directly to a networkport. The following is a summary of different interfaces that can be used to connect printers:
Parallel
A parallel interface (IEEE 1284) works by sending an 8-bit parallel data stream to the printer. It uses a parallel printer cable, which has a DB-25 connector for connection to the computer and a 36-pin Centronics connector for connection to the printer. The maximum length of the parallel cable is usually limited to 10 feet.
Serial
A serial interface sends data to the printer one bit at a time. These interfaces need to be configured to serial communication parameters such as baud rate, parity bit, or start and stop bits. Serial printers are rarely used these days.
Universal Serial Bus (USB)
The USB is the most common type of printer interface used on small and medium-range printers (and many other peripherals). USB is faster than other types of interfaces. USB printers come with PnP-compatibility and can be automatically detected and configured by the operating system.
IEEE 1394
The IEEE 1394 (also calledFirewire) interface is not built-in on many printers or computers and is mainly used where highly demanding printing applica- tions are used.
Network
Most high-end printers come with a built-in networkadapter or can be upgraded by installing one. These printers can be directly attached to one of the free networkports and can be assigned a networkidentification such as an IP address. The printer uses a standard networkcable with an RJ-45 connector. In busy offices, a dedicated computer that is used to route all print jobs to the printer is called aprint server.
Wireless
Computers and printers can also be connected using wireless connections that support 802.11, Bluetooth, or Infrared standards. The main advantage of wireless connections is that both the computer and the printer can be moved freely so long as they remain within the coverage area of the wireless network. Wireless networks mainly rely on radio signals and are prone to electromag-