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PC Repair 3 Lecture #4 - Introduction to Printers

Lecture Only
The student will become familiar with:
The various types of printer technologies,
Each technology's functions and capabilities,
The technical details of each technology,
Know how to install and troubleshoot a printer and its drivers.

The student will become familiar with the various printer technologies including their features, capabilities, limitations and be able to choose the best one for a given business situation as well as be able to select the best solutions and be able to install and troubleshoot these devices.

  1. Since the PC revolution the phrase "paperless society" has emerged. However, society appears to be far from paperless especially in the business industry. The single most persistent obstacle to a paperless society is security and demonstrability. That is, no one can truly assure you that your purely digital money (data values within a computer) is safe from "crackers" or even computer "glitches". And no one can demonstrate a single bit in a court of law like they can a piece of paper. We have seen digital evidence in courts of law but a good lawyer could always argue that "How do you know where that bit on that hard drive came from? My client's fingerprints are not encoded in it."

  2. There exist two forms of result or presentation of data from an application software: softcopy and hardcopy. Softcopy is the information in the form of a file or even on screen. In softcopy the information is still in digital storage whether it is in RAM or disk drive tracks. Hardcopy means the data has exited the computer and is immediately available to the human user. It can be handled. This is usually in the form of printer information on paper.

  3. Printers therefore are the primary form of hardcopy output device for the computer. There exist four major categories of printer: impact, thermal, inkjet, and electrophotographic (EP).

  4. There are two main types of impact printer: the dot-matrix and the daisy wheel. Daisy wheels used a spinning wheel with raised letters in a ring near the outer edge facing the paper. When the letter was in position a hammer would strike the wheel and knock the raised letter into the ribbon and into the paper thus printing it onto the paper. They are in essence a glorified computer controlled mechanical typewriter. Daisy wheels are considered entirely obsolete. Daisy wheels featured LQ or Letter Quality printing but could not easily change font, color, underline, etc.

  5. Dot-Matrix printers feature a print head with 9 pins to as many as 24 pins in a vertical arrangement. Servos force the pins out which strike the ribbon and force it into the page. As the head sweeps across the page patterns of the dots printed by the pins form the letters or any other symbols desired.

  6. Dot-Matrix printers are the only impact printers still manufactured and their main feature is that they can be fed multiform materials that need to printed with carbon duplicates like invoices and receipts. Businesses that need this capability amount to almost all of the market for these printers which are the slowest, noisiest, have the most moving parts, and the highest maintenance.

  7. Common printing problems of the dot-matrix include white streaks across the printed lines from a stuck or failed pin, torn pages from a pin stuck while extended out of the head, dark print on one side fading to unreadable on the other side from an improperly adjusted head or platen (the large hard cylinder that the paper rolls around on and which is the back stop of the print head). You turn the handle on the end of a typewriter's platen to eject the page.

  8. 9-pin Dot-Matrix printers letters are visibly granular. This is called Draft Quality or DQ. 24 pin heads print at NLQ or Near Letter Quality. Intermediate pin count heads are available but don't have clearly defined quality types.

  9. Dot-Matrix printers are generally capable of receiving a print to the LPT1 or PRN device from DOS with no drivers required. Dot-Matrix printers are character printers. They print one character at a time and respond to control characters in the stream like the [ENTER] key which translates into the carriage return and line feed control characters. The text mode video display responds to these just like a printer would, moving the cursor to the far left and scrolling the screen up one line.

  10. Thermal printers have had very little usage in the PC end user industry. If you have ever had a register receipt that went white after a few months you have seen the output of a thermal printer. These require special paper dipped with a dye that is clear at normal temperatures but turns dark at a higher temperature. The thermal printer focuses a heat lamp on a point to darken it and form the images. It hasn't been burned into the paper, the printer only raises the temperature taking advantage of the chemical properties of the dye in the paper. Over time, the images fade away. These are very cheap and popular for POS (Point – Of – Sale) systems, but see very few other applications because the hardcopy is so unreliable over time.

  11. Inkjets and bubble jets are based on a similar technology. Both rely on microscopic droplets of ink traveling from the head to the page through a magnetic field that can steer them into patterns on the page forming the images. The difference between the two technologies is how the ink is forced out of the aerosol print head tip. In Inkjets it is pumped out. This is mechanically more problematic (costlier) than the bubblejet and so there are probably no more inkjets being manufactured. Bubble jet printers have a heating element in the ink reservoir that boils the ink, the expanding bubble of gas at the bottom of the reservoir forces ink out of the print head. By carefully controlling the temperature the flow of ink can be forced with a slight rise in electricity to the heating element and stopped by a slight drop in power to the element.

  12. Ink jets are page printers. They will not print one character at a time. As such drivers must be installed into the system that will intercept prints to LPT1, read the stream of data until it is done or finishes one full page, then translate it into a stream of instructions out the port to the physical bubble jet printer. The language used is usually a form of Apple's Postscript. Though through the years and in the hands of various manufacturers this language has evolved into many different forms. Adobe developed another language called Page Description Language. Hewlett-Packard had a language called Printer Command Language used on dot-matrixes in the ‘70s. This language depended on escape sequences. Influenced by PostScript and PDL they modified it into a page desciption language for their laser printers. PCL 5 implemented in the HP LaserJet III was a significant improvement. 5e featured compression and 5c added new encoding features. The current language is PCL 6 introduced in the LaserJet 5 series EP printers.

  13. Electrophotographic or EP printers are laser printers. Laser printers do use a laser to draw the page but the actual break through technology in them is found in the electrophotographic drum. This drum is covered with a material that can be charged with static electricity, then loses that charge when exposed to light.

  14. There are eight standard component assemblies: toner cartridge, fusing assembly, laser scanner, high-voltage power supply, DC power supply, paper transport assembly, the corona, and printer controller circuitry.

  15. The Toner Cartridge holds the toner which is a black powder that the laser printer uses for its "ink". The toner is composed of carbon particles mixed with iron oxide particles. These two are attached to a bonding plastic some texts list polyester resins but it could be any form of plastic that the laser printer manufacturer develops. The particles of toner are small balls made of these three materials. The carbon provides the black color, the iron oxide provides the electrostatic affinity (jumps to objects that possess a static electrical charge on them) and the plastic bonds the two other substances together and bonds them to the paper during the fusing stage of the laser printing process. Toner also contains a substance called developer, which forms larger balls covered in the toner material until it is applied to the EP drums surface by the EP process. The toner cartridge usually houses the EP drum itself, which is drum that is coated with the electrophotographic material mentioned earlier. Some laser printers (most Hewlett-Packards in particular) include the drum and cleaning assembly within the toner cartridge, while other printers have the toner cartridge and the EP drum in two separate cartridges that can be replaced independently as needed.

  16. The laser scanning assembly. The EP drum only holds static electrical charges on areas not exposed to light. As a result, the laser is used to draw a negative of the desire image on the surface of the EP drum removing the static charges where it writes leaving the areas that represent the desired image untouched (by the laser) and therefore still charged. The laser does not actually move in its housing. Instead a mirror swivels and the laser pulses. The mirror only swivels from side to side allowing the laser to draw a single horizontal line of very fine dots. The drum then rotates slightly so the laser can draw the next line and so forth until the entire page has been drawn. The laser light also passes through focusing lenses to correct for the added distance the beam travels when pointed at the far edges versus the middle of the page.

  17. The high-voltage power supply (HVPS). This circuitry converts normal AC wall current (120 volts, 60Hz) into the much higher voltages needed by the printer to power the corona wires: the primary corona wire (or roller in some printers) and the transfer corona wire.

  18. DC power supply (DCPS). This power supply is very similar to the power supply of the PC itself and is used to convert AC wall current into the lower voltage required by the logic circuitry (+5Vdc and –5Vdc), and the paper transport motors (+24Vdc) and fan. Because of the different motor voltages the printer cannot use a PC's power supply.

  19. Paper transport assembly. The includes all of the rubberized rollers that actually move the paper from the storage tray through the printer processing and finally out of the printer. The first paper transport roller is the pickup (or feed) roller, which grabs the top sheet of paper out of the tray. This is followed by the registration rollers which coordinate the movement of the paper with a series of sensors so that the paper is in the exact position it is supposed to be in during the printing process. All of these rollers use electronic stepper motors which can move in very small increments rather than spin at high RPM.

  20. Transfer corona assembly. Once the toner has been attracted to the EP drum it must be attracted away from there and onto the paper. The transfer corona wire (or roller these days) imparts a large static charge to the paper for this purpose. This charge would also cause the paper to stick to the EP drum. So the transfer corona assembly also includes a static discharge strip which completely discharges the surface of the paper as it passes the point in contact with the EP drum.

  21. Fusing assembly. The paper passes between two rollers immediately after picking up the image from the EP drum. The two rollers mash the paper and heat it causing the image, which before this point consisted of powder resting on the surface of the paper, to melt into the paper. The fusing assembly consists of the halogen heating lamp, a Teflon-coated aluminum fusing roller, and a rubberized pressure roller. The halogen lamp heats the fusing roller to 165°C – 180°C.

  22. Printer controller circuitry. The laser printer has a main logic board which is essentially a small special purpose computer. It is designed to communicate with the PC either through the parallel port or a NIC that can be added to an expansion slot under a protective plastic door when not in use. This board controls all of the printer including warming up the fusers, converting a PCL transmission into the rasterized bit map image, firing the laser, moving the mirror, advancing the paper and so forth.

  23. The laser printing process consists of six phases: Cleaning, Conditioning, Writing, Developing, Transferring, and Fusing. Each of these will be addressed one at a time.

  24. The Cleaning phase. A rubber blade is extended into the surface of the EP drum and an erasure lamp is turned on. The EP drum is then rotated a full turn (at least) so that the entire surface has been exposed to bright light thoroughly discharging all residual static charges. The cleaning blade removes all toner that might have been left on it from the previous print job. The excess is scraped into a receptacle in the toner cartridge itself.

  25. The Conditioning phase. The primary corona wire is energized with high voltage electricity provided by the HVPS. This forms a field around it which the EP drum rotates within accumulating a strong uniform static electrical charge of approximately –600V.

  26. The Writing phase. The laser and mirror coordinate to illuminate narrow horizontal lines across the drum as it rotates. The laser illuminates small spots which represent the white area on the printed page, so it writes the negative of the desired printed page onto the surface of the EP drum. Where the laser writes the ?600V charge is dramatically reduced to about –100V leaving the other areas (where the toner will stick) at the original –600V.

  27. The Developing phase. The printer charges the developer roller which attracts the toner particles with about –100V. It is very close to the surface of the EP drum and the toner will leave the developer roller and cling to the surface of the EP drum only at the strongly (-600V) locations. After a full turn the toner has covered the EP drum with the desired printed pages image.

  28. The Transferring phase. The paper is now advanced by the registration rollers toward the EP drum. The transfer corona wire imparts a large positive charge to the paper and as it passes between the EP drum and the opposing pressure roll the toner clings to the page. However, the paper with this large positive charge would cling to the EP drum, so the paper crosses a discharge ground strip right beyond the point where it touches the EP drum. Having discharged the paper at that spot, the paper will follow the rollers and not wrap around the drum.

  29. The Fusing phase. The paper now has the image lying on it with no static electrical force holding it to the paper, only gravity. It is now advanced into the fusing assembly. The top roller is the heated one and the bottom roller is rubberized and presses the toner into the paper as it is melted by the fusing roller. During the printing process the fusing halogen lamp has heated the fusing roller to 350°F in order to melt the toner particles into the page. The reason the toner does not stick to the fusing roller is that it is made of Teflon.

    At this point the paper is advanced out of the printer and the page print is complete. The printer could then initiate the printing of the next page.

  30. Installing a printer. Character printers (like dot-matrix printers) do not have to be installed in DOS except to print beyond the ASCII character set. For graphics or special fonts or subsets (like bold and/or italic) drivers would have to be installed. Page printers like Inkjets and EP printers drivers must always be installed. In Windows this is done by clicking Start > Settings > Printers > Add a New Printer. This Wizard will attempt to detect the printer and load drivers. In the cases when it does not have the drivers included in the installation CABs you would click, "Show me a list..." rather than "Allow Windows to detect..." And in the next window click on "Have Disk" then specify whether it is a floppy or CD-ROM. Some drivers are written so that when you insert the CD-ROM the "AUTORUN" program starts inviting you to install the drivers. You can follow the installer program which functions like a Windows wizard. In the open lab you can practice installing and using a Dot-Matrix printer in DOS and Windows and you can practice installing a laser printer's drivers for Windows.

  31. Printer interfaces: Printers are commonly attached to the PC with a standard printer cable from the printer to the parallel port. The PC's parallel port is also called LPT1 which means Line Printer #1. LPT1 is the virtual device set up by the drivers for the actual physical parallel port circuitry and connector on the rear panel of the system. LPT1 is treated similarly to a file in DOS. Copying a text file like this: COPY EXAMPLE.TXT LPT1 will cause the file to be printed if a character-by-character (or line by line, hence the name LPT) printer is attached to the parallel port. The driver schema for Windows is more complicated because of its ability to interface to more complex printers that cannot be treated like a simple ASCII text file stream, but instead must capture the entire file and then convert it to the printer's control language and then transmit this on to the printer across the parallel port. In this scenario, the driver attaches to the LPT1 and captures all transmissions to it, rebuilds the information and then sends them back to the low level structures within the same LPT1 virtual device for transmission to the printer through the physical port. Serial ports (like COM1) were often used by printers in the 80's but fell into disfavor as printers became faster and the printer control languages that could print graphics needed to transmit large amounts of information to the printer. Modern day printers would literally take several minutes to transmit a graphic page to the printer over the serial cable and this technology is almost obsolete. USB has become the home printer interface of choice because the technology is built around the Plug-n-Play concept. Since USB is fully PnP and resides on the fully PnP PCI bus, installing a printer is as easy as plugging it to the USB port and turning it on. At that point Windows will detect the device and start the Add/New Hardware wizard which will lead the user through the installation process. Often the manufacturer recommends attaching the printer and leaving it off (because of this PnP feature) and then insert the CD-ROM which begins the installation process itself. The other major type of printer is SCSI. A SCSI printer adds the need for proper device ID configuration and the added ASPI driver layer for the host controller. A SCSI printer would not be recommended for the average user because of the cost and added complexity in the configuration. A SCSI printer would not be recommended on a mission critical system that has internal hard drives attached to the same host controller since a malfunction of the printer could possibly affect the hard drive(s) of the system.

Review Questions
  1. Name and briefly describe the four main types of printer:

  2. What are the two types of impact printer?

  3. Which type of impact printer is obsolete?

  4. Which main type of printer is seen mostly in POS systems?

  5. Which printer technology is the best (price vs. quality) for color printing?

  6. Which type of printer has the highest quality?

  7. Which technology used in the PC industry has the lowest quality?

  8. How many pins are in the print head of an NLQ dot matrix printer?

  9. Explain the difference between an inkjet and a bubble jet printer

  10. DOS can print to what type of printer with no additional driver support installed, Why?

  11. Inkjets and laser printers print an entire ___________________________________ at a time.

  12. List and briefly describe the major subsystems of a laser printer:

  13. List the laser printing process phases:

  14. Describe each step of the laser printing process:







  15. List and describe the possible standard printer interfaces to the PC (include advantages and disadvantages:

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