Never connect a laser printer to a manual switchbox. Taking the laser printer on and offline by changing the active location with the dial creates line noise spikes on the cables that can damage the laser printer or the paralel port.

LPT Port System Resources
Parallel Port	I/O Address	IRQ
LPT1	378-37Fh	7
LPT2	278-27Fh	5
LPT3	3BC-3BFh	7 or 5

     Most PC printers do not use an IRQ in either DOS or Windows operating systems. However, devices such as external tape or storage drives, that do use interrupts also connected to parallel ports. Two types of connectors are used to connect printers and PCs:

• 25-pin Female: This connector is usually mounted on the motherboard or on an adapter card and is used to connect with the male DB 25-pin connector of the printer cable.
  
  
• 36-pin Centronics: This common connector is used at the printer end of the connecting cable.
  
  

Some of the common types of printers are:

• Daisy-wheel: These printers are still in use but are becoming rare. Daisy-wheel printers are an early type of impact printer that uses a print wheel that works much like the ball on an electric typewriter.
  
  
• Dot-matrix: These printers create characters by forming a group of hard-wire pins into the pattern of the letter, number, or special character and then striking the entire pin group through a ribbon, forming the character on paper.
  
  
• Inkjet, or bubble jet: These printers are probably the most popular printer type in use. They are able to produce a better-quality print but without the noise of both the dot-matrix and daisy-wheel printers. In brief, inkjet printers produce an image by heating ink into steam and then "jetting" it onto the paper.
  
  
• Laser: These printers use a complex printing process to produce very high-quality documents. Laser printers are becoming more common on the desktop.
  
  

Daisy-wheel Printers

     Daisy-wheel printers produce what is called letter-quality (LQ) print. Letter quality is the quality standard achieved by a typewriter. It is the standard which all impact printers are measured. The daisy-wheel printer is very, very noisy.

Dot-matrix Printers

     The dot-matrix printer is an impact printer that creates its printed characters using a matrix of very fine printwires to form a pattern of dots. As the number of printwires used to create the character increases, so does its resolution (dots-per-inch) and the quality of the printed image. The resulting character of a dot-matrix printer is not as good as that created with a daisy-wheel printer, but given the trade-off in noise and speed, you can live with less quality for many documents. The resolution is measured by the number of dots that can be printed in a square inch (dots-per-inch or DPI).

     The most common numbers of printwires (pins) in the dot-matrix printer's printhead are 9, 18, and 24. A 24-pin printhead produces near-letter quality (NLQ) print. Printers with less than 24 pins are capable of draft quality print, which produces characters with lots of wide-open speces between dots.

     To produce a character, the print mechanism extends all the printwires needed to create a character. Behind each printwire is a solenoid coil that causes the pin to extend and impact the inked ribbon. A spring then pulls each printwire back into the printhead. Because of the impact used to strike the ink ribbon onto the page, dot-matrix printers are commonly used in situations where forms or documents with many carbon copies are created.

     Much like a typewriter, dot-matrix printers use a platen, a large rubberized roller under which the paper is fed. Dot-matrix printers also support form tractors, or pin-feed tractors, which attach to the platen are are also driven by the platen motor. Form tractors provide a more consistent feed mechanism by using the pin-feed holes along the side of the continuous form paper.

     The speed of a dot-matrix printer is rated in characters-per-second (CPS). Common speed ratings range from 32 to 72 CPS. The actual speed realized from the printer depends largely on its mode of operation. Dot-matrix printers operate in either font (normal text, numbers, and symbols) and dot-addressable (graphics and charts) modes.

     The printhead in a dot-matrix printer can get extremely hot and should not be touched while in use. Also, the printhead is never cleaned during preventative maintenance.

Inkjet Printers

     Inkjet, or bubble jet, printers are probably the most popular printer type in use for two main reasons:

• They are quiet - the inkjet printer is a nonimpact printer.
  
• Their ink reservoir is included in a disposable cartridge along with the printing mechanism. This means that each time the ink reservoir is replaced, a new print mechanism is also supplied.
  

     Inkjet printers form characters by squirting ink, using an elaborate ink-stream process with as many as 50 tiny nozzles, to form characters on the paper. The print quality of an inkjet printer is rated in dots-per-inch (dpi). The more dots in the image, the better the image. Inkjets range from 150 dpi to over 1400 dpi on photo-quality printers. Inkjet printer speeds are rated in pager per minute (PPM). Inkjet printer speeds range from 2 PPM to 9 PPM.

Laser Printers

     Laser printers are page printers in that they form and print all the text and graphics for one full sheet or page at a time. Three different printing processes are used in laser printers, each directly attributable to one or more manufacturers: EP, HP, and LED. Will will cover EP and HP:

• EP: The electrophotographic (EP) process, developed by Xerox and Canon, was the first laser printer technology, and all laser printers use this technology in one form or another. Its characteristics are the use of a laser beam to produce an electrostatic charge and a dry toner to create the "printed" image.
  
  
• HP: The Hewlett-Packard (HP) process is essentially the same as the EP process, with the exception of some minor operating procedures. It's similar enough to be considered the same process, but different enough to get its own name.
  
  

     Laser printers use toner to create the image on the printed page. The toner, a dry powder that consists of iron particles coated with a plastic resin, is bonded to the paper during the print process. Toner is supplied to the printer in a removable cartridge that also contains many of the most important parts used in the printing process. Inside the toner cartridge are located the photosensitive drum, a mechanism used to place a charge on the drum, a roller used to develop the final image on the page, and, of course, the toner.

     Including the toner cartridge, an additional eight standard assemblies exist in a laser printer. These assemblies are:

• The drum: The drum inside the toner cartridge is photosensitive, which means it reacts to light. The drum holds an electrostatic charge except where it is exposed to light. The laser beam is reflected onto the surface of the drum to create a pattern of charged and not-so-charged spots, representing the image of the page to be printed.
  
  
• High-voltage power supply: The EP process uses very high voltage to charge the drum and transfer and hold the toner on the paper. The high-voltage power supply converts AC current into the higher voltages used by the printer.
  
  
• DC power supply: Like a computer, most of the electronic components in the laser printer use direct current. Like the computer's power supply, the laser printer's DC power supply also contains the cooling system fan.
  
  
• Paper transport: Inside the laser printer are four types of rollers that move the paper through the printer. Each rubberized roller or set of rollers is driven by its own motor. The four roller types in the paper transport system are the feed roller or the paper pickup roller, the registration roller, the fuser roller, and the exit roller. Most paper jams occur in the paper transport area.
  
  
• Primary corona: Also called the main corona or the primary grid, this device forms an electrical field that uniformly charges the photosensitive drum to -600V as a way to reset it prior to receiving the print image and toner.
  
  
• Transfer corona: This mechanism moves a page image from the drum to the paper. The transfer corona charges the paper and the charge pulls the toner from the drum onto the paper. As the paper exits the transfer corona, a static charge eliminator strip reduces the charge on the paper so that it won't stick to the drum. Not all printers use a transfer corona; sume use a transfer roller instead. Be careful not to touch the transfer roller with your bare hands or arm when working on this type of printer. The oil from your skin will spot the roller.
  
  
• Fusing roller: The toner is melted permanently to the page by the fusing rollers that apply pressure and heat, between 165 and 180 degrees Celsius. The fuser - not the use of a laser - is why pages come out of a laser printer hot.
  
  
• Controller: This is the 'motherboard' of the laser printer, and it carries many of the same architecture and components of a PC motherboard. It's the controller that communicates with the PC, houses the memory in the printer, and forms the image printed on the page. Memory expansion is possible on virtually all laser printers. Adding memory allows the printer to reproduce larger documents or graphics in higher resolutions or to support additional soft fonts.
  
  

     A printer that experiences frequent memory overflow errors has a bad memory board, a memory board that was installed incorrectly, or a memory board that needs additional memory. Diagnose this condition in this order.

     Six major steps are involved in printing a page on a laser printer. It is important that you know the sequence and activities of each step in the process:

• Cleaning: Before a page is started, any remnants of a previous page are cleared away. The drum is swept free of any lingering toner with a rubber blade, and a fluorescent lamp removes any electrical charge remaining on the drum. Any toner removed in this step is not reused but is put into a used-toner compartment on the cartridge.
  
  
• Conditioning: The entire drum is uniformly charged to -600V by the primary corona wire located inside the toner cartridge. This charge conditions the drum for the next step.
  
  
• Writing: The laser printer controller uses a laser beam and a series of mirrors to create the image of the page on the drum. The laser beam is turned on and off in accordance with the image to be created on the drum. Where the laser's light contacts the photosensitive drum, the charge at that spot is reduced to about -100V. After the image has been transferred to the drum, the controller also starts the page sheet through the printer, stopping it at the registration rollers.
  
  
• Developing: The developing roller, located inside the toner cartridge, has a magnet inside of it that attracts the iron particles in the toner. As the developing roller rotates by the drum, the toner is attracted to the areas of the drum that have been exposed by the laser, creating the print image on the drum.
  
  
• Transferring: The back of the paper sheet, the one that has been waiting at the registration rollers, is given a positive charge that attracts the nagatively charged toner from the drum onto the paper as it passes. After this step, the paper has the image of the page on it, but the toner, which is held only by simple magnetism, is not yet bonded to it.
  
  
• Fusing: The fusing rollers apply heat and pressure to the toner, melting and pressing it into the paper to create a permanent bond. The fusing rollers are covered with Teflon and treated with a light silicon oil to keep the paper from sticking to them.
  
  

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Preventative Maintaince

     Never vacuum spilt toner, the particles are so fine they can seep through the vacuum bag and melt inside the vacuum motor. Special types of vacuum cleaner bags are made for working with toner. Never wash toner off your hands with hot water, its best to wipe as much off your hands as possible with a paper towel, rinse with cold water, and finish by washing with soap and cold water.

     Usually packed with the toner cartridge is a cleaning brush or cotton swab that you can use to clean the transfer corona wire. During the print process, the laser produces a gas called ozone. Most laser printers have an ozone filter that also captures toner and paper dust. Replace or clean this filter in accordance with the manufacturer's instructions.

     Inside the laser priter are two or more mirrors that reflect the laser onto the drum. Using lint-free cloths, periodically clean the laser mirrors. The fuser cleaning pad and the fusing roller can also become contaminated and leave unwanted toner blobs on the paper. Check these printer parts regularly and clean them as necessary.

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Troubleshooting Common Printer Problems

     The first real sign of a printer problem is that paper with printing on it isn't coming out of the printer. When this happens, you can look in four places:

• Printer: First check the power is on, next check to see if the printer is on-line. Make sure that the printer has paper and that the feed tray, roller, or slide is in its proper position for operation.
  
  
• Cable: If the printer seems to be generally ok, ensure that the cable is the proper type of cable. If the cable is the correct type, then be sure that it's solidly connected at each end.
  
  
• Port: To check the port, use loopback plugs and diagnostic software. After the printer, the port has the next highest failure rate.
  
  
• Software: In the Windows environment, printers stall for just about any reason. If the printer status shows no problems, and you can't find any other problem, try restarting the system.