Lesson #16 - Printers

A large portion of the A+ exam centers on printers, mainly because they are the primary accessory that a computer uses that you will be required to service as a technician.  You MUST be able to state the parts, the inner workings, and the different terminology in order to be able to get a high score on the A+ exams.

Connections

Although there are exceptions to the rule, for the A+ exam you can assume that all printers hook up to the computer through either the serial or parallel port.  (Printers also can hook up through networking cables, and some older printers used proprietary ports.  You won't see questions on those printers on the exam)  A serial connector is a connection that sends information one bit at a time, where a parallel connection sends multiple bits at a single time.  Most current printers use the parallel port, or the LPT port.

It is possible to have multiple LPT ports on a computer, but most only come with one built in.  You should assume that a question regarding printers is going to be about the parallel port on the exam unless it mentions something about the serial port.  It's easier to remember one connection than two.

LPT ports have specific I/O addresses and IRQ values.  LPT 1 (The default LPT port) uses I/O 378-37Fh and IRQ 7.  LPT 2 uses I/O 278-27Fh and IRQ 5.  LPT 3 uses I/O 3BC-3BFh and either IRQ 5 or 7.  Notice that you can not use LPT 3 and another LPT port set to the same IRQ value at the same time, or the computer will create a conflict and freeze.  Although printers don't often use the IRQ values, devices like Tape back-up drives and external storage devices often will.  If you have a printer and an external drive conflicting, check the IRQ's.

Parallel ports use two different kinds of connectors.  The port on the computer itself is generally a 25-pin DB female connector.  The cable will have a 25-pin DB male connector that fits over this connection.

The printer itself will generally have a 36-pin Centronics connector.  Centronics connectors look different that DB connectors, as they connect in a much different way.  The male Centronics connector consists of two rows of metal contacts anchored by a center strip down the middle of the connector  .  The female connector has the exact opposite center strip, with metal connectors attached so they will contact with the pins on the male connector.  (This is hard to explain, but easy to see.  Disconnect your printer from your computer and look at the two ends.  The one at the printer is the Centronics.)

Important Notes about Printer Cables and Accessories

There are several easy questions to get right on the A+ exam.  The first deals with cables.  Never use a cable longer than 15 feet on a printer, as the signal degrades quickly over longer distances.

The second easy question will deal with switch boxes.  It will ask something about switchboxes and Laser printers.  Just remember that you NEVER connect a laser printer to a switchbox.  When a switchbox throws a switch, it sends a spike of electricity through the box that can damage the printer.

The last question will deal with the IEEE 1284 Parallel port standard.  The IEEE 1284 Parallel Port Standard deals with bi-directional communications over parallel ports, and is made up of three standards;

• Standard Parallel Port (SPP) - One-Way transmission of Data

• Enhanced Parallel Port (EPP) - Allows data to transmit both ways over the cable, but only in one direction at a time.

• Enhanced Capabilities Port (ECP) - Allows full bi-directional simultaneous communications, but only when used with an ECP (IEEE1284) cable.

Impact and Jets

There are two basic variations of the common printer.  They are distinguished by the method used to transfer the mark from the printer to the paper.  Any printer that uses a print head that strikes a mark on a printer is called an Impact Printer.  These include Daisy Wheel printers and Dot-Matrix Printers.  All impact printers use a ribbon to transfer the image on to the paper.

Any printer that doesn't use a ribbon is called a non-impact printer.  Ink Jet, Bubble Jet, laser, and LED printers are all examples of non-impact printers.  They use a storage medium to store the ink or toner until it is attached to the printer.  In the next section we will deal with exactly how this transfer takes place.  This is important to know, as it is on the exam.

Impact Printers

As stated before, impact printers use a ribbon that is struck by a print head in order to transfer an image onto the paper.  The oldest type of Impact printer is the Daisy Wheel Printer.  The print head on a Daisy wheel printer is nothing more than a metal wheel with each possible character the printer can reproduce engraved on it.  The wheel spins to strike the ribbon with the appropriate character facing the ribbon.  Because the quality of the print is dependant on the quality of the engraving, it is called letter quality printing.  (LQ)

That is the only question you MAY see on the exam about Daisy wheels.

Dot Matrix printers also use a ribbon to create characters, but use a much different impact system to strike the ribbon.  Dot Matrix printers do exactly what their name says; they print out a matrix of dots to create letters.  This is done with a series of wires attached to pins that are controlled by the print controller.  There can be 9, 18, or 24 print wires and pins on a dot matrix printer, with 24 pin printers capable of producing Near-Letter Quality printing (NLQ) and 9 and 18 pin printers producing Draft Quality printing.

In order to make it's impact, a dot matrix printer's print head pulls on the print wires in a format to produce a specific character.  A pin in front of each print wire extends out from the print head, and strikes the ribbon against the paper. A spring then pulls the pin back into the print head after the mark is made.  

Dot Matrix printers are measured by their resolution, or how many dots per inch (DPI) they can create.  A low-resolution printer will have large spaces in between each dot, where a higher resolution printer will overlap the dots to reduce these spaces.  Because they are an impact printer, they also are used in situations where carbon is used to make multiple copies.  (A non-impact printer can't push on the carbon to make a second copy, and thus can't be used for invoice printers where multiple copies are made on one sheet)

There are also two different paper feeding mechanisms used by Dot Matrix printers.  A large rubber roller, called a platen, is used is form feed mode to pull a sheet of paper through the printer and past the print head.  Form Feed uses single sheets at a time.  Tractor Feed mode uses rubber or plastic pins that fit holes cut into the side of the paper to push the paper through the printer.  This allows pages to be linked together in a chain so that a continuous stream of paper can be fed into the printer without having to insert a new page each time.

Two important aspects of Dot Matrix printers; Never clean a dot matrix print head, as they are not user-serviceable.  And Print heads get extremely hot during printing, and should not be handled when in use.

Non-Impact Printers

There are between 6 and 8 printer questions asked on the A+ exam, and no more than two will deal with the preceding information.  That means that the following information will make up 70% of your printer questions, so please read carefully.

Inkjet printers, also known as Bubble Jet and Desk Jet printers, are the most popular printers used today.  They are cheap, and they are quiet, so they are perfect for almost every home and some office uses.  

Inkjet printers use nozzles to "jet" ink from a reservoir onto a page.  Here is how the process occurs;

1. A nozzle has an opening inside that is filled with ink from the ink storage.

2. Either heat (thermal) or an electric current (Piezo-electric) is passed through the nozzle, causing the ink to expand.

3. When the ink expands, the excess ink is sprayed out on to the page.

4. When the ink returns to normal size, it draws more ink from the reservoir to fill the nozzle again.

There can be upwards of 100 nozzles on an inkjet print head.  The more nozzles there are, the better the resolution the printer can create.  Inkjet printers are rating in Dots Per Inch (DPI) like dot matrix printers, but range to upwards of 2400 DPI for photo-quality printers.  Inkjet printers are also rated on speed, counted in Pages Per Minute. (PPM)

The difference between Piezo-Electric nozzles and Thermal nozzles is significant.  Thermal nozzles create a set temperature inside the nozzle, forcing a set amount of ink out each time.  This means that the printer can only push out a certain size dot.  Piezo-Electric printers send an electric current through the ink.  By varying this current, they can push more or less ink out, creating different sized dots.  (Because heat isn't used in Piezo-Electric nozzles, they require less maintenance as well) Therefore, Piezo-Electric Inkjet Printers are more expensive than thermal Inkjet printers.

Lasers

Laser printers and Light-Emitting Diode (LED) printers are the most important printing format to understand for the A+ exam.  Since they are mostly used in commercial offices, they are the most likely printer format you will see when working in the field.  We will devote the last of this week and all of next week to Laser printers.

There are 6 processes that go into a laser printer producing an image on a sheet of paper.  You must remember these processes IN ORDER for the exam.  Try to remember an acronym that will help you remember the letters CCWDTF.  (Something like Charlie Choked While Drinking Thick Fluids...  It's morbid, but you'll remember it)

1. Cleaning - The first step in printing a page is to clean off all the elements of the last page.  A rubber blade called a "Doctor" blade cleans all the remaining toner off the drum, and a fluorescent lamp removes all electrical charges that may remain on the drum.  Left-Over toned is not reused, as it has already been exposed to the fusing process.

2. Conditioning - The entire drum is given an equal -600 Volt charge by a corona wire.  

3. Writing - Using a series of mirrors, the laser "draws" the image onto the drum.  Where the laser hits the drum, the charge is reduced by about -100 Volts. 

4. Developing - A magnet inside a roller called the "Developer Roller" attracts toner from inside the toner cartridge.  The developing roller is then passed across the drum, where the ink sticks to the parts of the drum that have been exposed to the laser.

5. Transferring - The back side of the paper the image is to be produced on receives a positive charge.  This is done by a registration roller, and it attracts the negatively-charged toner to the front of the sheet.

6. Fusing - The toner up to this point is held on through magnetism, and the plastic elements in the toner must be melted to the paper.  The Fusing roller heats the paper and toner to about 180 Degrees Celsius, and fuses the toner right to the paper.

It is important to realize that the drum is a constantly revolving mechanism, and does not carry a charge for an entire sheet of paper at a time.  While the last part of the drum may be in the cleaning process, the images from the middle of the page may still be fusing or transferring.  A laser printer does everything line-by-line.  This means the entire process can be occurring all at the same time during a single page.

There are also two formats used during writing and developing.  The A+ exam uses the  example where the image is written to the drum, and you should remember it as I have written it above.  Some printers however write the opposite of what is to be printed on the drum, like a picture negative.  They then attract toner to the parts of the drum NOT exposed to the laser.  

Next week We'll discuss the corona wire, drum, paper transports, and other elements of laser printers, and discuss exactly how the laser mechanism works.