Absolute Beginners Guide to A+ Certification. Covers the Hardware and Operating Systems Exam

2017-07-07 02:10:07

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Dot-Matrix Printers

The dot-matrix printer is so named because it creates the appearance of fully formed characters from dots placed on the page.

The print mechanism of the dot-matrix printer is almost always an impact mechanism: A printhead containing 9 “24 fine wires (called pins ) arranged in one, two, or three columns is used along with a fabric ribbon, similar to typewriter technology. The wires are moved by an electromagnet at high speed against the ribbon to form dot patterns that form words, special characters, or graphics.

Dot-Matrix Printhead Types

The two major types of dot-matrix printheads are

9-pin

24-pin

A third type of dot-matrix printhead uses 18 pins and basically acts like a faster version of a 9-pin printhead.

To create a character, a dot-matrix printer uses a predefined series of columns called a matrix , in which each character is formed. As the printhead moves across the paper, commands from the computer rapidly move the 9, 18, or 24 pins in special sequences to form characters or graphics.

Figure 10.5 shows actual print samples from a typical 9-pin printer's draft mode, a typical 24-pin printer's draft mode, and the Near Letter Quality (NLQ) mode of the same 24-pin printer.

Figure 10.5. Actual print samples illustrating the differences in 24-pin and 9-pin printers. The narrower pins of the 24-pin printhead produce a reasonably good NLQ printout but hard-to-read results in draft mode.

Figure 10.6 shows how small the printhead is in relation to the rest of the printer. Size is misleading, though, because a damaged printhead will render the rest of the printer useless.

Figure 10.6. Components of a typical dot-matrix printer. The model pictured is a wide-carriage version, but its features are typical of models using either standard or wide- carriage paper.

Although 24-pin printheads produce much better-looking text in NLQ mode, their capability to print on multipart forms is limited because of their narrow wires. The smaller diameter causes a lighter impact on the top page, and subsequently even lighter impact on all the remaining pages. Typical form limits for a 24-pin printer are the original plus three non-carbon copies. 9-pin and 18-pin printheads use wider wires, and high-end printers with these printheads can handle up to four or more non-carbon copies as well as the original.

Durability in the field is also better with 9-pin/18-pin printheads; a broken pin in the printhead is more likely to result with the narrower wires used in the 24-pin printhead.

Parts of a Dot-Matrix Printer

The components of a typical dot-matrix printer are identified in Figure 10.6.

Impact dot-matrix printers have the following parts moving in coordination with each other during the printing process:

The paper is moved past the printhead vertically by pull or push tractors or by a platen.

The printhead moves across the paper horizontally, propelled along the printhead carriage by a drive belt, printing as it moves from left to right. Bidirectional printing prints in both directions but is often disabled for high-quality printing because it can be difficult to align the printing precisely.

The pins in the printhead are moving in and out against an inked ribbon as the printhead travels across the paper to form the text or create graphics.

The ribbon is also moving to reduce wear during the printing process.

Standard-Carriage Versus Wide-Carriage Models

Many printer manufacturers produce both standard-carriage (8.5x11-inch paper) and wide-carriage models (15.5 inches wide) of some of their dot-matrix printers. The wide-carriage models are preferred for accounting or other occupations requiring very wide printouts. Wide-carriage printers can fit about 80% more text on a line with the same- sized typeface.

Wide-carriage models can be adjusted to use standard paper, but often feature options such as a bottom paper feed for running high volumes of printing.

Periodic Maintenance and Care for Impact Dot-Matrix Printers

To keep an impact dot-matrix printer in top condition

Change ribbons when the ribbon begins to dry out; the ink in the ribbon also helps lubricate the printhead. Discard frayed ribbons because the fraying can snag a printhead's pins and break them.

Use platen conditioner to keep the rubber platen supple; a platen that becomes hard can break printheads.

Adjust the head gap whenever you change from ordinary paper to multipart forms, envelopes, labels, or other thicker-than-normal items, and when you return to normal paper; failure to set the head gap properly can result in smudged printing and broken pins in the printhead.

Periodically clean out hair, dust, and paper shreds from the printer.

Impact dot-matrix printers thus have a lot of moving parts and traditionally high noise levels. This, along with their incapability to create truly high-quality, letter-quality text except at low speeds and high noise levels, has moved them out of most homes and offices. They are largely used for utility tasks (bank receipts, point-of-sale receipts, and warehouse reports ) where their capability to print multipart forms is more important than print quality or noise level. Because impact dot-matrix printers use inexpensive ribbons designed to print millions of characters and can use fan-fold or single-sheet papers of all types, they have the lowest cost per page of all printers.

Dot-Matrix Printer Troubleshooting

The following problems are typical of dot-matrix printers; use this information to prepare for troubleshooting questions on the A+ Certification Exam and day-to-day printer troubleshooting.

Typical dot-matrix printer problems include gaps in printed letters , paper jams and torn perforations, and faded printing.

Gaps in Printed Letters

Because of how dot-matrix printers work, a slight amount of space between the dots that make up a dot-matrix letter is normal, especially in draft mode (refer to Figure 10.5). However, if horizontal white space through a letter is noticeable in NLQ mode, this usually indicates that the printer has a bent or broken pin.

When a pin in the printhead has become bent or broken, the printhead must be repaired or replaced . Incorrect head gap settings are a typical cause of bent or broken pins; the head gap must be adjusted to match the thickness of the paper, forms, or label stock inserted in the printer. Another typical cause is the use of a dried -out or damaged printer ribbon; replace the ribbon when print quality fades to protect the printhead and produce sharper, easier-to-read printing.

Paper Jams and Tears at Perforations

Incorrect tractor-feed width and position settings are the typical causes of paper jams and torn sheets. Make sure the tractor feed is adjusted to the correct width, which will make the paper lay flat without putting undue stress on the tear-off perforations. Printers with push and pull tractors must have the tractor-feed and paper-feed options selected correctly to avoid jams and torn sheets.

Faded Printing

If the print is evenly faded, the ribbon is dried out. Replace the ribbon to achieve better print quality and protect the printhead. If the print appears more faded on the top of each line than on the bottom, the head gap is set too wide for the paper type in use. Adjust the head gap to the correct width to improve printing and protect the printhead from damage.

Printhead Won't Move

The printhead should move back and forth during printing; if it won't move, check the drive belt and the gear mechanism. Jammed gears in the printer or a broken drive belt will prevent the drive belt from moving the printhead. Check the drive belt first to see if it is broken, and then check the gears that move the printhead. You might need to disassemble the printer to check the gears.

Paper Won't Advance

The paper advance, whether single-sheet or tractor feed, is also gear driven. Jammed gears will prevent the paper advance from working. You might need to disassemble the printer to check the gears.

Some printers require special tools to remove the plastic shell; contact the printer's manufacturer for detailed disassembly instructions and recommended tools.

Most impact dot-matrix printers have a self-test feature onboard. Use this to determine

Which firmware the printer is using

Which fonts and typefaces the printer includes

Normally, the self test is activated by holding down a button, usually the LF (line feed) button, while the printer is turned on. If the printer is wide-carriage, make sure the paper in place is also wide-carriage; if you don't use wide-carriage paper, the printer will try to print on the platen, which could damage the printhead. Note that the printer shown in Figure 10.3 is a wide-carriage model loaded with standard 8.5x11-inch paper.