PC Hardware —

Dot-Matrix Printers

The most popular impact printer is the dot-matrix variety. Dot-matrix printers form characters as raster images on paper by pressing pins onto an inked ribbon, which then is pressed onto paper. Dot-matrix printers use an array of pins (commonly 9 or 24 pins) which are made of stiff wire. The higher the number of pins, the more dots per square inch and the higher the print quality. The pins are held in a print head that travels on a rail in front of a roller that transports the paper. The pins are controlled by electromagnets; dots are created when power is applied to selected electromagnets in the print head, forcing the desired pin away from a magnet in the print head. The pins strike an inked ribbon, which then strikes the paper. As the individual dots are struck, a character is formed. Each character produced by the print head is made up of several rows and columns of dots. (See Figure 12.2.) High-resolution dot-matrix printers use more dots to form one character.

The dot-matrix printer is still used in some commercial environments where its ability to inexpensively create multipage forms is appreciated. The pins have enough impact to be used with carbon paper, making it easy to obtain two or more copies so that one can be given to a customer or kept for accounting, filing, or other purposes.

Dot-matrix letters formed from dots.

Maintaining a dot-matrix printer is very simple:

  • Change the ribbon.

  • Keep the printer clean.

  • Keep the print head clean.

  • Replace the print head if it fails.

Troubleshooting Dot-Matrix Printers

Troubleshooting a dot-matrix printer usually requires a reference manual. There are so many printers on the market that no single guide will suffice to help a computer technician troubleshoot all printer problems. If a manual for a particular printer is unavailable, check the printer itself for instructions (sometimes there are diagrams inside the printer). Usually, a thorough inspection of the mechanical parts will uncover the problem. The following table lists common problems encountered with dot-matrix printers, and possible causes.

Symptom Possible Cause
Printer does not function at all. No AC power is getting to printer. Fuse is blown.
Device does not print although power is on. Printer is not online. Printer is out of paper. Printer cable is disconnected.
Printer won't go online. Printer is out of paper. (Check connections.)
Paper slips around platen Paper is not being gripped properly. (Adjust paper-feed selector for size and type of paper.)
Head moves, but does not print. Ribbon is not installed properly or is out of ink.
Head tears paper as it moves. Pins are not operating properly. (Check pins; if any are frozen, the head needs to be replaced.)
Paper bunches up around platen. There is no reverse tension on paper.
Paper has "dimples." Paper is misaligned, or the tractor feed wheels are not locked in place.
"Paper/Error" indicator flashes continuously. There is an overload condition.
Printout is double-spaced or there is no spacing between lines. Printer configuration switch is improperly set. (Make sure it isn't set to output a carriage return or linefeed after each line.)
Printer cannot print characters above ASCII code 127. Printer configuration switch is improperly set.
Print mode cannot be changed. Printer configuration switch is improperly set.