PC Hardware A Beginner’s Guide

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more of the print data file can be sent from the computer to the printer. This continues until
theentirefileistransferred.Afterthefileiscompletelytransferredtotheprinter,thecom-
puter disengages and moves on to other tasks.
The printer’s processor reads the instructions for one line of print from the buffer and
translates it into the dot patterns needed to form each character on the print line. The pro-
cessor also decides the travel direction of the printhead to print the line, which means
whether the line will be printed when the head is traveling left to right or right to left, or
both. The processor must also control the movement of the paper, advancing it a single
line, to the top of the next page, or feeding an entire page.


Forming a Dot Matrix Character


The key to understanding how a dot matrix printer works is to understand how it forms
its characters. As described earlier and illustrated in Figure 17-5, a dot matrix printer
forms its characters with a pattern of dots. Characters are formed in stages by the pins;
there are typically 9, 18, or 24 pins in the printhead. A 9-pin printhead, in which the pins are
arranged in a single column, as shown in Figure 17-9, forms characters by printing the ap-
propriate dots in a series of connecting columns to complete the character. Dot matrix
printers with 9-pins are capable of producing only draft quality print and are usually
used for forms. Eighteen-pin printers have largely disappeared, and 24-pin printers are
used in high-end dot matrix applications.


The Printhead


As with all printers, the printhead is the most important part of the dot matrix printer. It
forms the characters and prints them on the page. The major components in the
printhead, as illustrated in Figure 17-10, are the solenoids and pins, along with a perma-
nent magnet and wire coils and springs on each pin.
Inside the printhead is a large permanent magnet that exerts a magnetic force on the
pins that draws them back and away from the printhead. Each pin is attached to a wire
thatiswrappedwithacoilofwireandaspringthatpushesagainstthepullofthemagnet.
Whenpowerisputonthecoilofwire,itselectromagneticforceneutralizesthatoftheper-
manent magnet and the spring forces the pin forward to strike the ribbon. When the
power is removed from the wire coil, the magnet retracts the pin.
The speed at which the pins are moving and the constant energizing and de-energiz-
ing of the coil electromagnets, along with the friction of the moving parts, creates heat.
The printhead of a dot matrix printer gets very hot when it is printing, which is why most
have a heat sink attached or designed into the housing of the printhead. The heat sink
provides cooling to the printhead.


Dot Matrix Print Speeds and Resolutions


Dot matrix printer speeds have dramatically increased over the past few years to the
point that their speeds range from 200cps to 1200 and 1400cps. These printers that origi-
nally featured only 10dpi (dots per inch) now boast resolutions of 150dpi and higher. Dot
matrix printers that are capable of producing high-resolution graphics offer Super High


Chapter 17: Printers^413

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