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November 28, 2013

Dot Matrix Printer (DMP) - Introduction

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Friends, we have already discussed about the introduction of Printers. You can read that post from here. In this post we shall discuss about dot matrix printers or DMPs. A Dot Matrix Printer is the most popular printer used for personal computers (PCs). In this type of printers, each character is formed from a set of dots in a rectangular matrix. DMP uses impact technology.The print head contains a physical array of pins. The print head moves across the paper and the selected pins are activated to strike the paper through an inked ribbon to produce an impression (consists of dots) on paper. To print the full character, the print head moves a predetremined number of positions to complete the matrix. The matrix usually contains 7 rows and 5 columns. However, the number of pins in the vertical array of print head are 9. The bottom 2 pins are used to form the descenders of lower-case letters, such as g, p, q and y. 

The figure shows the letters formed by 7x5 matrix. At normal reading distance, the dots blur together to appear as semisolid characters. If we observe closely you can observe the dots. In the below image you can see the difference. The letters S T and U are zoomed so that you can observe the 7x5 dots in them.

Many dot matrix printers are bidirectional, that is, they can print the characters from left to right and right to left. Dot matrix printers are very versatile since the character set can be changed by software or firmware. Also because they do not have fixed character font, they are capable of printing in various languages with fonts of different types and sizes.

The Dot Matrix Printers are very less expensive. The main disadvantage of these printers is that the print quality is low. Dot matrix printer with 24 pins in the print head are also available. These printers have a better printing quality compared to 9 pin dot matrix printers. The print quality can also be improved by stetting the printer to NLQ (near letter quality) mode. This is a double strike mode with each character is printed twice with the second print slightly being displaced with respect to the first.

Dot matrix printers can be either a 80 column printer or a 132 column printer depending on the maximum number of characters that can be printed in a line. The printing speed of typical dot matrix printer ranges from 150 CPS (Character Per Second) to about 1000 CPS. So a typical specification of dot matrix printers may be like 24 pin, 132 column, 300 characters per second.

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