If the CPU is the heart of a computer, then the printhead is the engine of
a dot matrix printer. Any matrix printhead uses electro-magnetic field to fire
the print head wires. There are two main printhead engineering technologies - in the first
one electro-magnetic field shoots the print head's wire. In the second one, the
so called permanent magnet printheads, a spring shoots the printhead wire and
the magnetic field just holds the spring in stressed and ready to shoot printhead
wire position. When the electro-magnetic field equalizes the magnetic field, the
spring is released to shoot the wire. To see all this in action, take a look at the picture bellow.
Dot matrix print head mechanisms in action:
Classical printhead mechanism is showed from the left side. The permanent magnet print head mechanism you may see at right.
In general the permanent magnet printheads are faster and are used in heavy-duty printers. Some of the most popular print
heads of this type are Epson DFX, IBM 4226, Fujitsu 5600 and 6400, and all Oki print heads.
In serial dot matrix printers the characters are formed by the print head as it moves in horizontal direction. Such a print head has a number of print wires arranged in vertical column. As it moves in horizontal direction, the print wires are fired selectively to form the desired characters. The most commonly used print heads has 9 print wires in one column or 24 print wires in two columns, for better print quality. In some heavy duty printers there are also used 18 wire print heads which have 2 columns, 9 wires in each.
The above discribed printing process is shown at the picture bellow:
Serial dot matrix printer in action
The distance between wires in column may give us vertical printing resolution. For example: 9 wire print head with distance 0.35 mm between adjacent wires will result in 25.4/0.35=72.5 dots/inch vertical printing resolution for one pass printed line of characters. 24 wires print heads has 2 columns, 12 wires each, with a vertical displacement of ½ step. So if the distance between adjacent wires is 0.21 mm, then one column will print with 25.4/0.21=120.9 dots/inch vertical resolution, but since the second column print between the dots printed from the first one, the overall vertical resolution will be 240 dots/inch.