Epson 4500 - Manual (Page 118)

Graphics
newspaper photograph, you will find that it is made up of thousands of
small dots. Your printer also forms its images with patterns of dots, as
many as 360 dots per inch horizontally and 360 dots per inch vertically.
The images printed by this printer can, therefore, be as finely detailed
as the ones at the beginning of this section.
Twenty-four-pin graphics
The graphics mode that takes full advantage of this printer’s print head
is 24-pin graphics. This mode has five densities, but for simplicity this
explanation deals with only one of them, triple-density.
Triple-density prints up to 180 dots per inch horizontally. As the print
head moves across the paper, every 1/180th of an inch it must receive
instructions about which of its 24 pins to fire. At each position it can
fire any number of pins, from none to 24. This means that the printer
must receive 24 bits of information for each column it prints. Since the
printer uses S-bit bytes of information in communicating with the
computer, it needs three bytes of information for each position.
Pin labels
To tell the printer which pins to fire in each column, you first divide
each of the vertical columns into three sections of eight pins each and
consider each section separately. Since there are 256 possible
combinations of the eight pins in each section, you need a numbering
system that allows you to use a single number to specify which of the
256 possible patterns you want. This numbering system is shown
below :
Software and Graphics 4-11