Epson 4500 - Manual (Page 119)

Graphics
To fire any one pin, you send its number to the printer. To fire more
than one pin at the same time, add up the numbers of the pins and send
the total. Using these labels for the pins, you fire the top pin by sending
128. To fire the bottom pin, you send 1. If you want to fire only the top
and bottom pins, you simply add 128 and 1, and send 129.
By adding the appropriate label numbers together, you can fire any
combination of pins. Below are three examples of how to calculate the
number that will fire a particular pattern of pins.
128
3 2
8
2
1 7 0
With this numbering system, any combination of the eight pins adds up
to a decimal number between 0 and 255, and no numbers are
duplicated.
Since there are 24 pins in each column, you must make a calculation for
each of the three sections in each column. As you can see, this method
of planning and printing dot graphics requires considerable calculation.
Because triple-density uses 180 columns per inch, printing a single line
of triple-density graphics only one inch long requires 540 numbers.
Fortunately, commercial graphics software is available to do these
calculations for you.
Before you can put these numbers into a graphics program, however,
you need to know the format of the graphics command.
4-12 Software and Graphics