Epson LQ-850 User Manual page 87

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:
To fire any one pin, you send its number. To fire more than one
pin at the same time, add up the numbers of the pins and send the
sum to the printer. With 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, then send 129.
By adding the appropriate label numbers together, you can fire
any combination of pins. Below are shown three examples of how
to calculate the number that will fire a particular pattern of pins.
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.
Graphics
4-11