Epson Apex-80 - Manual (Page 63)

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. Therefore, 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. Figure 6-1 shows three examples of how to
calculate the number that fires a particular pattern of pins.
Figure 6-1
Pin numbering system
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. Before you can put these numbers in a graphics program,
however, you need to know the format of the graphics commands.
Graphics Commands
The graphics mode commands are quite different from most other
commands. For most of the other modes, such as emphasized and
double-wide, one command turns the mode on and another turns it
off. For graphics, the command is more complicated because the
command that turns on a graphics mode also specifies how many
columns of graphics will be printed. After the printer receives this
command, it interprets the next numbers as pin patterns and prints
them on the paper.
6.4 Introduction to Dot Graphics