Epson Apex-80 - Manual (Page 61)

If you use commercial software that produces graphics, all you need
to know about dot graphics is how to use the software. If, on the other
hand, you wish to do your own programming or merely wish to
understand how the Apex80 prints graphics, read on.
The Print Head
To understand dot graphics you need to know a little about how the
Apex80’s print head works.
The Apex80's print head has nine pins. As it moves across the page,
electrical impulses cause the pins to fire. Each time a pin fires, it strikes
the inked ribbon and presses it against the paper to produce a small
dot. As the head moves across the paper, the pins fire time after time in
different patterns to produce letters, numbers, or symbols.
6.2 Introduction to Dot Graphics