Epson Apex-80 - Manual (Page 70)

Now look at the high-speed doubledensity design in Figure 6-3. It
should point you in the right direction for your own work.
Figure 6-3.
Arrow design
After plotting the dots on a grid, you calculate the numbers for each
pin pattern by dividing the design grid into separate print lines. For the
arrow design, the grid was divided into two lines, each seven dots high.
Then each column was examined to calculate the graphics data. The
results for the first line are shown in Figure 6-4. The pin values are on
the left and the sums at the bottom of each column.
Figure 6-4.
Calculating data
Introduction to Dot Graphics 6.11