OK, so let’s look at some wave forms. In the middle of printing a page of black-only text, I trace the signals for 10 seconds. The first image shows pin 1 to 8 (named 00 to 07 in the software). Pin 4, 5, 7, and 8 show a short spike every 1.5 secs. More on that later. Channel 5 shows blocks of pulses.
Now for a close-up on pin 4. The signal sits at 1.5V, is then pulled down for 12ms, then bounces to 3V for a very short time, and goes back to 1.5V.
Now for pin 6. This looks like a 0-3V clock signal at 66kHz. It comes in 5 or so bursts per Pin 4, 5, 7, 8 cycle. Printing a single line takes less than 1.5 seconds. It seems that instead one burst corresponds to one printed line.