True, however, a 'normal' implementation would just the one clock signal to clock in the three data lines; to lower the clock frequency you could use the rising

True, however, a 'normal' implementation would just the one clock signal to clock in the three data lines; to lower the clock frequency you could use the rising edges to clock in the even bits and falling edges to clock in the odd bits. That's not what they do here, though: they use the rising edge of the clock to clock in the even bits, as you'd expect, but the S1-S5 lines to clock in the odd bits. That is unexpected. Same with the existence of the S1-S5 lines all by itself: you could also have fixed that with a counter clocked by the existing clock line and a separate reset line; that'd simplify the connections between printer and cart. I'm still not sure why they didn't do that.