Summary

You built a colour plotter and connected it to a computer. Everything it can do is a few formulas, and you derived each one from what the machine has to do:

• a step per arrow — LXS = CKL AND (LEFT OR RIGHT): the 60 Hz clock reaches the motor while you hold the arrow;
• the pen — a toggle flip-flop: D = NOT Q, every PEN UP/DOWN press flips it;
• the turret motor — two flip-flops in a ring: AB → BC → CD → DA;
• a quarter turn — 6 steps on the CD4017, the carry counts the pens: CP1 CP0;
• turn until they match — MOVE = (C0 ⊕ CP0) OR (C1 ⊕ CP1);
• pen down only in place — PDN = PEN AND NOT BUSY, BUSY = MOVE OR MOVED;
• our address — «none of the 14 bits differs»: SELP = NOT (G1 OR G2 OR G3 OR G4);
• who drives — the CD4019: D = A·REMOTE + B·NOT REMOTE.

And the program knows only three addresses: $C090 — steps and the pen, $C091 — limit switches and BUSY, $C092 — the colour.

What next

• Draw your own picture: add DATA lines in the plotter language (P, C, M, L, E) and a menu item.
• Change the spirograph: a different ratio of the radii (4 * T / 3 in lines 2636 and 2640) — a different pattern.
• Speed up the turret: the CKT clock at 60 Hz instead of 10 (does the table keep up? watch BUSY).
• A fifth pen will not fit — but why are there only four colours? Count the bits in the $C092 register.