A bit of history

The first plotter, 1959

The computers of the 50s could only print letters. In 1959 the Californian company CalComp released the CalComp 565 — a drum plotter: the paper was wound on a drum, the drum turned with a stepper motor (that is the Y axis), and the pen rode along it on a rail (the X axis). Each step was a hundredth of an inch. Aircraft drawings, maps, charts — everything that used to be drawn by hand, the computer now drew itself.

the drum turns — the Y axis; the pen rides along the rail — the X axisCalComp 5651959
The CalComp 565 drum plotter: paper on a drum, the pen on a rail, two stepper motors. One step is a hundredth of an inch.

A straight line from steps, 1962

A motor steps only along its own axis. How do you draw a slanted line? In 1962 the IBM engineer Jack Bresenham came up with a way for a CalComp plotter attached to an IBM 1401: step on the long axis every time and on the short one when enough has built up, with additions only, no division. His algorithm still draws lines on screens — and in our plotter’s program (line 1000).

16 steps in X, 6 in Ya Y step — when enough has built up
A straight line from steps (Bresenham, 1962): on the long axis a step every time, on the short one when the counter reaches 16. Additions instead of division.

The head changes the pen, the 70s–80s

A colour picture means several pens. At first they were changed by hand, then plotters learned to pick a pen themselves: some kept the pens in a «garage» on the side, others in a circle in a turret head. In the mid-80s Apple sold a colour plotter with such a head for four pens: the computer said «colour 2», the head turned, and green came to the paper. On the panel — the same buttons as in our lesson 1: arrows, PEN UP/DOWN, SELECT, REMOTE/LOCAL and the POWER and ERROR lamps.

d0d1d2d3«colour 2»POKE 49298,2the head turnedgreen to the paperd0d1d2d3
A four-pen turret head: the computer names a colour — the head turns and the right pen comes to the paper.

The Spirograph, 1965

The British engineer Denys Fisher invented the Spirograph toy: a toothed wheel rolls inside a ring, a pencil sits in a hole of the wheel — and a pattern of loops appears. Mathematicians call this curve a hypotrochoid. The seventh picture of our program is a spirograph: a wheel of radius 45 rolls in a ring of radius 105, the pen 60 from the wheel’s centre.

ring 105, wheel 45, the pen 60 from the wheel centre
The Spirograph: a wheel rolls inside a ring, a pen in a hole of the wheel draws loops. The seventh picture of the program — three laps, each with its own pen.

What became of plotters

In the 90s inkjet printers learned to print colour drawings faster than pens, and pen plotters nearly vanished. But the drawing mechanics stayed: cutting plotters cut film for signs, CNC machines lead a cutter along the same two axes, and a 3D printer is the same plotter with a third axis that «draws» with plastic layer by layer.