HP-GL, the language of pen plotters

HP-GL (Hewlett-Packard Graphics Language) is the set of commands Hewlett-Packard's pen plotters understand: two letters, then numbers, sent to the plotter as plain text. HP began marketing its two-letter language under that name in 1977, with the 9872A plotter.¹

A short program

IN;
SP1;
PU0,0;
PD4000,0,4000,4000,0,4000,0,0;
PU;

IN initialises the plotter. SP1 selects pen 1. PU (pen up) lifts the pen and moves it to the point given, here 0,0. PD (pen down) lowers the pen and draws to each pair of numbers in turn, so this one command draws four connected lines: a square. The last PU lifts the pen again. Each command ends with a semicolon or a line break, and the numbers must be separated by commas, not spaces.²

The numbers are plotter units. One unit is 0.025 mm, the smallest move the plotter can make, so 40 units make a millimetre and the square above is 10 cm across.³

PA (plot absolute) measures the coordinates that come after it from the plotter's origin, as above; PR (plot relative) measures them from where the pen is.²

New drawing

The plotters

The 9872A cost $4,200 in 1977. It held four pens; the 9872C of 1981 held eight.¹

The 7470A of 1982 was smaller and cost $1,550. Earlier plotters held the paper still and moved the pen over it on a mechanical arm; the 7470A moved the paper instead. The HP Computer Museum calls it the world's first small-format paper-moving plotter. It held two pens and drew on A4.⁴

The 7475A followed in 1983 at $1,895. It had a carousel of six pens and took A4 and A3. In 1984, helped by the IBM PC and Lotus 1-2-3, it was the most profitable single product HP made. The HP Computer Museum calls it "the most popular pen plotter ever made"; it was not discontinued until 1994.⁵

HP-GL today

The command-line tool vpype can turn an SVG into HP-GL for vintage plotters, for example with vpype read input.svg write --device hp7475a output.hpgl. It has to be told the plotter and the paper size, because, as its documentation says, HP-GL coordinate systems "vary widely from plotter to plotter and even for different physical paper format".⁶

Olof Broström's Unverified Air at JAEV is stored as files for a pen plotter, in black, red and blue ink.⁷ The machines that came before HP-GL, and the artists who used them, are described in the notes on the CalComp 565 and the Zuse Graphomat Z64 and in JAEV's text on plotter art.