The CalComp plotters which were a standard item of equipment in many computers of the 1960's and 1970's responded to octal commands, whose three bits indicated a raised or lowered position for the pen, rotation of the drum up or down signifying increments in the x-axis, and sliding the pen carriage left or right, along the y-axis.
Above and beyond the data stream arriving from the computer, manual controls turned the instrument on or off, and allowed positioning of the pen. An interlock interrupted horizontal motion, preventing the pen carriage from being forced into the margins, but no mechanical check existed for the presence of paper, or running off the end of a roll.
Programs supplied with the plotters changed absolute coordinates, taken as floating point numbers supplied in terms of inches, into a succession of increments through which stepper motors placed the pen where it was desired. Digital increments distinguished the CalComp line from plotters which used digital-analogue converters and servo motors; placement of the pen was more accurate and reproducible, varying somewhat with the quality of electronics employed.
There were essentially four basic programs accompanying the plotter:
initialize the plotter
Strictly, the last two could have been created from pen movements alone; the combination of their utility and complexity meant that they were included amongst the basic offerings.