A digital plotter is a computer output device which draws curves and other computer graphics data on ordinary 8-1/2 × 11 inch (21.6 × 27.9 cm) paper. See Fig. 2-24.

Data from the computer is translated into signals which cause tiny incremental movements of the plotter's pen. The pen on the plotter (a felt-tip ink pen) is moved by a combination of X and Y carriage motions. The pen can move in eight different directions. These eight directions are used to simulate all the movements necessary to create curves. Since the incremental movement of the plotter is small (0.005 inch or 127μm) proper selection of directional moves combine into what the eye perceives as a smooth curve or line. See Fig. 2-25.


Fig. 2-24. A digital plotter provides up to 200 incremental movements per inch, and requires RC-232C serial or parallel 6-bit TTL data.

Because of the extremely small movements that the plotter can make, drawings on it are very precise and detailed. Fig. 2-26 shows some examples of plots made with the Houston Instrument HI PLOTTM plotter.

Using the plotter from BASIC requires that the output program send the plotter ASCII characters that stand for the incremental directions. For example, to draw a line 0.1 inch (0.25 cm) long in the +X direction the code string “z sp-sp rrrrrrrrrry” is sent to the plotter. The z stands for pen down, sp stands for an ASCII space character, r means move right, and y means pen up.


Fig. 2-25. Example of resolution of digital plotter.
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