Saturday, March 9, 2013



A plotter (or graph plotter) is a device for producing graphical output on paper. By graphical output we mean graphs, designs and drawings output by the computer.

On most plotters characters have to be produced by tracing out their shapes, rather than by printing them. However, there are available printer/plotters designed to overcome this problem.

Plotters can be used on-line or off-line.

1 On-line

A plotting program running in the computer produces binary data which is sent directly to the plotter to control its movement.

2 Off-line

The plotting program produces the same data as before but the data is usually stored on magnetic tape (sometimes on paper tape). This tape can be used to produce a plot later, without the computer being involved, by using a reader attached to the plotter.

Advantage of off-line use of a plotter

Because of the slow movement of the pen, plotters produce output fairly slowly in comparison with other output devices. The off-line method allows plot data to be output to a high-speed medium, thus saving computer time.


1 Flat-bed plotter

The paper is held stationary on a flat surface. For large plotters (sometimes they are several metres in length and width) the paper may be held in place by suction from underneath.


The pen is held above the paper and can move:

(a) Up or down - When down it contacts the paper and draws lines.

(b) Across the width in either direction.

(c) Along the length in either direction.

By combining these movements lines can be drawn, broken or continuous, curved or straight, anywhere on the paper.

Plotters  2

Fig 9 A Hat-bed plotter in use-being driven by a microcomputer

2 Drum plotter

The paper, usually a roll, is held on a drum which can be rotated backwards and forwards.

The pen is held above the paper and can move:

(a) Up (to move without drawing) or down (to draw).

(b) Left and right across the paper.

Lines can be drawn anywhere on the paper by combining drum movements with pen movements.

Plotters 3

Uses of plotters

In any situation where hard copy of graphical output is required, for example to output designs which have been developed using a graphical display unit, or to produce maps.