Overview

Gerber data (RS274D) and extended Gerber (RS274X) provide an effective and generic way of providing manufacturing data for use by PCB manufacturers; the files are human readable ASCII text which means that they can be modified and generated by hand when necessary.

Gerber data files were designed to convey control information to GSI photo plotters and as such the data structures that they use are based on the way that the hardware of the Gerber Scientific Photo Plotters worked.

Essentially the photo plotter was a flat bed onto which a piece of photographic film was attached; the plotter then moved a light source over the film, turning it on and off as required to expose areas of the film. In order to make different width, a wheel with holes of different diameter in it was placed between the light source and the film; this wheel, called the aperture wheel, could be rotated by a stepper motor to select the required hole. If a pad was required rather than a track the source could be flashed, producing a mark on the film that was of the same shape as the hole in the wheel. The number of apertures that the machine could select from was limited by the size of the wheel. Since the wheel was fixed to the machine a table of apertures was usually provided to allow the PCB layout software to export data using the available apertures.

At its simplest, Gerber data consists of program control instructions,...