APERTURES, D CODES AND THINGS THAT GO BUMPIN THE NIGHT


	We have been feilding a number of questions about Gerber files
lately so this is written in an effort to give a history and overview
of plotting, in the electronic CAD enviorment.

	The name "Gerber" came from the company, Gerber Scientific, who
to the best of my knowledge were the first to plot from a computer file
directly to film. The way this was accomplished was by a flat bed plotter
complete with a moving light head. The apertures were machined in a metal
disk and the disk was rotated to position the reqired aperture in the light
beam. The light was either flashed for a pad or maintained on as the head
was moved to create a track. The Gerber conventions were D1 for flash
D2 for light on and D3 for light off. These Dcodes are taken directly from
RS274D which is the standard plotting convention. So now you can see
that a Gerber file is made up of a D code which tells the plotter
which aperture to use, a D code to tell the plotter to flash the light
or to maintain it on or off and a X coordinate and Y coordinate to move to.
These older plotters had a limited number of apertures that were
available, and each aperture had a specific D code. In the modern
raster plotter there is an unlimited range of aperture sizes and shapes
available because the aperture file is treated only as a lookup table.
Therefore you can specify any D code (10 or above) as any shape or size
of aperture your CAD will allow. Maximum size of flash is .5 inches.

	Now lets go to the drafting system you have. There is a great
variety ways in which drafting systems get the circuit you have just
drafted to a Gerber file that can be plotted. The more advanced CADs
allow you to specify the shape and size of the pads you wish to use and they
will generate an aperture list and assign D codes to each different
pad size for you. The less sofisticated CADs will give you a list
of the pad shapes and sizes you can use for drafting and usually give you
an aperture file that already has the Dcodes assigned to the sizes you
can use, or lets you edit the aperture file so that you can put in the
sizes that you used. This allows the possibility of changing the pad
sizes by altering the aperture file after you have created the Gerber
files. If you wanted a .075 inch pad and the cad system only had a
.070 inch pad, you can change the aperture file after you have created
the Gerber files and the plotter will plot the .075 inch pad. However
if you are going to do this type of an "end run" make sure you view the
gerber files for shorts etc. before you send the file away.

	Another fuction performed by the Gerber section of the CADs
is the generation of some sort of a drill data file. The varieties of
these are huge, so rather than try to explain all the varieties I will
explain what we need. The first and most important of the concerns
is to provide us with a list of the pad sizes you want drilled
and what drill we should use for each different pad size. If your program
will create an ascii drill file and tool file then once you have located
and edited the file which specifies which drill size for what pad size
(only one drill size per tool number) the problem is solved.
Remember our software can use only drill files and tool files in
ascii text. If you can't read them, neither can we. If on the other hand
your program will not create a drill file and tool file the drilling
information can be conveyed to us using a column in the aperture table
to specify which D codes you want drilled with what size drill.
As you may have guessed you can only have one drill size for each
D code, but you can use the same size drill on as many D codes as you want.

	 Since this service is new, and our custommers range from the
newest of CAD users to the most seasoned of the professionals, we are
watching for the problems encountered by our users and writing information
bulletins in an effort to help. If you have any further questions leave us
some mail on the BBS. All help articles will have a .inf suffix.