Right: Viewer set to incremental when data is absolute. You see a "starburst" display similar to figure 3 on your CAM station you are probably trying to read a file written with incremental coordinates.įigure 3 Left: correctly displayed. What's worse is that many incremental databases don't bother to use the G91 command either. You may never encounter the G90 command in any Gerber file since most programs assume that absolute coordinates are the default and do not bother to write the G90 command. The converse to absolute is incremental - each coordinate is measured relative the previous coordinate value and is set by issuing the G91 command. The G90 command in line 1 tells the machine that data coordinates are absolute. Used to prepare the machine's state prior to drawing. Gerber calls the Gxx commands preparatory codes. There seems to be different kinds of commands: instructions beginning Without any prior knowledge of Gerber one would correctly deduce that each line representsĪ particular machine command and that the asterisk (*) is the end of commandĬharacter. The line numbers at far right are not part of the file. One man's feature is the next man's unsupported problem.īelow is a simple Gerber file illustrating the structure and content of the format:
Plus whatever features they feel necessary to differentiate their machine. Every photoplotter manufacturer supports the basic Gerber commands Further, the temptation to create non-standard extensions This missing data is handed from the designer to the plotter operator separately,Īnd is often the source of errors. The Gerber file lacks essential information necessary to Space is measured in hundreds of megabytes instead of hundreds of bytes.īut simplicity exacts its own price. Into as few bytes as possible explains many of the "problems" we encounter today when storage The need to compress as much information as possible The database had to be simple and compact because the first machines wereĭriven by a fragile punched paper tape. There are only four essential commands plusĬoordinate data. The beauty of the Gerber database is its simplicity. Given the proper commands for table motion, aperture selection and shutter operations, one can construct When the table simultaneously moves while the shutter is open, a line or stripe is imaged onto the film. When the shutter is open light shines through the aperture exposing the aperture's image onto the film. A controller converts Gerber commands into the appropriate table movements, wheel rotation and shutter opening. A bright light source is directed through a shutter, through an aperture wheel and focused onto the film. Early photoplotters consisted of a precision servo controlled X-Y table to which a piece of high contrast film is attached. Misunderstandingsīetween the PCB designer and photoplot operator account for a large percent ofīefore diving into the details of the photoplot file a short discussion of photoplotters will make the following material easier to digest.
Aggravation can be reducedīy better understanding what is contained in a photoplot file.
No matter how powerful your design software may be, you must eventually createĪ photoplot file in Gerber format to create film.