The last step in circuit design is the generation of artwork (or gerber) data. In 1980, Ucamco released an ASCII vector format for PCB manufacturing that describes each layer of a circuit via an image. This includes all conductor layers, mask layers, legends, drill data, and so on. Artwork data is used in the manufacturing of a PCB with special tools that rely on EDA (Electronic Design Automation) and CAD (Computer-aided Design) systems. It is used as the standard input format of assisting machines ranging from photoplotters up to AOI (Automated Optical Inspections).
There are two types of artwork data: RS-274-D (standard type), which is obsolete and not widely used anymore; and RS-274X (or extended type), which is the current standard that now includes coordinate units and apertures. As such, RS-274X is a superset of RS-274-D that no longer requires additional files to represent the PCB data to be manufactured. The image in blue is how a typical artwork file looks like.
The above example is for a conductor (copper) layer. However, as mentioned, other types of data can also be included in the artwork files. For instance, the image in orange shows the artwork file for the top solder mask layer of the same circuit.
Allegro PCB Editor reads film control records to determine the number of artwork files to produce, their names, and list of classes and subclasses to include in each file. The Color Dialog form sets the required layers and then the Export Artwork form configures features like rotation, offset, mirroring, etc. This is also where the gerber format is chosen, as well as size limits, name affixes, units and apertures, amongst others
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