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March 2016 • The PCB Design Magazine 73 Including the netlist in the PCB fabrication data set increases the security of data transfer by an order of magnitude. The first thing a fabricator does after reading a fabrication data set into his CAM system is to generate a netlist—the reference netlist—from the image. During the CAM process the CAM engineer will regularly check the job data against the reference netlist to protect against operator or software errors. When a netlist is present in the incoming fabrication data set, he will also check his reference netlist against the supplied netlist. Any serious errors in the images or drill files will inevitably result in netlist differences and set off an alarm. The presence of the netlist in the fabrication data sets protects against mistakes in data transfer, whether these are due to software or operator error, in CAD output or CAM input. Adding the netlist to the fabrication data sets extends the regular netlist check performed by the CAM operator to encompass not only the CAM process, but the complete CAD-to- fabrication data transfer. Now, errors in the transfer of image data from CAD to CAM are rare, but they do happen. And they are very costly when they happen. Without the netlist, the fabricator cannot know he is working from a wrong image. He will faithfully manufacture the wrong PCB, which will pass his electrical test as it is tested against the netlist created from the wrong data. The error will only become apparent after the PCB is loaded with components, at which point the costs are staggering. All deadlines are missed, recriminations fly, and the search for the guilty starts. Simply including a netlist largely protects against such a rare but dramatic event – it is like installing smoke detectors and sprinklers. Fires the gerBer guide, chapter 7 & 8 Figure 2: Confusing a drawing and digital data.