Tim's Takeaways: Rules Keep You from Crossing the Line


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I have the privilege of living in a neighborhood along with a lot of other families. In the winter, we enjoy indoor holiday celebrations, and during the warmer months, we enjoy outdoor potluck dinners. We help each other out with various chores, and routinely celebrate our triumphs and victories together. No, this isn’t the fictional TV town of Mayberry, but at times it almost seems like it. And one of the greatest joys of living here is all of the children. These little ones laugh and play and want to tell me all about their day at school. While waiting for their parents to finish barbequing the hot dogs, they ride their bikes, trikes, and scooters all over our cul-de-sac. It really is perfect.

In 10 years though, this story will be a little different. All of these little ones will have grown into teenagers, and they’re going to exchange their bikes, trikes, and scooters for cars, trucks, and motorcycles. You can bet that these same parents who take delight in sneaking their kids an extra cookie during the potluck dinner will be creating detailed lists of rules and constraints to keep their teen drivers safe. These rules won’t be limited to just attending driver’s education and successfully passing their driving test though. I’m sure that school grades will also have to be maintained, curfews will be in effect, and of course, their driving skills will be monitored. There will be a zero-tolerance policy regarding unsafe driving practices or breaking the rules of the road.

Driving Rules

Strict regulations like these demonstrate how much parents truly love and care for their kids by trying to keep them safe, even if the kids don’t really believe it themselves. Driving rules are designed to keep drivers between the lines of traffic instead of crossing over those lines into dangerous situations. Similarly, design rules are also intended to keep PCB trace routing between the lines instead of crossing over them as well. But you might be surprised how many people refuse to use the full potential of their DRCs to protect themselves, and in some cases, refuse to use them at all.

When online design rule checking first appeared in PCB design tools many, many years ago, there were a lot of problems associated with them. At first, they only had a few (if that many) clearance settings, and those didn’t always work as they should. I remember being warned of clearance errors on some tools that actually weren’t errors at all; the tools just weren’t reporting them correctly. This would happen more often if you were using oddly angled traces on too fine of a grid. It was not unexpected then that PCB designers back then were sometimes leery of using or relying on DRCs.

Another problem during that time was PCB design was largely converting over from hand-taped designs to computer-aided design tools. If you aren’t familiar with the process of hand-taping a design, PCBs were laid out at two, four, or even 10 times the actual size working on a transparent grid over a light table. The designer would apply opaque tape and dots to create the traces and vias of the layout using the grid for a reference. The grid, plus the designer’s experience, was the only clearance checking available, and designers would maintain the correct trace to via spacing by eyeballing it. If something was too close together, an X-Acto knife would be used to trim the tape or dot down a bit.

To read this entire column, which appeared in the May 2019 issue of Design007 Magazine, click here.

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