The plotting thickens

The shortest distance between two points is …

It’s the late 1970s and pilot fish is working for a small company that produces custom solutions involving computer hardware, software or both. A large customer wants a pair of drafting machines — pen plotters that can produce large-scale physical engineering drawings from CAD systems. 

And fish’s company comes through, integrating the plotters with DEC PDP-11 computers, which had a pair of DEC RX-01 8-inch floppy disk drives to import and export the CAD data. The flatbed plotters measured about 4 by 10 feet, and the huge sheets of BoPET polyester film that are used for the drawings rather than paper are held in place by tiny holes in the flatbed that lead to a vacuum chamber, like an air hockey table running in reverse.

Everything works perfectly, and everyone is happy. Until …

After several months, the client has a complaint: The machines are no longer drawing correctly. As a demonstration, it produces a simple test drawing: a straight line drawn diagonally across the plotter, from Point A to Point B, and then back again, from Point B to Point A. The result should be a single line, but instead there are two, looking like a cross-section of a convex lens.

This doesn’t sound like a software issue, so fish lets his hardware colleagues head to the client’s site to study the problem. And the team watches as that line gets plotted (and fouled up) again and again, until someone finally notices something: While the plotter pen appears to be moving in a straight line, the plastic sheet seems to shift from side to side. In other words, it’s not the system; it’s the plastic sheet that’s at fault. In fact, it’s slightly domed, which pushes the pen to one side when it goes in one direction, and then to the other when it returns.

The solution: Get a new vendor for those sheets of drawing plastic.

Hardware, software — Sharky loves all your true tales of IT life. Send them to me at You can also subscribe to the Daily Shark Newsletter and read some great old tales in the Sharkives.

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