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Testing Todd: Looking at Digital With 11111100100 Vision

Sitting down to my first exposure to the modern technology of the era was an original IBM PC (4.77 MHz.) If you could find the 5¼ inch floppy disk with DOS version 0.92 (which I still have for nostalgia), you were up and running. Insert that disk into drive “A,” flip the big red power switch on the side, and after some “whirring and buzzing,” you would be given the date/time prompt. It works! Now what? I’m staring at an A> on the screen with a flashing cursor. That’s what it was just a mere 34 years ago.

Circuit boards of the time were mostly double-sided, with some larger manufacturers delving into the 4, 6, 8, and 10 layers. All were plated through-hole only (PTH). The big push of the time with the computer explosion was the motherboards (18” X 18” boards with all drilled holes). These were the PCs and workstations of the time. The electrical test of the time was extremely easy compared to now. CAD systems were laying out the boards on 100 mil grids for the DIP packages of time. Universal grid testers (fixture testers) were all 100- mil grid as well. There was no such thing as “Netlist” testing.

Back then, it was all self-learn and compare. Creating the test fixtures was a process of manually digitizing the outer layer film. There were no electronic drill files. Remember that we are playing with 360K DSDD floppy disks. Once digitized, the result would be plotted on white paper/mylar and then overlaid on the outer layer film to check for accuracy and/or missing holes. When approved, the drill file would be punched to tape. It would then be read into the drill machine, and the plate would be drilled. These fixtures were typically 1” tall and had a reusable 100 mil base plate.

Once drilled, the plate would again be checked for accuracy, and along with standoffs, it would be assembled. Once complete, the fixture would be placed on the fixture tester. A single board from the lot would be “learned” by the machine. Once learned, subsequent boards would be tested against the learn. If the majority of the lot “passed,” the learn would be considered “gold” and could be saved for future lots.

To read this entire column, which appeared in the February 2020 issue of PCB007 Magazine, click here.