On my second job from college in the 1970s, I was working at a company that supplied bare PCB test systems. These systems were calibrated with a known-good board, then could test subsequent production boards for shorts and opens.

We were coming out with a new version, and my assignment was to design the digital logic to run the high-voltage “opens” and “shorts” test instruments. At that time there were no microprocessors, only TTL logic ICs; LSTTL and CMOS logic were not available yet. Also, the company used large wire-wrapped boards – not PCBs – to mount the ICs in their systems. My design had over 150 ICs, mostly 14-pin and 16-pin DIPs. The finished prototype looked something like this:

I started debugging sections of the circuitry, and all of the sections worked as designed except one critical section. I worked late several nights in a row trying to find the problem; I substituted ICs, re‑wire‑wrapped several ICs, removed wraps to isolate sections, all to no avail. My boss and the main engineer spent some hours trying to help. I ohm’d-out every section, and finally the section started working, but intermittently.

After working the weekend, I came in on Monday morning, frustrated to the point of giving up. I borrowed a flashlight and saw it: a 1-inch long crumpled piece of bare wire-wrap wire stuffed part‑way down into the rats nest. Upon the removal of the wire the system starting working perfectly. I was elated, my boss was happy, and everything seemed right with the world.

What bothered me though was, where did that wire come from? I finally decided that there was a saboteur in the group. I never found out who it was, and I had several successful designs afterwards. I never forgot the lesson; if everything else fails, put on your Sherlock Holmes hat and start considering what someone with evil intent might do to your design!

Mike McNatt manages EMC and safety compliance activity at Advantech B+B SmartWorx.

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