On Jul 27, 10:31 pm, David Forbes <[email protected]> wrote: > You're not the first. > > I once designed a PC board that was missing a few pull-downs on CMOS > input pins. The board worked just fine in Arizona where we built it, but > failed in Dallas at a trade show. Humidity went from 10% to 95% on that > journey, which caused some unwanted connections.
Back in the early 80's, I designed a Z80-based controller board that was used at the radio transmitter site for FM subcarrier-based paging. Based on both the wire-wrap prototype and the first 10 pre-production PCBs working without a hitch, 100's of units were ordered. Then one of the 10 pre-production boards developed a problem, and would randomly transmit gibberish. After poking around for a couple days with a scope (we were too poor to have a logic analyzer), I discovered that one of the bus pins was being driven by a normal TTL driver, not a tri-state driver. The surprise was that all 10 PCBs and the prototype had worked for months without any problems. According to the databook, this was impossible. This became known in the company as "Terry's bumblebee" (at the time, bumblebees were thought to be aerodynamically incapable of flight, yet they obviously could fly). I don't have the original wire-wrap board (the client ran off with it, despite the contract saying it would be returned to me), but I do have 4 of the original 10 pre-production boards, including one in the complete system that we exhibited at NAB that year. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
