Back in the day, when microprocessors were just beginning to rule the world, I was on a design team that included a very senior, extremely detail-oriented designer. His schematics were works of art, done with technical skill and achieving near perfection.
Many times, we needed no changes between his initial hand-drawn design and the final one, except when a vendor’s part did not actually perform as claimed on the data sheets, or when there were unforeseeable mechanical-fit problems in layout which required some adjustment of the design. All this was in the days before modeling, simulation, and the other tools we routinely use now, which made his work even more impressive.
As was common at the time, his designs used many 7400 TTL digital-logic ICs, available as second sources from about a dozen vendors (and exemplified at the time to most EEs by the Texas Instruments’ mustard-colored data books which the really senior engineers had in a hardcover version).
That’s how I learned an important lesson about the transition between engineering and production/manufacturing.
At one of the final design reviews, someone asked if a diode wasn’t needed between the TTL IC’s output and the next stage in this particular configuration. In response, this senior engineer explained how he had ensured that the output of the gate would properly interface with the subsequent circuit: he saw that one — just one — of the many sources for the 74XX IC he was using incorporated an internal diode in their implementation, thereby eliminating the need for an external diode. Pretty smart! Here is a closer look at the TTL devices, courtesy of Wikipedia.
That’s where the project leader jumped up: “You mean we are counting on the fact that this vendor internally uses a certain topology? And they could change that at any time with a different die design, but still meet their output specs?”
That’s a real warning sign, of course. Then the purchasing liaison pointed out that they did not like to be locked into a single vendor for parts which were otherwise widely available, and 74XX parts were pretty much in the “buy the cheapest ones available, from anyone on approved vendor list” group.
Both the project leader and the purchasing fellow were absolutely right. It’s unwise to count on an internal aspect of a part in a case such as this one, even if it cleverly saves the cost of an external diode. What would have happened is that the prototypes would have worked, and maybe even some early production units. Then at some random time in the future, we’d have gotten mysterious failures in production test or the field which would consume days and weeks to figure out — if we ever did.
Sometimes you can be so smart that you outsmart yourself, or inadvertently plant a delayed-action design bomb in your product. I’ve seen similar situations, though not quite as drastic, with mainstream analog parts such as op amps, as there are so many of them that seem so similar, offered by a single vendor as well as multiple vendors. So when your design counts on a second- or third-tier parameter or specification in order to work “just so,” and somehow that value changes or the part’s source is changed (and maybe you even OK’d the change, having forgotten why you wanted that particular part in the first place), it could also mean you’ll be spending long nights firefighting in crisis-mode a few months or years later.
Have you ever seen a designer be a little too clever in hardware or software? And confess: Have you ever done it yourself, only to later regret it?