When I was working on semiconductor machinery, we used TUV to get CE certification so we could sell the machines in Europe. We got through emissions alright, it’s similar to the FCC testing we already did, but immunity testing was brutal. When we broadcast RF at a machine, the wafer elevators went nuts and started breaking wafers. We had managed to convince the TUV guy that the speckles and snow on the monitor were not technically a failure, since you could still read it. But robots going open-loop? No, nobody could talk that past TUV. Turns out the cabling was the culprit. There was shielded twisted pair to the Banner sensors that located the elevator stops. In fact, I think they even used braid+foil shielded wire. But the semiconductor machinery company connected the cables with those red-brick AMP connectors, the MR series.
MR These MR (miniature rectangular) connectors work great for appliance wiring, but they provide no continuous shielded path for radio frequency interference (RFI).
Now designing cabling is often thought of as a mechanical engineering function. But mechanical engineers often don’t understand the principles of RF shielding. Get this— they cut the cable shielding about 2 inches back, connected the power, ground, and signal to pins, and yeah, they connected the braid to a pin, and sent it into the connector, to mate with another cable that had 2 inches pulled back. The cables were all dressed beautifully and shrink tubing everywhere. But like my buddy says—“4 inches of untwisted unshielded wire is a nice antenna.”
D-sub The D-sub connector was developed for military applications and then picked up by PC makers for serial, parallel and video ports. One reason is its good RF performance. Make sure your cable braid contacts the metal shell.
I switched them to D-subs using 9 pins with a metal shell, and we finally passed. So remember, RF energy is like light—it can leak into the smallest spaces and screw things up. Make sure the EE department revises the detailed design of the cable, or your machine might get held up in certification too.
Atmel | Bits & Pieces 