I made up a number of F628 circuits that have been installed (not
by me) in small planes, and all seemed to be well until a couple of
the pilots reported back that, when using the radio, now and then
the PICs acted as though one of the circuit switches had been
operated. The symptoms indicate that noise is getting into at least
Portb4 and being interpreted as a switch closure. I've not yet been
able to get to the either of the circuits to physically check how they've
been installed, mainly because they're overseas. It couldn't have
been the ones just up the road that went funny could it !!

All circuits are in closed aluminium cases bolted to the plane's
chassis and I recommended that grounded screened cable be
used for signals (about 2m long) and power lines. The cable
plugs into a 9 pin D. Portb4, like other inputs, already has pulldowns,
ceramic caps, Zeners and Schottkys on it. Portb4 is the first
tested in a switch checking loop, which is why I'm assuming (oh-oh)
that's the reason for the faulty operation

The radios causing the problem are 5W @ 100-150MHz. I've sent
the installer some ferrite beads to see if that cures it, and someone
has suggested winding the cable once or twice around a ferrite ring.
Maybe it needs a ferrite + choke as well, or something similar to
the surrounding ferrite on a printer cable

If anyone has comments that would be appreciated

TIA

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