As I said the inputs are opto isolated, I would expect these to fail as well. Likewise with power, non of the regulators etc have ever failed. Only way I can think of, is for the supply to dip below ground and cause the pic to conduct, as it is a battery backed supply, it seems unlikely that is the case. There appears to be very little RF that I can measure. I know the circuits are ok, as we have many near identical system, that run without issue. There is just one change in that data gets corrupted without termination at the control and the end of one of the runs. This is not normally needed on other systems as the rate is only 300 baud. Cannot think that this is causing that sort of problem though. maybe I should consider opto isolation in sections. Baffling!