Quote Originally Posted by BobP View Post
Hi,

Had a quick look at the schematic and can see nothing that I would expect to kill a PIC.

But I have some ideas for you to consider?
The 24vac supply for the power and the switches, are they independent or the same supply?
If independent are the earths connected? Can you isolate the switch earths from the rest of the circuit?

Having AC around with floating inputs definitely not recommended. Is there a good clearance between the 24vac and the PIC circuit? Can you tie the unused pins to ground with resistors?

Are the PICs killed within seconds of powering up? Are they killed on the bench or after connecting to its intended 'world'?

Hope this is of some help!

Bob

PS If the diagnostic input gets a static charge or voltage spike this would be relayed via the PICs internal port diodes to the 5v rail.
Everything uses the same power transformer and a common ground. The PICs are killed instantly when power is applied to the board on the bench. The diagnostic input is only used to bypass an internal startup delay timer. There is no connection to this pin other than a header socket so this should not cause any problems. Last week 4 PICs went south after passing a functionality test on the bench. The only thing that I can think is it has something to do with the floating inputs.