At 06:16 PM 9/13/2011, Brent Brown wrote: >What I strongly suspect is happening is that plugging into header=20 >pins the GND pins >are not contacting first (there is nothing to guarantee the order of=20 >connections) and >when +12V pin contacts is couples power through discharged=20 >elelctrloytic caps on >daughter board to GND rail, then "backfeeds" to +3.3V rail through=20 >internal body >diodes of chips on that rail, pushing up the +3.3V rail and popping=20 >the processor on >the main board. Similar could be happening on RXD, TXD and +5V=20 >rail... but nature >of 2 failures so far suggest +3.3V rail more likely. Just a thought - can you add a power switch (MOSFET or whatever) to=20 the 12V line as it enters the daughter board? The 12V rail would be=20 dead until something tells it that its OK to power-up. Note that this switch is on the daughter board, since you mentioned=20 that you would like to avoid making design changes to the main board. For example, you could make a simple one-transistor controller that=20 monitors for about 3V between the daughter-board ground and 3.3V=20 rail. If either ground or 3v3 was missing, don't allow the MOSFET to turn = on. dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .