Presuming that you are talking about the bottom transistor, my guess is 
that you punched its gate:  There is nothing to prevent the gate from 
floating up/down out of the maximum voltage range.  The upper transistor 
at least has the 100k resistor to keep it from accumulating a charge.  
(The 2N7000 isn't rated for more than about +-40 volts under any 
circumstances.)

A similar resistor (to pull up or down) on the gate would probably work 
fine - as would a (more expensive) zener in the range of >5 to 15 
volts.  (Just a plain diode would clamp it in one direction - but not both.)

Clint


>I am trying to design an rs232 circuit from 2 transisitors. The circuit shown has worked well, except one day when changing the MPU, the MPU receiver transisitor popped.
>
>http://flowperformance.com/images/rs232circuit.gif
>
>Are there any major flaws with this circuit, and how can I prevent the MPU receiving transistor from popping?
>
>Thanks
>
>John
>
>  
>
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