7i77 burned, cause?

On a retrofited Biesse I have a situation where both 7i71 cards burned out all same chips are damaged on both. 7i77 and 7i70 cards are fully functional.
7i71 outputs are wired to factory plc mosfet outputs. Two cards have nothing in common except 24v power supply that they also share with 7i70 which are ok.
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If you had multiple cards damaged at once that suggest's
some common issue (the 24V power?)

Note that a 7I70 (an input only card) will withstand up to 50V on its field power
input but the driver chips on a 7I71 will be damaged at about 36V

Could it be that some voltage or short came back on some of output pins. Manually applying 24v on outputs activates all plc outputs and all solenoids activate 

IT looks like you connected your mesa outputs trying to drive the mosfets on the Biesse machine directly.

This has been in use for 4 years now

you should know what kind of plc and how it was connected .. if it worked for so long it means it wasn't bad. maybe a shorted mos brought back voltage from the gate.
regards
Mauro