Dear Reza,

Yes the points mentioned by you are quite relevant.

1.Software bug: As far as I have checked up there is no statement in the area shown by you that writes to the crucial bit by mistake. Anyway the other ideas of checking DIN_SYSOK and multiple checking can be some insurance against such faults.

2. EMI : The PLC output to the MCU unit is at 24V level and there is good opto isolation at the input of the MCU unit.

3. Power supply spkies : A strong possibility and the only one at this point of time. The whole set up is in operation for almost two years now and this is the first such event. Thanks for the link to the Intel document.

4. The PLC checks something more than a dozen inputs to synthesise the SYS_OK output.

Regards

raghu