The PC uses a 300Watt SMPS that has a whole lot of certifications punched on it ( meaning it is NOT home made )

The MCU uses a good linear power supply built around 317 regulator.

The Earth - Neutral voltage is always monitored and the trip circuit issues a warning if this crosses 2.0V AC.

The 230V supply to the PC is via an isolation transformer in all our PC based DAS systems.

And I do not know why the D Connector case is being brought into this discussion. In my application it is Plastic and let us stop discussing that. Even if it is of metal, I would not dare to use THAT for a signal path.

Anyway I am trying a few options starting with the Opto Isolated RS232 and shall post the results once we are through.

Raghu