I also face same issue (the subject). The project is Dynamic Check weigher(0-2kg/1gram,
speed 120 packs/min). In one of the posts Mr. Eric pointed out that system ground should be connected to Earth. I have not done that.

I have checked RS232 with a LABTOP computer, found no issues. With RS232 cable connected to a Computer(check weigher and computer both powered from same outlet), the sensors for pack detects erroneous signals with other problems.

Without the rs232 cable connected to external computer, the check weigher works very fine.

The erthing is good with 3V B/W NEUTRAL and EARTH. And also satisfies, V(L-N) = V(L-E) + V(N-E).

some more information may help me

Thanks