I don't know many things about this subject, so I'm searching information only.
I think that in equipments with linear power supplies these things are different to switched power supplies ones.
In MY case, earth MUST be connected to the chassis of the device, this is clear. Its serial port connector (DB9) is metalic, so its chassis is connected to earth. If I put a serial cable to a PC, the chassis of it, and the pin 5 are joined in its PCB (it's proved), so through the cable, in MY device, the pin 5 is connected to earth. I think it's better that this connection is done into MY device, not through 15 meters of unpredictable cable.
I can't explain this, but if two devices must share a reference it's better that this is acomplished with the earth line (the other option could be through a thin wire into the RS232 cable).
One more time this is withcraft to me.
What are your opinios ????