I have a circuit with a dsPIC which communicates to a computer using a MAX232 (there ain't any optical isolation).
The circuit ground is not bounded to the earth. But the equipment case is earthed. The computer is connected to the equipment through a DB9 connector on the rear panel of he equipment.
It works fine most of the time. But some clients (not much) after receiving the equipment complain about the RS232 link, which doesn't work. When I get back the equipment I see that the MAX232 is not working, and I have to replace it (but it worked before, I have tested it).
Is there a "good practice" of putting some protection for the RX, TX after the MAX232, like a TVS or something? Or my mistake is to not bound the circuit GND to the earth to prevent voltage differences when the serial cable connects the equipment's GND to the computer GND?
edit: Device in question (from comments)