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)
Best Answer
Ideally on an interface you want signal ground to make before any signal lines and break after any signal lines. If it doesn't you can get surges on the signal lines as any potential difference between the grounds of the two circuits discharges through the signal lines rather than through the ground wiring.
Unfortunately rs232 is an old interface. The plugs and sockets do have a shell that can be grounded but there is no gaurantee that any given serial cable or port will actually have wired that up. The main pins are all the same length so there is no gaurantee which will mate first or break last.
Normally I wouldn't worry too much about it but the fact your device is a hipot tester probablly means a greatly increased risk.
You say you have "no opto isolation", does that mean that one side of your hipot output is tied to the signal ground of your interface? if so then I really think you need to build isolation into your device.