RS232 and ESD

Hi everyone, we are building a equipment powered by batteries. It has RS232 interface. When we plug/unplug rs232 cable (connecting to PC on another end) with the equipment on, the MCU inside the equipment sometimes crashed. I observed the 5V power line inside the equipment, high voltage spike can be caputred by the scope. I think it is caused by the different ground between the equipment and the PC.

Any idea to prevent such kind failure? The rs232 transceiver inside the equipment has ESD protection function.

Thanks for any suggestion.

Ground the PC. I've done this many times - get some decent ground braid, put a couple of spade lugs on it, and use a screw on the metal PC case to physically ground the PC to the equipment. It solves a lot of problems like this one.