One of our technicians accidentally connected a laptop PC serial port to a RS 485 comm port on a compressor controller. He has since utilised a signal converter to confirm that the port does not function, but... the manufacturer is claiming that we caused the comm port failure even though the communications did not work right from day one. Is it possible that this caused any permanent damage to the comm port on the controller? Do we have any defense against this accusation?

 

It isn't clear which port is dysfunctional, the RS-232 COM port on the PC or the RS-485 on the compressor controller.

From memory, RS-232 works at something around 10 or 12 volts, and RS-485 works at something around 5V, I suppose that excessive voltage from the RS-232 port could damage RS-485 electronics.

I have witnessed 2 laptop serial RS-232 COM ports whose owners had never used the port and which turned out to be dysfunctional when I attempted to use the ports, leading me to believe that serial COM ports are never tested by a PC manufacturer.

It isn't clear to me how much overvoltage RS-485 can handle, but I routinely supply isolators for all embedded serial ports, whether RS-232 or RS-485, because the replacement costs are so high with usually non-stock parts.

My serial isolator vendor also carries other lines and after every electrical storm here in the midwest they get calls for replacement serial port parts, sometimes expensive controller cards or controller modules, otherwise functional except that the serial port is bad.

Isolation pays for itself.

Carl