A mission critical fire alarm system has a GSM module that is connected via RS232 to a computer whose task is to notify emergency response teams. In case the computer fails, an off-site operator is notified and he needs to switch to a backup computer within a minute. What I'm looking for is a solution this operator can use to remotely "unplug" the RS232 cable and "plug" it to the other computer. (I was informed that splitting the cable was not a reliable option.)

I'm looking for cheap commercial solutions or building something with Arduino. The important part is that it after a handshake is established, it should forward communication transparently and reliably.

The remote control part is nothing fancy: it is entirely possible to plug the device to a third computer on site and access its interface program via remote desktop. So USB control is fine as well as Ethernet access and a mini Web server.

My electronics knowledge level is rather low: I can solder some well documented digital chips together with some capacitors and resistors on a protoboard but I could never figure out their values myself or design any analog circuit containing more than one transistor. Please take that into account in your response.