Tuner has an ID (3 or 4 bit, I don't remember) settable via one of those tiny rotating things one uses to set SCSI address, and is capable of reporting it via its serial interface. So, idea was, if I understood correctly, to either have tuner set to a particular ID which would mean "tuner at home", or, if it is not connected, to plug into its connector a kind of loop-back thing which would connect Tx and Rx pins, which is also software-detectable. So, the SW would first test for 'tuner has "at home" ID set' condition, and if it finds no tuner at all, for 'loopback connector present'.