Communication system, converter for use therein and method of signalling

A communication system comprises first and second devices ( 2,3 ) having respective housings ( 5,6 ); a data trans-mission line ( 8 ) for transmitting data between the devices in a reflective signalling format; and conversion means ( 4 ) connectable to the line externally of the housings ( 5,6 ) for converting data between a reflective signalling format and another format. A method of signalling between first and second devices ( 2,3 ) linked by a transmission line ( 8 ) and of sensing a security violation of the line ( 8 ) comprises the steps of (a) transmitting a signal from the first device to the second device, reflecting the signal back to the first device in a manner corresponding to a first bit sequence, receiving the signal thus reflected at the first device, and comparing this signal with the transmitted signal to determine whether there has been a security violation of the line and to extract the first bit sequence.