Signalling system for locating error source

Claim 1

In a line transmission system comprising a transmitting terminal, a receiving terminal, and a plurality of signal handling arrangements betweEn the transmitting terminal and the receiving terminal, the combination of: a plurality of tone generator means connected to said transmitting terminal for producing a set of tones less than all of which are transmitted to said receiving terminal at one time in which the tones of said set are different so as uniquely to identify each of said signal handling arrangements; a plurality of tone transmitting loops, one associated with each signal handling arrangement, each loop including modulator means for modulating that tone unique to its associated signal handling arrangement, error detector means for causing said modulator means to modulate said unique tone in accord with the presence of a transmission error at the associated signal handling arrangement, and tone detector means for energizing said error detector means only in response to the presence of said unique tone; and means connected to said receiving terminal for determining the presence and location of an error source in response to the presence, if any, of said modulated unique tones.