TESTING TELEPHONE LINES

An automatic remote line testing device at subscribers premises enables a telephone line (2-wire or 4-wire) to be tested from the exchange when the subscriber has equipment connected to the other end via a line isolating arrangement (TRANS) even when the equipment is providing a short or open circuit, the device comprising a command tone validating circuit (3) and a response circuit (11, 26). The circuit (3) comprises a bounded frequency counter (28) whose operation is recycled (29) a known number of times (32), the output of counter (32) operating a relay (26; 11c) if a valid tone is received, but the counter (28) is reset (35) if the frequency is high (34) or low (31, 33) or if a break in transmission occurs. The processor (3) can be realised in integrated form as a custom ULA.