A handheld battery-powered telecommunications test instrument for monitoring the status of a pair of wires. A tone-generating circuit generates a tone signal in response to the user manually selecting operation in a tone-generating mode. The instrument monitors the pair of wires to determine whether...http://www.google.co.uk/patents/US5157708?utm_source=gb-gplus-sharePatent US5157708 - Portable telecommunications test instrument with line condition monitoring
