Abstract: A string of short pulses is impressed across the input circuit of a multipoint recorder and a pulse magnitude detecting circuit. If the input circuit is open, the magnitude of the pulses appearing at the pulse magnitude detecting circuit is higher than would be the case if the input circuit were completed by a normal input device. The pulse magnitude detecting circuit is operable to apply to the measuring circuit of the multipoint recorder a positive potential whenever the pulse detecting circuit detects pulses of a magnitude indicating that the input circuit is open. As a result, the multipoint recorder is driven upscale to its limit where the particular point which is open-circuited is identified by a printed indicia.