Patent GB-1241585-A (GB)
Title: A method of transmitting supervisory and alarm signals in an electric alarm system and a system for carrying the method into effect
IPC: H04M11/04
Filed: 1968-09-06
Abstract: 1,241,585. Electric alarm signalling systems. DANSKE SECURITAS A.S. 6 Sept., 1968 [7 Sept., 1967], No. 42654/68. Heading G4H. In an alarm system each of a number of alarm stations has a source of direct or alternating current and is connected through an alarm line 3 to a control station 4, connected in turn to a supervisory station, the alarm lines being monitored periodically by means of synchronous selectors at the control station and the supervisory stations to detect supervisory and alarm signals on the alarm lines, means at the control Station generating timed pulses corresponding to the supervisory and alarm signals which are transmitted to the supervisory station and registered, being compared with expected conditions and used to generate control pulses for stepping the selectors to monitor the next alarm line. A tone generator 7 is connceted to the line 3, which are telephone lines through link 8 and the telephone 2 is connected to line 3 through filter section 9 which blocks alternating current from generator 7. The generator 7 may produce a signal of about 1 volt at a frequency of 5000 H2 which is transmitted over lines 3, being connected, for example, by contacts not shown to indicate an alarm condition. When there is no alarm the tone voltage is transmitted via link 8 and reduced to about 10 millivolts to provide a supervisory signal. This low voltage signal does not disturb any telephone conversation on line 3. The signals on lines 3 are transferred through a link 11 to a selector system 10. Line 12 to the telephone exchange is isolated by a filter 13 which prevents alternating current flowing into the exchange. The selector consists of a tens selector 10a and units selector 10b which apply the signals on lines 3 in turn to a selective amplifier connected via rectifier 15 to Schmitt triggers 16, 17 responding to the supervisory signal and the alarm signal respectively. Each Schmitt trigger is connected to an integrator 18, 19 connected in turn to a pulse. forming network 20. The output pulse is generated at a time depending upon which integrator is operative. The time is reckoned from the moment that the lines are selected by the selector 10. The pulses produced by network 20 are passed over line 21 to a supervisory station 22, Fig. 2. The system may be arranged to transmit other indications such as whether the alarm station is connected or not, whether the alarm is due to fire or burglary. Such extra indications may be signalled by modulating the tone frequency at a lower frequency. Such a modulated signal is detected by circuit 23 and the output is connected through rectifier 24, Schmitt trigger 25 and integrator 26 to pulse-forming circuit 20. The output of circuit 20 therefore generates a third timed pulse indicating that the signal is modulated. The supervisory station 22 is synchronized with the selector 10 by a timed sync. pulse Is transmitted first in each cycle. This is followed by an alarm signal I A  and by a " fault on the alarm line signal I u . This is followed by a system connected" signal I M  after which may come a special alarm signal I AM . The different signals are distributed at the station 22 to different inputs 38 of a register 37. The sync. signal Is is applied to a circuit 53 controlling the synchronization between the selector 10 at the station 4 and selector 42 at supervisory station 22. The selector 42 is connected to scan the columns of a plugboard 41 which is plugged according to the expected conditions at each station. The actual and expected conditions are compared at 44 and if there is agreement pulse transmitter 45 steps both scanners to the next line. If an alarm signal appears in any line, amplifier 54 applies a signal to line 21 to receiver 36, Fig. 2, which returns a signal to cause the selectors to scan rapidly until the alarm line is found. Instead of using A.C. sources 7, dircet current may be used for signalling, rectifiers, then being unnecessary.