Patent GB-1462096-A (GB)
Title: Electrical signalling system
IPC: G08B26/00
Filed: 1974-04-11
Abstract: 1462096 Selective signalling AMERICAN DISTRICT TELEGRAPH CO 11 April 1974 [16 April 1973] 16354/74 Heading G4H In an electrical signalling system in which a plurality of monitor stations signal the status of conditions to a supervisory central station, a plurality of receiving devices at the central station each have an output to indicate the status of a selected condition and which feeds a clocked flip-flop 136 and an EXCL-OR gate 140, this gate also receiving the output of the flip-flop, the gate providing to a scanning circuit an output signal which is a function of the status of the selected condition. One or more line circuits link the monitor stations to the central station, transmitters in the monitor stations signalling a status by turning off a frequency for a duration (which may be selectively long or short). Control signals can be sent from the central station to other remote stations in the same way. The signals received at the central station are recognized (e.g. long duration distinguished from short), each possible signal, if recognized, feeding a respective flipflop 136 and EXCL-OR 140 (Fig. 3) in a respective input circuit 130. An oscillator 94 drives a counter 96 and through it a counter 100, the latter counter selecting up to 16 scanners 104 (only one shown) in turn and the former counter selecting input circuits 130 (only one shown) connected to the selected scanner 104. When an input circuit 130 is selected and the condition (represented by switch 90) that it corresponds to is abnormal, but the flip-flop is set to "normal", the output of EXCL-OR 140 will be 1 and this will be passed by a print data selector 106 in scanner 104 and inverted at 120 to inhibit oscillator 94 temporarily via AND 144, the inhibition being maintained by a flip-flop 142 set via a delay 150 assuming the signal is not a momentary false signal (due to electrical noise or relay contact bounce). Scanning thus stops, and flip-flop 142 also causes printing of a warning message to be initiated at printer 154 and a pulse to be applied via a demultiplexer 110 to clock the flip-flop 136 in the input circuit 130 so that it stores the input from 90, EXCL-OR 140 output going to 0. The output of flip-flop 136 now, via print red data selector 108, causes the initiated printing at 154 to be in red. After printing, printer 154 resets flip-flop 142, scanning being then permitted to continue. When an input condition returns to normal, stopping of scan and printing &c. occur as before except this time the printing is in black due to the state of flip-flop 136. The message to be printed in either case is selected from read-only memories such as 180a-d, selected and addressed by the counters 100, 96, the characters being compared with the print drum position 186 as at 182a-h to actuate the print solenoids. A signal 1111 from the memories will prevent printing at the corresponding position in the line. Fig. 2 shows one of the line circuits with its monitor stations 61 and associated central station equipment. A power supply 12, isolated from earth ground, supplies power to the remote stations over the loop lines 64, 66 which are used for communication between the remote stations 61 and central equipment 63. A break in the transmission path will deenergize a relay coil 62 to give an alarm and move contacts 62a, b from the positions shown to preserve power supply to the remote stations 61. A push-button controlling contacts 67a, b permits the relay coil 62 to be reenergized after the break has been corrected. An earth ground fault will energize a relay coil 68 to give an alarm. A short between the loops 64, 66, will increase current in a resistor 71 to make transistor 72 conduct and energize a relay coil 74 to give an alarm, capacitor 78 inhibiting energization for increased currents of short duration. A.C. signals are blocked by 80, 82.