Patent GB-535300-A (GB)
Title: Improvements in electrical signalling systems
IPC: G08B27/00
Filed: 1939-10-03
Abstract: 535,300. Electric selective signalling systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., IRELAND, E. B., and McCABE, F. Oct. 3, 1939, No. 27116. [Class 40 (i)] [Also in Group XXXIX] Switching operations at a selected one or all stations on a common line which may be used for conversation are effected by transmission of signals of different cadences (slowrate pulses), a particular operation being performed by two mechanically locking contactors one of which is operated in response to one cadence to prepare the other for operation by a second cadence. The cadences may be pulses of common A.C., or A.C. of different frequencies may be used to increase the selection. The switching operations may be applied to selective calling of air raid wardens' posts, and may comprise the flashing of lamps. Current limiters may be used to prevent operation by surges. The invention is shown applied to selective police telephone calling over closed fire-alarm loops, the central fire station being equipped as shown in Figs. 1 and 2. The cadence signals are generated by machines SCG, GCG driven by motor FM which also drives A.C. generator FG from which the cadence signals are applied to 'the fire alarm loop over conductors 11, 12. Four different cadences are used in combinations of two for selective calling, two further individual cadences being used for clearing and general calling. When a selective call locking key 1K-10K is pressed, the cam contacts SCC of SCG generating the appropriate sequence of two cadences are connected to relay CD, and start relay ST pulls up whereupon F energizes over the off-normal contacts ons1 of SCG and operates TA followed by T when F relapses on the opening of onsl. Relay ST also operates MA and CC which start the motor FM and cadence generator SCG, and connect the signalling loop 11-12 to A.C. generator FG. The connection of +ve battery by CC to lead 13 causes the disconnection of the fire alarm equipment from the loop. The selected cadences of SCG are'repeated over the loop as A.C. impulses by relay CD and when SCG reaches normal the re-operation of F operates H which disconnects CD in case a call is being sent .out by SCG over another loop, and releases CC to restore the loop to fire alarm conditions. Relay MA also falls back if there is no other call to stop the motor and generators. When the call has been attended to, 'the calling key K is restored whereupon ST falls back followed by T and TA, but it is held so that STA pulls up, operates CC and MB to reconnect the signalling equipment to the loop and start the cadence generator GCG which over contacts CDC sends the clearing signal to CD. Relays F, -T and TA function as previously described and when F re-operates on closure of ons2 after GCG has performed one revolution; H is shunted down and all the operated relays fall back. General call.-Key GCK operates relay GC and also functions similarly to a selective calling key except that cadence generator GCG is started - and its general call contacts GCC are connected to relay CD. relays F and ST being in this case controlled by off-normal contacts ons2 of GCG. Prevention of interference with fire calls.- The fire calling equipment removes +ve battery from lead 14 and terminals indicated by concentric circles so that even if ST, F and TA have been operated in a police call, CC, MA or MB do not operate or, if the transmission has been started, CC falls back to disconnect the signals and MA or MB remains held until the generator SCG or GCG has completed a revolution, the police signals being sent out when the loop becomes available. The holding circuits of MA, MB are controlled by NA, NB which operate when the respective cadence generators move off-normal. NA; NB also operate for the same purpose if the generators are inadvertently moved off-normal. Responding equipment, Fig. 3, at a substation.-Four cadence responsive movements GM, SM, GCM, CDM, which may be as described in Specifications 518,877 or 524,685, are provided, together with two mechanically locking contactors PC, SC. In response to the first cadence GM operates PC which replaces the shunt on SM by one on GM so that the latter responds to the second cadence and operates SC which connects a mercury tube flasher MTF in circuit with lamp L. The clearing cadence operates CDM which energizes the restoring windings of PC, SC at the station under consideration and also of contactors at other stations which have been operated by the first cadence. The general call . cadence operates GCM which directly operates contactor SC.