Patent GB-671348-A (GB)
Title: Improvements in or relating to intercommunication telephone systems
Inventor: ADLER ADOLF ERWIN
IPC: H04M9/00
Filed: 1949-02-03
Abstract: 671,348. Direct-call systems. MODERN TELEPHONES (GREAT BRITAIN), Ltd. Feb. 3, 1950 [Feb. 3, 1949], No. 3002/49. Class 40 (iv). In a telephone intercommunication system comprising both master stations and substations, the home line of each substation is multipled to a master station and to all the substations for signal and control purposes only in the case of communication with the master station, and for providing in addition a communication circuit in conjunction with a pair of common battery leads TB + and TB - when the substation considered is called by another substation. In the system shown in Fig. 1 calls are set up as follows:- Substation to substation.-The caller at substation ST(a) lifts his handset HS, thus operating the cradle switch SH, and depresses a push button PB appropriate to the required station to connect the line, say b, to both a common speaking bar SS and a common ringing bar RR. The line b is part of the multiple of the home line HL(b) of a substation ST(b) which is identical with the home line HL(a) shown for substation ST(a), so that connection to the common ringing bar operates a relay S at the called station over its coils a and b to energize a buzzer BZ. The called party in answering cuts out the buzzer, releases the relay S and inserts a rectifier RIc to prevent interruption by negative battery from other calling substations. Master stations may, however, signal over the home line and light a signalling lamp LSMI at the wanted station. The push button PB is mechanically held in contact with the speaking bar during conversation and is released by the calling party replacing his handset. Substation to master stations.-To call the master station MI, the key KI at the substation ST(a) is thrown to prepare a connection over a three - wire multiple MMI to disconnect the substation calling circuit and to apply positive battery over line HL(a) to light calling lamp LSa and energize a calling relay CI which energizes a buzzer BE. The master station replies by throwing a key KS(a) which disconnects the circuit of the lamp LSa and the relay CI opens the buzzer circuit against further operation, energizes a relay TI over a high impedance winding a and in removing the low resistance lamp circuit LSa energizes the relay S at the calling station. The relays S and T complete the communication circuit, the latter switching in a microphone battery MTB. For a call in the reverse direction the master station throws the key KS(a)1 which operates the relay S over coil (a) and the operator at the called station replies by throwing key KI which sets up the same circuit as above. Master stations MII and MIII communicate with substations over their appropriate multiples MMII and MMIII but the connections are set up over separate signalling lines SL(a), SL(b) ..., positive battery calling station MII and negative battery calling station MIII in accordance with the rectifiers in the calling circuits. Master station to master station.-Each master station has a three-wire line to each of the other master stations for communication, and in the case of master stations MII and MIII changeover contacts of a relay R switch the microphone and loud-speaker or handset MH from the multiples MII and MIII to the intermaster station lines when the stations reply to a call or make a call to another master. The circuit to master station MIII is completed by the operator at that station throwing the appropriate KM key either to initiate a call or in response to a call and the inter-master station signalling is carried out over lines MLI and MLII, stations MI and MII being called by +ve battery and station 3 by - ve. A similar system including only one master station which is called by negative battery on the home line of the calling substation is described with reference to Fig. 2 (not shown), and the system described in Specification 650,070 is referred to.