Patent GB-705723-A (GB)
Title: Improvements in or relating to signal impulse receiving devices for telephone lines
IPC: H04Q1/48
Filed: 1951-11-21
Abstract: 705,723. Automatic exchange systems. TELEFONAKTIEBOLAGET L. M. ERICSSON. Nov. 21, 1951 [Nov. 21, 1950], No. 27331/51. Class 40 (4)  In a signalling system in which the signals consist of current impulses of different polarities sent over a two wire junction provided with a line transformer, the receiver comprises two relays, responding to the two polarities respectively, connected in parallel to the line and each when operated disconnecting the other by means of a break contact, one side of a winding of the transformer being connected to both break contacts and to the centre tap of a battery, and each break contact being connected via an impedance to a different one of the poles of the battery, so that the disconnected relay is subjected to a voltage which counteracts its operation. The Specifi- . cation also describes a transmitter for use in conjunction with the receiver. Transmitter -Fig. 1. The equipment shown is associated with a junction F and is controlled from a register by the extension of positive battery to wires t1-t7 in combinations as indicated in Fig. 3. Thus if digit " 1 " is to be sent the wires t1, t3, and t6 are marked. The chokes D1-D3 are initially energized, and the immediate operation of R2 connects C1 and D1 to line transformer L1. R1 pulls up thus disconnecting D1 and R2, and during the release time of R2 condenser C4 is charged and thereupon discharges through L1 to emit an impulse to line. The impulse is a damped alternating current of which only the first half-cycle is effective at the receiver. R6 then pulls up followed by R5 and D2 and C2 are connected to L1 and a second similar impulse is transmitted, R6 releasing. The subsequent operation of R9 and R8 causes D3 to control the sending of the third impulse, R9 releasing. The removal of the markings releases R1, R5 and R8. For the transmission of digit " 9 " the markings on t2, t5 and t6 ensure that R3 operates in place of R2 so that C4 is charged in the opposite direction and the first impulse sent is of the other polarity. R4 comes up, after R1 has released R3, and being slow to operate introduces a pause after the first impulse before operating R9 which controls the sending of an impulse of the first polarity as before. Receiver-Fig. 2. Initially R17 is momentarily energized from the register over wire V13 so that R11-R14 and R19 are up. The reception of an impulse of the first polarity over junction F results in the operation of R21 over rectifier e1. The resultant disconnection of R11 and of the lower winding of T1 induces a current in the upper winding of R21 which holds that relay, but rectifier e3 is backbiased during the release period of R11 to prevent false operation of R22 to the second half-cycle of the transmitted impulse. With R11 down the second impulse (of digit " 1 ") brings up R23 via e5, the resultant disconnection of the lower winding of R13 inducing a holding current in the lower winding of R23, e7 being back-biased during the release time of R13, and R24 being disconnected thereafter to prevent its false operation. R13 holds the slow-to-release R19. The third impulse brings up R25 which removes the short-circuit from its lower winding, back-biases e10, and locks up, at the same time operating R15. Relay R18 then - operates over contacts of R15, R13, R11 and wire VI in series with the No. 1 register relay (not shown). R17 pulls up re-operating Rll and R13 and releasing R15 and R25, whereupon R18 and R17 release in turn. The first impulse of digit " 9 " brings up R22 via e3 so that R12 releases and-e1 is back-biased. During the pause corresponding to the operate time of R4 (Fig. 1), R 19 falls away. The second impulse operates R23 and the subsequent release of R13 allows R18 to operate over wire V9 in series with the No. 9 register relay (not shown). The receiver is reset as before.