Patent GB-1487534-A (GB)
Title: System for addressed time-multiplexed telecommunication
IPC: H04Q1/38
Filed: 1975-05-15
Abstract: 1487534 Automatic exchange systems MAXMAN AG 15 May 1975 [16 May 1974] 20750/75 Heading H4K A TDM communications switching system in which subscribers transmit the identity of a wanted party using an impedance dialling technique, incorporates an analogue/digital converter which in response to the first digit dialled causes the identity of the caller to be inserted in binary code in a shift register during a free time slot, whereafter the value of the dialled digit is determined by comparing a stepped voltage waveform with the voltage presented across the loop and is then recorded, also in binary form, in the same register, subsequent digits being similarly assessed and recorded, and then the recorded addresses are used for operating line gates which serve to connect the calling and the called parties together. Each substation (Fig. 1A) comprises a microphone M and loud-speaker H coupled via an amplifier and hybrid arrangement F to line L via a full-wave rectifier bridge D2-D5. A transistor T2 serves to turn on a constantcurrent source T1, when the line is looped by opening contact AV. (With AV closed, resistor R11 effectively provides an open circuit loop.) A light emitting diode LED indicates the operative state of the subset. Each subscriber's line circuit (Fig. 1B) comprises a 2/4 wire converter HY and a D.C. isolating-transformer TR. In operation, closure of a digit dialling-key 0-1 causes a voltage comparator K1 to produce a logic 1 signal which enables an encoder ENCl associated with ten such line circuits to mark its individual output lead F and to send in binary code on wires T the identity of the particular line that is currently in the calling condition. A control circuit (Fig. 2, not shown) chooses a free time slot and inserts the identity in a pair of shift registers (one for each digit of the address). Every connection is governed by a set of 16 status conditions and 16 time phases. The effect of storing the caller's identity changes the status from S0 to S1. The latter status causes the shift register to be read-out after an exchange clock cycle whereby the previously stored binary signals are fed to a decoder DECl. This enables transistor Ta which remains on via latch INT between successive appearances of the address on leads I, Ei during the time slot allocated to the call. The effect of Ta is to disable the request for service via comparator K1. In addition the output of DEC1 enables comparator K2 to effect (in conjunction with not shown control circuitry) a comparison between the line voltage which corresponds to the operated digit selection key and a stepped voltage waveform. The referred to 16 time phases each correspond to one voltage step for digit 1-0, (phases T0-T11), hang-up (T12-T15) and a special condition (T16) during which the call may be effected in simplex mode by operation of key S in a substation. The translation from voltage level to time phase and then to binary code is effected in the control circuit and recorded during the appropriate time slot in the shift register. The second digit is dealt with similarly but as the status of the connection differs, the calling party address is no longer required. At all stages during dialling timeouts are operative to prevent unnecessary holding due to dilatory dialling. With the calling and called party identities registered, the connection is cut up by supplying these identities to decoders DEC2 whereby the Go and Return speech gates ANU, ANI are opened during respective portions of the allocated time slot. The central control includes a speech level detector for controlling the direction of speech. Hang up is recognized via comparator K2 of either party.