Patent GB-1263528-A (GB)
Title: Electronic telephone exchange based on the time division principle
IPC: H04Q11/04
Filed: 1969-02-17
Abstract: 1,263,528. Automatic exchange systems. SOC. ITALIANA TELECOMUNICAZIONI SIEMENS S.p.A. 17 Feb., 1969 [26 Sept., 1968], No. 8386/69. Heading H4K. In a T.D.M. exchange each subscribers line circuit separates speech currents from dialling signals and gates them to different common highways, the numbers of calling and called subscribers are stored in different recirculating stores and when a wanted subscriber is busy the busy line is tested periodically. As shown (Fig. 6) a single speech highway Va interconnects all the subscribers such as X 1  and X 2  and the trunk circuits such as Lu. There are in addition three signalling highways; Vb used to monitor the busy/free states of the trunk circuits, Vc used to monitor the busy free states of the line circuits and Vd used to send dialling pulses to the trunk circuits. The exchange time cycle is divided into 100 time slots to provide 100 channels 96 of which are used for speech. One channel is used to test lines (one per cycle) for calling condition or if already involved in a call for dialling signals and on hook (receiver replaced) condition, and the three remaining channels are used for automatic routine testing. The main logical and storage units (detailed below and shown in Fig. 4) are identical. Nine such units are used one of which is on standby and is automatically switched in in place of a faulty unit when a fault is detected. The units are arranged in groups of three and during a test period the three units of a group are set identically. The outputs are compared two units at a time, a faulty unit giving lack of identity in both comparisons in which it is involved. A subscribers line circuit such as CU1 comprises a transformer Tr, a low pass filter, a speech transfer circuit Rs connected when a gate A*1 is open to the speech highway Va and a line current detector Rj connected when a gate C1 is opened to the signalling highway Vc. The gates are both opened simultaneously when the line is scanned or tested. A trunk Lu has a somewhat similar circuit CUu but in place of the line current detector the trunk side of the gate Bu is connected permanently to positive and to a contact controlled by a circuit Rt repeats dialling pulses arriving from the highway Vd through a gate 6, to the trunk if the gate 6 is enabled. Logic and storage units.-Each of these units which form the main components of the exchange comprises (Fig. 4) a recirculating store including four magnetostrictive delay lines L 1  to L 4  in parallel, each delay line storing 100 serial bits. The delay lines are synchronized by signals applied at C K1  to C K3 . Each four bit word represents a digit and as it arrives at the output of the delay lines is converted to eight bit form U 1  to U 4  by adding complementary signals and applied to a logic unit RL (not detailed) which has other inputs shown on the left of the figure. The logic unit is arranged such that if a signal is present at input Ca "one" is added to the digit before it is recirculated. If a signal is instead present at input Ci "one" is subtracted. An input at T E  causes a number applied at inputs E 1  to E 4  to replace the recirculating number. An input at T 1  causes the recirculating number to be replaced by "one" and an input at To causes it to be replaced by "zero". Call processing.-Figs. 5 and 6 together show the general arrangement of the exchange. It includes a first store I comprising three parallel sections I, II and III each consisting of a logic and storage unit as detailed above. Thus the first store stores for each time slot three digits (one per section) in parallel and is used to store the numbers of calling subscribers. A second store N of identical construction stores the numbers of wanted (or called) subscribers. The two stores are synchronized so that the calling and wanted subscribers for a call are read out at the same time. A single logic and storage unit MP operates in synchronism with the stores and for each call stores a digit characteristic of the state of the call. A further storage and logic unit is used in a circuit TC which receives dial pulses and gives end of digit signals used to distribute the dial pulses which are also applied to the store N, such that one digit is entered in each section I, II and III. The final storage and logic unit is held in reserve in MR and a test unit CO switches in this reserve unit when a faulty unit is detected as set out above. A unit T provides timing signals. Once during every cycle in the time slot reserved for testing the lines, the number of the line to be tested or scanned (which number is stored in that time slot location of store I or N and is incremented by one for each cycle) is entered into a register RA. If that subscriber is off-hook a flip-flop R is set to the state 1 and the digit being recirculated in MP in association with that time slot and subsequently referred to as the "call state digit", is set to "one" (0, 0, 0, 1) to assign the channel to the calling subscriber. During the remainder of the cycle a comparator J I  compares the number of the calling subscriber (which is still in RA) with the numbers of subscribers in I and N. In the event that a match is detected indicating that the calling subscriber is engaged in a call previously detected a flip-flop D is set and the "call state digit" is reset to zero. If D is not set then the calling subscriber has initiated a new call and his number is entered in store I, the "call state digit" is advanced to "two" and a tone generator TO (Fig. 6) is instructed to return dial tone to the caller over the speech highway Va. As the dialling pulses are received they are accumulated one digit in each of the three sections I, II and III of the store N in the same time slot as the calling subscriber number in the store I. The call state digit is advanced by one for each digit dialled up to the total of three digits. The call is not processed further until the called lined is tested or scanned. When this occurs the "call stage digit" advances from its current state "five" to state "six" and a comparator J N  compares the called number (now in RA) with the numbers in N to see if the called subscriber is already involved in a call. If a match is detected flip-flop D is again set to the state 1 and the call state digit is returned to the state "five". By alternating between states "five" and "six" the called line is tested periodically until that line becomes free or until the caller replaces his receiver. If the called line is not involved in a call the flip-flop R is not set and the call state digit steps through the states "seven", "eight" and "nine" during which ring tones from TO are sent to the called line and ring back tones are returned to the caller. When the called subscriber lifts his receiver the call state digit advances to ten and the call is established. The call is tested periodically during the normal scanning cycle and the memories I and N cleared when the subscribers replace their receivers.