Patent GB-1230928-A (GB)
IPC: H04M1/515
Filed: 1969-09-15
Abstract: 1,230,928. Automatic exchange systems. M. LE DORH, Y. LE GOFFIC, and F. ARNOUAT. 15 Sept., 1969 [13 Sept., 1968; 24 Jan., 1969], No. 45304/69. Heading H4K. A time - division - multiplex pulse-code-modulation switching system in which substations have pcm modulators 102 and demodulators 106, Fig. 1, and pushbutton number senders 308 which for each digit transmit at the pcm bit rate a given number of bits of one polarity followed by fixed four bit code alternating with a four bit code representing the digit. Operation of key 1 to 0 of the number sender, Fig. 2, injects into circulating stepping register stages 3011 to 3018, the four bits abed representing the keyed digit, and a fixed code 1110. Each operation of a key is reckoned to be effective for at least 30 ms. and for the first 10 ms. a monostable 306 controls gates 302, 303 to ensure isolation of the substation pcm encoder and to connect a series of 560 consecutive zero bits to the send pair of the four-wire line to the exchange. Instead of using an eight-stage stepping register at the substation to produce the alternating pattern of digit code and fixed code, a three-stage binary counter may be employed, Figs. 11 and 12 (not shown). A two-digit numbering scheme is postulated and, at the exchange a digit detector 60, Fig. 3, is capable of processing eight time channels of the exchange highway 2100 for calling data at the same time and scans these channels at intervals of 4À56 ms. and for a duration of 570 Ásec. for each channel. Each fourwire line 11, 12 terminates on line circuits 50, 500, received #-modulated pulses being stretched at 200 for adjustment to the exchange cycle and being transmitted to the appropriate substation by way of the exchange highway 2100 and a retransmission circuit 2000. Receive and send highway gating complexes 2101, 2102 are controlled from an address register 2103 governed from a call store 2104 referred to as a multiregister. To connect a calling line to the digit detector 60 the multiregister 2104 operates a bi-stable of the group 21051 to 21058 so as to gate the exchange highway 2100 to the digit detector in one of the detector scan periods t<SP>1</SP>1 to t<SP>1</SP>8. If on a calling line a digit is being heralded by a series of 560 zero bits at least two scan pulses should detect in counter 63 a consecutive sequence of 32 zero bits and in response to either will ready the first of two digit stores 67, 68. The bit stream received from the connected calling channel is set up on a twelvestage stepping register 66 and when two abcd digit codes, separated by the fixed code 1110, are present the digit detector circuit 65 extracts the digit code and transfers it to the first digit store. The multiregister, having a storage compartment recording progress of connections, stores the state of the digit connector between each scan period of 570 Ásecs. for each call being processed and sets up the detector at the start of each scan period according to the state of the call. With one digit stored in store 67 in respect of a call, the next detected digit is steered to the second digit store 68 and the completed call particulars are lodged in the multiregister and the associated bi-stable 2105 is reset to free the scanning period of the digit detector for service with another call. Ringing is effected under instruction from the multiregister 2104 which sets a bi-stable in the send line circuit 500 to couple ringing generator 510, Fig. 3. The generator drives one winding of a tuned transformer second and third windings of which are wired into the legs of the send pair 12. Battery potential is supplied to a substation over a phantom of the send and receive pairs 11, 12, the potential being developed over a Zener diode 404 shunted by capacitor 405, Fig. 1, the circuit being made by off-hook contacts 406. The ringer 408 is shunted by off-hook contacts 407.