Patent GB-769734-A (GB)
Title: Improvements in or relating to multi-channel pulse two-wire communication exchange systems
IPC: H04Q11/04
Filed: 1955-03-09
Abstract: 769,734. Automatic exchange systems. TELEFONAKTIEBOLAGET L. M. ERICSSON. March 9, 1955 [March 9, 1954], No. 6992/55. Class 40 (4). In a pulse multiplex automatic exchange system a two-way multiplex path is provided, each pulse position being divided into two halves which are used respectively to receive and transmit signals from and to the subscribers. Alternate cycles are used for establishing connection with the calling and called lines respectively, signals incoming to the multiplex path being gated to a circuit introducing a delay equal to the channel pulse repetition period plus half the duration of a pulse position and then being gated on to the multiplex path as an outgoing pulse. A small exchange is described capable of dealing with three simultaneous conversations. Completion of calling subscriber's loop causes a voltage drop over resistance 209 in the subscriber's line circuit L, and fires tube 223 to cause an identifier I to indicate the calling subscriber's identity in binary form on leads 412 ... 415. The identifier also applies positive potential to lead 411 to fire tube 416 thereby enabling valves 408, 511, 601. The magnetic drum 312 has magnetizable areas as shown in Fig. 8. The A and B areas are allotted respectively to the calling and called subscribers. The areas in columns C1 ... C4 are used to represent the subscriber's number in binary form. The areas D1 ... D3 are used to indicate the free or busy condition of the channels, the respective pick-ups giving a negative or positive pulse according as the channel is free or busy. The positions of the teeth on the associated discs 313 ... 318 are also indicated. Three discs like 318 are provided one for each channel. Selector 504 indicates a free channel, e.g. channel No. 1 as shown. Negative pulses are received from area D1 are amplified at 509 and applied to the D1 recording head to reverse the direction of magnetization so that the areas D1 henceforth produce positive busy indicating pulses. These amplified at 510 are passed to valve 408 which is also pulsed from disc 314 to cause the insertion of the calling subscriber's code in positions AI. Pulses picked up from these positions are passed to a device V which emits AI pulses on a lead 363, individual to the calling line, to open the subscriber's line gate 207. The pulses also feed a lengthener 217, 218 whose output fires tube 225, the resulting pulse extinguishing tube 223 to release the identifier I. The consequent pulse produced across condenser 420 fires tube 426 to bring up relay 430 which transfers the recording head leads to the digit register and the pulsing circuit for 408 to the disc 315. Pulses D1 are modulated by dial tone in valve 605 and fed to lead 208, whence they are passed by gate 207 to the calling subscriber's line circuit, demodulated at 206 and fed to the calling subscriber. The latter sends two digits each in the form of one of ten frequencies. These frequencies are sorted by band-pass filters 608 ... 610 and the digits successively set two sets of gas tubes 630 ... 633 and 650 ... 653. These mask wires 452 ... 455 with a binary indication of the wanted subscriber's number thereby causing the number to be registered in positions BI on the drum, the instant of energization of the recording head being controlled by valve 408. Completion of the registration operates relay 666 followed by relays 440 and 501. The former extinguishes tubes 416, 426 and the register gas tubes. The latter steps the selector 504 to a free channel as indicated by the reception of pulses from amplifier 509. The device V now receives digits picked up from the BI position on the drum and produces BI pulses on a lead 373 associated with the called subscriber's line circuit. These are applied to the called subscriber's line gate 207 and are also integrated at 217, 218 to prime tube 226. This tube is provided with intermittent D.C. to operate relay 230 to connect up ringing current to the called subscriber. When the latter answers the cathode potential of 226 rises to cut off the tube and interrupt the ringing current. Tube 225 fires to prevent tube 223 from calling in the identifier I. Calling subscriber's speech is now gated at 207 on to the multiplex path in pulse position AI during the first half of which it is gated at 301 on to the delay line FL whence it emerges during the second half a BI pulse. Gate 302 passes it back via the multiplex path to the called subscriber's line circuit where it is gated in at 207, which is opened in position BI, and thence to the called subscriber's demodulator 206 whose output is fed to line. The called subscriber's speech is treated similarly with the exception that it arrives in the BI position and is transmitted to the calling subscriber in the AI position. Supervision. When channel I is seized, DI pulses pass via selector bank 505 to fire tube 514 associated with the channel, thereby enabling valve 517. Whilst either subscriber's handset is off the hook, pulses are received in the AI or BI position on lead 208 have a lower amplitude than when both have hung up. These are gated by valve 517 into the integrator circuit 521 and when both subscribers have hung up the output from the integrator enables valve 522 to operate relay 523. Condenser 526 is thereby charged and during the charging period valve 409 is enabled to pass AI and BI pulses to the erazing head of the drum so obliterating the called and calling subscribers' numbers. Likewise valve 513 is enabled to energize the recording head for the DI pulses in such a manner as to restore the magnetization to its original state indicating that the channel is free. Relay 523 extinguishes tube 514 and relay 515 releases to block valve 517 again, whereupon 523 releases. Tube 225 in the subscriber's line circuit is extinguished by its increase in cathode potential when the subscriber hangs up.