Patent GB-625623-A (GB)
IPC: H04W84/02
Abstract: 625,623. Radio signalling. STANDARD TELEPHONES & CABLES, Ltd. July 28, 1947, No. 20269. Convention date, Sept. 1, 1945. [Class 40 (v)] - In a selective calling system for radio communication stations any two of which may intercommunicate on any one of a number of frequencies a common frequency not used for intercommunication is used for transmitting the call code of a station to be called and a code indicating the frequency to be employed. Two automatic telephone selectors at each receiving station may receive the dialled number of the called station and connect in to the called station a third selector indicating the conversation frequency. A time limiter brings the calling and receiving systems back to their rest positions in the case of a, call being uncompleted or of no reply being made. Calling arrangement.-The calling operator wishing to call a station coded, e.g. 45 rotates back a dial CA to 4, thereby closing contacts CAd and exciting the electromagnet ESC of a multi-bank 11 position step-by-step selector SC via its interrupter contact ISC, contact SCdl, contacts CAd and the rest contact of a relay RB: The selector therefore passes to position 2. One contact CAd also closes the circuit of a relay RC which operates and becomes fixed in operating position over its contact and SCe contacts 2-10. The selector SC automatically continues its movement to a predetermined position 5, whereupon the circuit of a relay RD is made via contacts SCf 5-10 and a contact of RC. RD cuts out the receiver RR and a microphone M of transmitter TR, replacing it by a modulating oscillator OSC, and switches the aerial to the transmitter. The return of the dial effects four interruptions of the transmitted signal, whereupon the contacts CAd open, the electromagnet ESC and opened contacts ISC, ISC<1> are released and the selector moves to position 6. The unit digit 5 is then dialled, the selector SC passing then to position 7 and finally the code number, e.g. 7 of the frequency to be employed, previously ascertained to be free, by listening in. SC then passes to position 8 but is unable to remain there, and continues to position 11 where it breaks the circuit of RC, and finally continues to the starting position 1. RD releases at position 11 of SC, thereby re-inserting the micro - phone. A switch P connects up the microphone circuit and aerial change-over relay. Dialling time limit control.-A control selector LT automatically returns the selector SC to starting position if the dialling is uncompleted or exceeds a fixed time, e.g. 15-20 seconds, by completing the circuit ELT, ILT, LTa1 and SCd5, 6 or 7 when SC is on position 5, 6 or 7, LT then passing to position 2. Alternate contacts of LTb are connected to differential windings X, Y of a relay RF, one of them, X, through a condenser CF, so that relay RF only operates after a delay due to partial charging of CF. Through the closed contact of RF, ELT is again energized and LT proceeds to position 3, cutting off current in X, Y. LT proceeds thus to position 11, whereupon the circuit of ESC is completed over ISC, SCb5, 6 or 7 and LTC 11 so that SC passes to position.8 8 and finally position 1. If, however, the call has been completed before LT reaches position 11, or LT is at 11 after setting SC to 1, SC restores LT to position 1 by energizing ELT over contacts ILT, LTa 2-11, and SCc1. Receiving arrangements.-All stations are initially tuned to and pick up the call frequency and upon detecting it cause it to operate a relay RA and relay RB. The latter operates (i) the call indicator lamp LO ; (ii) opens the circuit of CAd so that relays RC, RD do not function and transmission is impossible, and (iii) energizes ESC over contacts ISC, SCa1 so that SC passes to position 2. RB and a further relay RE are slow to release due to condenser CB and rectifier RS respectively and remain pulled up during interruptions of the modulation. During their occurrence the tens digit selector SD is energized through RE and the rest contact of RA and proceeds from its normal position N to the tens digit called. Enough time elapses between dialling the ten and unit digits for RE to release and ESC, previously energized so that its armature remains pulled up, is also released and passes to position 3, this movement cutting out SD and introducing a unit selector SU. On the selection of the unit digit being completed RE again falls back and SC passes to SC4. It is arranged that only station No. 45 called has SDb4 and SUb5 connected and in series with ESF (frequency selector electromagnet) and the rest contact of RA, SF of this station rings a bell on passing from its normal position N to that dialled and indicates the called frequency on a drum. On the frequency signal ceasing, RA and RB fall back, SC passes to and remains at position 5, and the circuit of ELT is closed over LTa1 and SCd5 so that LT begins to meter an allotted time for reply, failing which all calling and called selectors return to initial position. The called operator responds by (i) setting his transmitter and receiver to the frequency called for ; (ii) answering, and (iii) pressing switch BS so that selector SC passes to positions 6-11. When transmission of the received modulation ceases RA and RB fall back and SC and LT return to position 1. ESD is therefore energized over contacts ISD, SDa1 to 0, LTb1 and SCc1 so that SD returns to N, thus permitting SU and SF to return to N and cutting out the bell S. At other stations not wanted SC similarly passes to position 5 and, after RA falls back, to position 6, through the circuit of ESC being made through SFbN, and so to position 1, and so bringing back SD and SU to N. On a conversation ceasing the operators manually restore the transmitters and receivers to the calling frequency. By connecting SDb0 and SUb0 all stations may simultaneously be called by dialling 00. Stations in excess of 100 may intercommunicate by providing a hundreds selector.