Patent GB-1480223-A (GB)
Title: Telephone intercommunication system providing common access to a communication line
IPC: H04Q3/62
Filed: 1973-10-26
Abstract: 1480223 Intercommunication system GTE INTERNATIONAL Inc 10 July 1974 [26 Oct 1973] 50060/73 Heading H4K The substations (Fig. 1) have access to two exchange lines LCA, LCB and a local secret line SGC over 3-wire circuits ILS, TLA, TLB, by means of respective relay contact trees controlled by the keys KLS, KTA, KTB (the trees are shown schematically as rotary switches SWl-SWn). The existence of these calls is indicated to other subscribers by means of flashing or continuously lit light-emitting- diodes. At the start of any call access is made to a common line circuit CLC and for local calls the relevant wanted party (or parties) is called by actuating a key in keyboard KP so as to operate the wanted party's buzzer BZ. For a secret call, both (or all) parties operate their KLS buttons so signalling over single lead LTS to the secrecy guard circuit which causes light flashing signals to be reverted to all substations. The SGC circuit is released when the last party hangs up. For trunk calls, incoming or outgoing, the stations gain access by operating key KTA or KTB thereby signalling over the single leads LTTA or LTTB. An external call may be placed on hold by operating local key KL so that an interim call can be made over the common or secret local circuits. First embodiment (Figs. 2, 3). Each substation is normally connected to the common line circuit (Fig. 3) so that when a handset TSC is lifted battery is immediately supplied thereto. The off-normal contacts ON-1 close thereby supplying operating earth to the keypad Kp. Actuation of a selected key (of keys) permits energization of a buzzer BZR associated with a wanted party (or parties). All parties to the call are connected to the common line circuit. If a secret call is required, all the parties to the call actuate their keys KLS together. Each station thereby contributes to a current flow over the common lead LST (the current through relay TT in each station is ineffective) which turns on transistor T5 and then brings-up relay LC. The increase in current flow in LST caused by the short circuiting of resistor R4 brings-up each TT so as to switch the speech path to the secrecy guard circuit. Transistor T1 is energized such as to earth wire LSP and hold TT. In the guard circuit, SE comes-up following the enablement of transistors T3, T4. When all the KLS keys are restored, the current through LST ceases whereby T5 and LC fall off, whereas transistors T2, T6 energize so as to enable relay LP. If two TT relays are operative, SE drops back whereby a flashing signal is sent from generator FG over lead LST to pulse transistor T11 and LED1 on and off. If more than two TT relays are operative as on a conference call SE remains up whereby T11 is turned off and LED1 is turned on continuously. All relays restore when the holding earth supplied via the offnormal contacts ON1 is removed. In the exchange line circuits relay P is normally operative such as to direct incoming ringing to a detector RR which is effective to energize a common call bell CB and flash the individual LED2's or LED3's in each station in sympathy with the ringing signal. Any station may answer by going off-hook and actuating the relevant key KTA or KTB. Relay LT (and/or TT depending on the strapping in the diode field DSF) goes up to the resistance earth on lead TTA (c.f. lead LTTA in Fig. 1). The change of bias on the base of transistor T8 causes relay PU to rise. The off-normal-contact-earth is thus effective over wire TPA to turn-off transistor T7 and drop P thereby switching-through the speech path and bringing up relay PP. The lamp LED2 or LED3 glows continuously. In order to hold the exchange line, the substation closes his key KL (or KLS if he wishes a secret interim call) whereby LT drops so removing the earth on wire TPA (or wire TPB if the B exchange line circuit is involved). Relay P comes up to loop the exchange line via operated contacts PP1 while PU falls back. The flashing generator is connected to wire TPA so as to flash LED2 or LED3. Any party may gain access to the exchange line as described above by actuating his key KTA (or KTB) but usually of course only the person contacted during the interim call would do so. Release of exchange line.-The removal of the off-normal contact earth is not sufficient to drop LT which remains held by a current flow through wire TPA. This current turns-on T14 which brings up RE. T9 and T 10 are thus turnedoff so dropping PP. LT finally gives way to release the substation while P comes-up and PU followed in turn by RE drop so as to restore the exchange line circuit. An exchange line on hold can only be released by accessing it with a KTA or KTB key and then going on-hook. Outgoing call.-A subscriber accesses a free line circuit A or B by actuating his key KTA or KTB and then keys out a wanted number with the aid of an integrated circuit dial-pulse or MF dialler DPC. Second embodiment Figs. 4, 5.-Local calls are set up over the common line circuit indicated by line wires L using the key pad KP and strapping field SF for actuating the wanted part's buzzer 5 so as to alert him to the existence of an incoming call. There is no provision for secrecy in this system. Exchange line circuit 6, 7 incorporates a normally operative relay DB whose contacts direct incoming ringing to a common bell CB. They also connect one of the substations lines to the common local line circuit (dotted connections DBT, DB) so that in the event of a power failure this line would be directly connected to the exchange line. Any subscriber responds to the ringing by going off-hook and actuating his key KT. Switching relay LT comes up in a circuit extending over lead PU, and transistor T and relay P in parallel. The earth condition on PU is then extended via R1 to lead H such as to change the bias on T2, and thereby cut it off whence P can come up. The above operation can only take place with one KT key operative so that only one subscriber at a time is permitted access to the exchange line. The LED is illuminated to warn-off other substations. The exchange line is held by depressing key KL whence P drops and PP comes up and holds. A resistance loop R8 is placed across the exchange line while the flash generator FU is connected to the PU lead so as to flash the LED in every substation. When KL is released LT drops whereby an internal call via circuit L can be made. Any subscriber regains access to the exchange line by actuating his key KT thus bringing up LT and P and dropping PP. Release follows immediately on restoration of the handset and opening of off-normal contact ON2. (For Figures 4 and 5 see next page.)