Branched telephone system

Abstract: A branched telephone system having a plurality of telephones (18, 20) which are connected to a single subscriber line or to single radio apparatus (12) has a function of establishing a priority set condition in which the communicating function of particular one of the telephones is validated to prevent the other telephones from being used, and a function of automatically cancelling such a priority set condition.

Description: The present invention relates to a branched telephone system in which a plurality of telephones are connected to a single subscriber line or to single radio equipment and, more particularly, to a branched telephone system capable of automatically cancelling a priority set condition in which particular one of a plurality of telephones is usable. In most of prior art branched telephone systems, two or more telephones are simply connected in parallel with no special control provided over them. In such a configuration, all the telephones share entirely the same functions and, therefore, it is impossible to give priority to desired one of the telephones over the others or to positively invalidate the latter. In a vehicle-mounted telephone system, two independent telephones may be connected to single radio equipment which is mounted on a vehicle and individually disposed in the vicinity of a front seat and a rear seat of the vehicle. It often occurs with such a vehicle-mounted telephone system that an occupant in the vehicle desires to invalidate one of the two telephones. For example, a person originating a call on the telephone near the rear seat may desire to keep the telephone number of the other party and the contents of communication from the driver. In the specification of United States patent number 4,218,590, which was published on August 19 1980, there is described a key telephone system having a control unit at each of a plurality of stations which are operable to connect one or more telephone lines selectively to a phone set. An exclusion circuit in a phone set at each station is able to detect a busy condition on the line and prevent that set from being connected to the line. In the specification of German Federal Republic Offenlegungsschrift 3524523, which was published on January 15 1987, there is described a branched telephone system in which the lifting of one telephone receiver automatically causes that telephone set to be directly connected to the line and one other telephone set to be separated from the line. Objects of the present invention are the provision of a branched telephone system which is capable of establishing a priority set condition wherein the communication function of particular one of a plurality of telephones is validated while the other telephones are prevented from being used, and which is capable of automatically cancelling a priority set condition in which one of a plurality of telephones connected to a single line is validated, the others being invalidated. It is another object of the present invention to provide a generally improved branched telephone system. There will be described below, by way of example a branched telephone system having a plurality of telephones, each of which is connected to a single subscriber line or to a single radio equipment, priority setting means associated with a particular one of the telephones, but operable independently of the associated telephones for establishing a priority set condition in which the associated one of the telephones is usable, the other telephone or telephones being invalidated in function, and priority cancelling means for cancelling the priority set condition by detecting a change in a switch-hook condition of the telephone associated with the priority setting means. The following description and drawings disclose, by means of example, the invention which is characterised in the appended claims, whose terms determine the extent of protection conferred hereby. In the drawings:- Fig. 1 is a schematic block diagram showing a branched telephone system in accordance with the present invention; Fig. 2 is a circuit diagram showing a specific construction of a branching circuit and that of a switch unit as shown in Fig. 1; and Figs. 3A and 3B are waveform diagrams representative of signals which appear in various portions of Fig. 2. Referring to Fig. 1 of the drawings, a branched telephone system embodying the present invention is shown and generally designated by the reference numeral 10. As shown, the system 10 is made up of radio equipment 12 having an antenna 14, a branching circuit 16 provided with priority setting means, a master telephone 18, and a servant telephone 20. The telephones 18 and 20 are each connected to the radio equipment 12 via the branching circuit 16. A switch unit 22 located in the vicinity of the master telephone 18 is connected to the branching circuit 16 and is accessible for setting a priority set condition. Fig. 2 shows a specific construction of the branching circuit 16 and that of the switch unit 22. A speech signal and control signal line 12b and a power supply line 12c of the radio equipment 12 are directly connected to, respectively, a speech signal and control signal line 18b and a power supply line 18c of the master telephone 18. A switch-hook signal line 18a of the master telephone 18 is held in direct connection with a switch-hook signal line 12a of the radio equipment 12. On the other hand, a switch-hook signal line 20a, a speech signal and control signal line 20b and a power supply line 20c of the servant telephone 20 are connected to, respectively, the switch-hook signal line 20 a, a speech signal and control signal line 12b and power supply line 12c of the radio equipment 12 via a relay 24. A flip-flop 26 is connected between the switch unit 22 and the relay 24 while a transistor 28 is connected between the relay 24 and the flip-flop 26. Usually, the output Q of the flip-flop 26 is maintained low level so that the transistor 28 remains non-conductive. In this condition, no current flows through the winding 24d of the relay 24 to hold contacts 24a, 24b and 24c of the relay 24 closed, so that the servant telephone 20 is connected to the radio equipment 12 and usable in the same manner as the master telephone 18. When a contact 22a of the switch unit 22 is pressed, the output Q of the flip-flop 26 becomes high level, as shown in Fig. 3A. Then, the transistor 28 is turned on to cause a current to flow through the winding 24d of the relay 24, whereby the relay contacts 24a, 24b and 24c are opened. Consequently, the servant telephone 20 is electrically isolated from the radio equipment 12 and, therefore, unusable while the master telephone 18 alone is usable. In such a priority set condition, when the contact 22a of the switch unit 22 is pressed again, the output Q of the flip-flop 26 is inverted to low level restoring the servant telephone 20 to the usable condition. A light emitting diode 22b included in the switch unit 22 is adapted to display the state of the flip-flop 26 so that a person may see if the servant telephone 20 is usable through the switch unit 22. A differentiator 30 is made up of a capacitor 30a, a resistor 30b and a diode 30c. When a change of the switch-hook signal from low level to high level (see Fig. 3B) is detected by the capacitor 30a and resistor 30b, a reset signal is fed from the differentiator 30 to the flip-flop 26 to thereby reset the output Q to low level. Now, assume that a person who is holding a communication on the master telephone 18 while giving priority to the telephone 18 through the switch unit 22 hangs up the receiver of the telephone 18 to end the communication. Then, the differentiator 30 detects the resulting change of the switch-hook signal from low level to high level, resetting the flip-flop 26. As a result, the output Q of the flip-flop 26 becomes low level to close the contacts 24a, 24b and 24c of the relay 24 again, whereby the priority given to the main telephone 18 is cancelled. In Fig. 2, the reference numerals 32, 34 and 36 designate inverters, the reference numerals 40, 42, 44 and 46 designate resistors, and the reference numeral 48 designates a capacitor. In summary, it will be seen that the present invention provides a branched telephone system with a capability of automatically cancelling a priority set condition. Specifically, the system of the invention automatically cancels a priority set condition by referencing the hook state of a telephone to which priority is given, preventing various undesirable occurrences ascribable to an inadvertent failure of cancellation. Various changes and modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof, as defined in the appended claims. For example, the present invention is applicable to telephone systems other than a vehicle-mounted telephone system shown and described, and telephone systems having more than two telephones. Further, the branching circuit and switch unit may be implemented with any known arrangement in place of the circuitry shown in Fig. 2.

Claims: A branched telephone system (10) having a first (18) and a second (20) telephone each of which is connected to a single subscriber line or to a single radio equipment, characterised in that the system includes priority setting means (22) associated with the first (18) of the telephones only and operated independently thereof for establishing a priority set condition in which the first (18) of the telephones is usable and the second (20) telephone is rendered inoperable, and priority cancelling means (30) operable automatically to cancel the priority set condition upon a change in the switch-hook condition of the first telephone (18) associated with the priority setting means (22) from an off-hook condition to an on-hook condition. A branched telephone system as claimed in claim 1, wherein the function of the second telephone (20) includes at least one of a communicating function and a dial displaying function. A branched telephone system as claimed in either claim 1 or claim 2, including indicator means (22b) for displaying the priority set condition. A branched telephone system as claimed in any one of the preceding claims wherein the system is mounted on a vehicle and wherein the first telephone (18) associated with the priority setting means (22) is disposed in the vicinity of a rear seat and the second telephone (20) is disposed in the vicinity of a front seat.

Inventors: FUJISAKI HISASHI C O NEC CORPO, SHIMADA KEIKO C O NEC CORPORAT, FUJISAKI, HISASHI C/O NEC CORPORATION, SHIMADA, KEIKO C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1988
Main classification: H04M1
IPC codes: H04M1