Abstract: A global subscriber group service system capable of accommodating individual subscribers as a terminal within a global subscriber group. The system also permits the subscriber to access a service control node for new registration or modification of the types of services. The system, therefore, includes a customer access function in a local switching node that is controlled by the service control node. The customer access permits the subscriber to access the service control node to make a necessary registration or a change of the type of services.

Abstract: A key telephone system has an oscillation preventing circuit comprising a termination impedance element, said circuit is a switching element and connected between the ground and another circuit which includes an impedance compensating circuit, therein. The switching element is turned ON upon oscillation of the impedance compensating circuit due to any remarkable impedance fluctuation in the signal transmission line, thereby effectively suppressing such oscillation of the impedance compensating circuit to maintain a favorable transmission efficiency.

Abstract: A data transmission system between a main CPU and a plurality of sub-CPU's includes a data bus connected between them, a CPU select line for transmitting a CPU select signal from the main CPU to the sub-CPU's, a latch circuit for providing a wait signal to the main CPU upon write or read in the main CPU with respect to the sub-CPU's and a wait clear line connected between the respective CPU's to provide to the latch circuit a wait clear signal upon completion of input and output of the sub-CPU to release the waited state of the main CPU, the transmission system being thereby simplified to reduce installation space and wiring labor.

Abstract: A circuit for sending out a ringing signal of a high voltage and a low frequency to a telephone set through a telephone line repeatedly with a predetermined time interval. In order to omit use of a high power oscillator and relays, a D.C. power supply of a high voltage is operatively connected to the telephone line through phototransistors of photocouplers being driven by a pulse signal having a low voltage and the low frequency under control of pulses occurring at the predetermined time intervals so that the ringing signal is sent out to the telephone set repeatedly with the predetermined time interval.

Abstract: An extension ringing arrangement for a key telephone system wherein a plurality of key telephone sets are connected respectively through associated plurality of channel units to main equipment including a main CPU. The channel units as well as an interface unit, including a sub-CPU interconnected to the main CPU are connected in parallel to the main equipment. The interface unit is of the type used for interconnecting individual line telephone sets to the key telephone system, whereby a ringing of a selected one of the key telephone sets from a calling party is made possible, with a simple arrangement effectively utilizing existing components of the key telephone system.

Abstract: A key telephone system comprising a main equipment and a plurality of key telephones connected to the main equipment and comprising elements for operatively connecting another similar key telephone system. The main equipment in the key telephone system comprises a system call transmitting interface and a system call receiving interface which are connected through connecting lines to a system call receiving interface and a system call transmitting interface in a main equipment of another similar key telephone system, respectively. Each key telephone in both systems comprises an additional key for selecting the system call transmitting interface of its own main equipment. By use of the additional key in one key telephone, a speech path can be established between the one key telephone and another key telephone in another key telephone system through both main equipments and the connecting line.