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: 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.