Abstract: A channel switching network, in which each cross-point of a switch matrix is constituted as a channel switch of balanced type using two PNPN semiconductor elements. Each of the two PNPN semiconductor elements is arranged in a forward direction with respect to a direct current flowing from the channel switching network to a corresponding terminal equipment. A column of cross-points and a line of cross-points in the switch matrix are designated to select a simultaneously designated cross-point in response to a pair of selection signals. A control circuit is provided for each pair of the incoming lines and the outgoing lines of the switch matrix so that the terminal voltage of a power source for flowing the direct current in the PNPN elements is controlled to temporarily rise to turn ON the PNPN elements at the selected cross-point and thereafter to drop to such a value as only to maintain the PNPN elements in the ON states.

Abstract: A key telephone system, in which a plurality of key telephone sets are selectively connected to a plurality of CO lines by the use of a plurality of talking lines under control of a key service unit and a plurality of control lines. In order to seize a CO line, a CO line seizure signal from one of key telephone sets is time-divisionally transmitted at a low speed and received by a speed change memory of the key service unit. A high-speed output from the speed change memory is added with dial or hook information and then space-divided for seizing a desired one of the CO lines. A talking operation, a holding operation, a privacy operation and a synchronizing operation in this system are performed on the base of the above operation principle.