Abstract: A multi-stage connection switch frame comprising crossbar switch lattices arrayed in at least three stages which are interconnected through links. Two adjacent stages of the multi-stage connection switch frame comprise a plurality of vertical groups each group comprising a plurality of upper part of verticals connected to each other by horizontals thereof in a multi-stage configuration and a plurality of lower part of corresponding verticals in all of the vertical groups being connected to each other by horizontals thereof in a multi-stage configuration, thereby to form the lattices in two stages which are coupled to each other through the verticals.

Abstract: A camp-on detecting system for a private automatic branch exchange (PABX) is provided with the camp-on connecting function for connecting queuing or camp-on calls to a busy line circuit. On detecting that the control line of the line circuit is at a busy potential, the potentials on the control lines of all camping-on trunks are changed and the potential changes are detected by the detecting means connected to the line circuit to detect that a camp-on connection has already been made to the line circuit.