TIME DIVISION TELEPHONE SWITCHING EXCHANGE

A TDM telephone exchange serving a multiplicity of subscriber lines has a primary and a secondary processor each including a caller memory, a responder memory and a monitoring memory, stepped in synchronism through a multiplicity of phases including service phases for the establishment of a connection and conversation phases for enabling communication between two subscribers. The associated subscriber lines are periodically scanned in successive memory cycles; the address of any such subscriber found to be in the process of initiating the call, if not yet entered in a phase of the caller or the responder memory of either processor as determined during a service phase, is entered in the first available conversation phase of either caller memory whereupon the address of the called subscriber is registered in the corresponding phase of the associated responder memory to let the conversation proceed under the control of appropriate entries in the monitoring memory of the same processor. If the first vacant phases of the two caller memories coincide, preference is given to the memory of the first processor. If a fault develops in either processor, the other one is used on all calls. With subscriber lines arranged in groups along common branches of two main signal paths, entry in a vacant phase of either caller memory is possible only if the concurrent phases of the caller and responder memories of the other processor do not carry the address of another subscriber of the same group; otherwise, the address of the new caller is transferred to the next available phase not subject to this restriction.