Circuit for a central control time division multiplex PCM telecommunications system

A circuit for a central control PCM telecommunications exchange having a stand-by operating facility, said circuit including:

• a duplicate pair of central switching networks each capable of carrying out system operation for the switching of line connections, and a duplicate pair of central processors each associated with and operatively connected to a respective one of said central switching networks for system operation thereof, wherein one central switching network and associated one central processor are used for normal system operation and the other central switching network and associated other central processor are kept in stand-by for backup system operation, and
• a plurality of decentral terminal groups, each one of which is connected in parallel to the two central switching networks by respective link lines, and each one of which includes a plurality of terminal units, a subsidiary switching network associated with the terminal units for connecting them to the central switching networks via the respective link lines, and a programmable, decentral control device associated with the subsidiary switching network which is capable of receiving, processing and/or transmitting switching information to and from the central processors via the respective link lines and the respective central switching networks, wherein the decentral control device of each of said plurality of decentral terminal groups is loaded with program software and switching software from a central processor for initializing system operation with the central processor and associated central switching network,
• wherein the improvement comprises:
• said plurality of decentral terminal groups being assigned to each other in pairs and having switching lines connected between the decentral terminal groups of each assigned pair,
• said subsidiary switching network of each decentral terminal group being capable of connecting the terminal units of the other decentral terminal group of its assigned pair, as well as its associated terminal units, to one central switching network via a respective link line, in a stand-by mode during a changeover of system operation from the one central switching network and associated one central processor to the other central switching network and associated other central processor, and
• each decentral terminal group having a plurality of changeover switches which are switchable between a normal position, for respectively connecting its terminal units to its associated subsidiary switching network in normal operation, and a stand-by position, for respectively connecting its terminal units to the subsidiary switching network of the other decentral terminal group of its assigned pair via said switching lines in stand-by operation during a changeover of system operation,
• wherein a changeover of system operation is carried out without a substantial interruption of the switching functions of the decentral terminal groups by switching the terminal units of one decentral terminal group of each assigned pair, via the changeover switches in the stand-by position, for connection to the subsidiary switching network of the other decentral terminal group of each assigned pair while the one central processor continues in system operation, so as to remove the subsidiary switching network and associated decentral control device of said one decentral terminal group of each assigned pair from operation temporarily so that switching software and program software can be loaded to said decentral control device removed from operation from the other central processor kept in stand-by, and then switching back the terminal units of said one decentral control group to its associated subsidiary switching network, via the changeover switches in the normal position, when the loading of switching software and program software to said decentral control device removed from operation has been completed.