Line scanning circuits sequentially examined the state of each line, junctions and so on by means of a pulse many times a second and immediately after each pulse a data store (the Cyclic Store) offered the MCU permanent information relating to the line. When a calling condition was detected, the scanning pulse was passed to the MCU indicating to it that a new call had to be set up and busying it temporarily to further calls. As the first steps in dealing with the new call the MCU recorded the directory number and class of service (shared service, PABX line, incoming junction register, TOS and so on) information offered by the Cyclic Store and allocates one of its associated group of up to 30 registers. The registers were connected to peripheral terminals of the switching network, in the same way as subscribers' lines, junction circuits and other units and the MCU proceeded to issue instructions to the network to connect the subscriber and register terminals.