se become free, thus busying them against seizure stored program control techniques. Up to 20 of these units may be by other MCU. The equipment numbers of the calling line and provided dependent on the number of calls to be handled. The register are then sent to all interrogators to identify free paths processors operate simultaneously but independently in a load shar- through the switching network. Interrogators with no free paths ing mode so that, in the event of a fault, the processor concerned is available release. Each of the others identifies the optimum path busied out and the traffic is distributed over the remaining proces- through its own sub-unit and signals this to the MCU which selects sors at only a slightly reduced grade of service. one of these for the actual connection. The appropriate markers are The overall operation of the main control processor and the then instructed to set up a path. The register will signal to the MCU manipulation of the information within it is controlled by a stored that the path has been completed and the MCU can then release. program held on read only memories. The use of non-volatile Should this path not be completed successfully, a second attempt is memory for program storage gives complete security against mali- made automatically and the identity of the faulty path is printed out. cious or accidental corruption of the program. Changes are not When sufficient information has been received by the register, the required in service since the essential variable parameters are lo- MCU is recalled and examines the dialled information to establish cated in the data store with keyboard access. the destination of the call. Registers are processed in groups by the main control units, so that dialled digits from the calling termination, which are detected by the register, are stored in a common working store within the main 32.4.3.2 Local call processor. The register itself contains only the limited amount of equipment required to count the dial pulses under microprocessor For