ormation etc. The stores consist of read only shift registers which markers. The supervisory processor is program controlled using continuously circulate data and present this on common highways reprogrammable read only memories. to all main control units. Associated with the data store is scanning equipment, including 32.4.3 System operation additional storage for maintaining a record of the state-of-line information for customers. This is synchronised with the main data 32.4.3.1 Connection to a register store so that all information relating to a particular termination is available simultaneously. When a call is originated either from a customer or an incoming The basic store module provides for 5760 customers and a propor- junction, the calling condition is detected via the scanning by one of tion of incoming and outgoing circuits. Up to seven data stores can the main control units (MCU) which has been pre-allotted. At the be provided in an exchange giving a maximum size of over 40000 same time, the equipment number, directory number and class of lines. For security, the complete shift register data files are dupli- service relating to the calling line, available on the cyclic store cated and parity coding is used to enable single errors to be detected highways, are stored in the MCU. and corrected without any effect on exchange operation. A complete Under the control of its program, the MCU analyses the class of backup store is also provided on magnetic tape and this can be used service of the calling line to determine if any special requirements to reload the data files. apply, such as calls barred. If the call is to proceed, the MCU initiates a search over all registers available to it on the cyclic store highways. A free register is selected and its equipment number 32.4.2.2 Main control units and registers stored by the MCU in readiness for setting up the connection. The MCU then seizes the common highways to the interrogator The main control units are special purpose processors employing markers when the