Control Unit System

The goal of this paper is to provide a general view of the organization and structure of the No. 2 ESS control unit. An attempt is made to emphasize the background and philosophy behind some of the design choices and their relation to the broad requirements of an economical, flexible, and reliable stored program control system. l.i Definition of Control Unit System The role of the control unit is to direct the processing of calls and maintenance in the system. It is comprised of the central processing unit, the stores, and the interface to the peripheral units. Two control units and the maintenance and administration center are referred to as the No. 2 ESS control complex. The various subunits which form a control unit include the program control, the input-output control, the call store, and the program store. The program control is the central processing unit which executes the instructions read from the program store. The input-output control performs the function of interfacing with the peripheral units by providing circuits such as a central pulse distributor, address bus drivers, and scan answer bus receivers. In addition, the input-output control autonomously performs certain processing functions associated with high rate, time critical, peripheral communication. The call store is 2619