Stored Program Control - SPC

The switching procedures in SPC exchanges arc controlled by means of stored programs (a program is a sequence of instructions stored in the computer memory). The SPC uses a computer as the time-shared control. The computer is defined by the program stored in its memory. The computer is programmed to test the conditions of the inputs and old states and decide on new outputs and states. Stored program control of switching systems is based on this principle. The computer controllers provide the time-shared decision maker, expressed as programs, which can be used to change or extend the functions of a control system.

Stored Program Control - SPC
Stored Program Control - SPC

In the figure registers are connected to the processor with input/output interfaces to the register access switch. Each interface has an address and may be connected to the input/ output buses by loading that address into the address buffer. Programming of the computer is such that its memory is divided into two:

  1. Program memory, for the sequence of instructions.
  2. Call memory, for the state of each register.

There is a third memory called translation memory, which translate the information dialed by the subscriber into specific actions. It is part of the program but is treated independently, so that the program may be used for any similar system. The register interfaces have one or more inputs and are scanned at regular intervals. These scan points are accessed by loading the address of the required register into the address buffer. Decoding the address returns the state of the required scan points. The outputs of the interfaces are treated similarly.

In operation, an external clock pulse triggers the computer. An overall control program, after loading the address and setting one of the call memories to point to the block of the memory associated with the first register, then hands over to the register program that decides whether any action is required for the current state and input. The next register is addressed, when the register program has completed its actions and the overall control memory modifies the addresses of the input/output buffer and call memory index. When all the registers have been dealt with, the computer waits for the next clock pulse before restarting a new scan. The SPC has the following advantages over other types of control:

  1. By changing the program, changes in the working of the exchange
    can be made at any time. Thus new services for the used and new
    maintenance and monitoring procedures can be introduced.
  2. It provides cheaper inter-exchange signaling, i.e., Common Channel
  3. No compromise between the efficiency of the switching network and
    the cost of a marker.

The SPC exchange does introduce certain problems also. For example, its cost is independent of the size of the exchange and in case of break down of the computer ihe exchange will stop functioning.


A computer is programmable electronic device that can store, retrieve, and process data. The use of computer and, more recently, microprocessor has brought a revolution in the design and operation of switching systems. In modern exchanges, particularly those using S.P.C., the use of a powerful and highly reliable computer control all the functions of an exchange.

All the information used in the translation processes are stored in the computer, thus it is easier to update the system when required. The computer has a complete overview of the system, so is able to analyse traffic patterns and diagnose faults more easily. Details of traffic and faults along with information for the preparation of user's bills can also be recorded in the computer. Special services, which would be used by only a few users, can be provided more efficiently.

The computer can also share between its own exchange and the remote exchange by a control signaling link. Thus functions which are expensive and inconvenient to perform locally can be controlled from a central computer.