A High-Speed Line Scanner for Use in an Electronic Switching System

An earlier article 1 on the experimental electronic switching system described the role of a scanner in an electronic telephone office. The scanner, which is the real-time data input circuit for the system, can be thought of as a multiposition electronic switch. Directed by a binarycoded address from the central control, the scanner samples the voltage present at any one of its input terminals and transmits a quantized one or zero signal to central control. All lines and trunks are examined periodically at a rate fast enough to detect dial pulses as well as supervisory changes in state. In addition to lines and trunks, the scanner connects to a number of test points in other functional units of the electronic switching system to simplify automatic testing for preventative or corrective maintenance. The place of the scanner within the electronic switching system is shown in the block diagram, Fig. 1. l.i Line and Trunk Circuits The scanner is a voltage-operated device and as such draws very little current from the circuits it samples. Scanner voltages are developed in line and trunk circuits as shown in Fig. 2. Line circuits, which are associated with the great majority of scanner inputs, operate in a straightforward manner. Very little current flows from the -- oO-volt central 1383