ELECTRONICS IN TELEPHONE SWITCHING SYSTEMS 1011 



the place of the conventional make and break contacts in the electro- 

 mechanical switching art (see Fig. 23 for the "AND" function). 

 Magnetic core circuitry is also being exploited to perform high speed 

 switching functions-^ (Fig. 24). 



There are a number of differences between the circuit configuration 

 used for relay contacts and diode or magnetic core gates for switching 

 logic. When interconnecting such gates to realize complex logic func- 

 tions other gates are required when circuit elements are placed in series 

 or parallel, whereas in the wiring of relay contacts in series or in parallel 

 no additional circuit elements are required (Fig. 25). Pulse signals 

 passing through diode gate circuits are usually attenuated since the 

 electronic device is not a perfect switcher (infinite impedance open cir- 

 cuit to zero impedance closed circuit). Some minute currents flow 

 when open and some resistance is encountered when closed. Therefore, 

 some amplification is needed at various places in logic circuits and this 

 can be provided by transistor amplifiers. The use of transistors as the 

 gating element eliminates this shortcoming by providing amplification 

 in each gate (see Fig. 24). Transistors have also been successfully used 

 in a new form of logic to provide relay contact like logic thus eliminating 

 the need for gate elements to represent the series of paralleling func- 

 tions^^ (see Fig. 26). 



The processing of information usually requires a sequence of logic 

 actions. To provide such sequences, momentary elements similar to 

 locking relays but with microsecond action times are required. When 

 this condition obtains a bistable or "flip-flop" circuit using transistors 

 may be emplo3^ed. Several forms of transistor circuits have been de- 

 vised using either the Eccles-Jordan principle,^^ negative resistance 

 properties,'^ such as achieved with a gas tube, or a regenerative ap- 

 proach.2^ Some suggestions have been made on the use of semiconductor 

 diodes in special energy storing circuits to amplify pulses instead of the 

 more conventional transistor amplifiers.^" 



CYLINDRICAL 

 LENS 



^ EMULSION 



TUBE "^ COLLECTOR 



CATHODE RAY _„,. „^^ ^ f" 



SLIT 



.^OUTPUT 

 VOLTAGE 



-- 1 ■) 



CATHODE pHOTOMULTIPLIER 

 FIELD LENS TUBE 



ROTATING PLATE 

 FLYING SPOT SCANNING A ROTATING DISC 



Fig. 21 — Photographic storage (from Proc. I.R.E., Oct. 1953). 



