AUTOMATIC SWITCHING SYSTEM 4GI 



If a talking path is now desired between line A on line frame 1 in Fig. 

 10 and trunk B on trunk frame 2, a +80-volt power source is connected 

 to the control leads at these points. These applied voltages are called 

 "marks" and originate in a number group circuit. The +80-volt mark 

 at line A in conjunction with the — 45-volts supplied to the primary- 

 secondary switch links causes the cold cathode gas diodes of the line A 

 vertical to fire and conduct at low current. The substantially constant 

 voltage-drop characteristic of gas diodes causes the voltage on the two 

 horizontal outlets of this primary switch to shift to +20 volts thereby 

 "marking" one input lead on each secondary switch of this line frame. 

 These -f20-volt marks in conjunction with the —105 volts supplied 

 from the junctors causes the gas diodes between the marked secondary 

 switch inlets and the junctor outlets to fire, to conduct at low current 

 and thereby to mark the associated junctors with —40 volts again by 

 virtue of the gas diode characteristic. As indicated by the shaded diodes 

 in Fig. 10 a mark on line A results in marks on junctors 1, 2, 3 & 4 and 

 thus reveals all the idle paths from hne A through the line frame. 



In a similar manner the -f 80- volt mark applied to trunk B results in 

 ihe firing of the diodes along the idle paths from this trunk to junctors 2, 

 4 and 7. The path to junctor 5, which is in use on the connection between 

 line C and trunk D, is not marked in this case. The —24 volts presented 

 by junctor 5 on its trunk control lead is not sufficient when combined 

 ^vith the +20-volt mark on the trunk primary-secondary link which 

 leads to this junctor to fire the associated crosspoint diode. 



For tliis desired connection there are two possible paths, either through 

 junctor 2 or through junctor 4, as indicated by the —40- volt marks 

 existing on both the line and trunk sides of these junctors. Selection 

 between these paths is automatically accomplished by use of a lockout 

 circuit which is common to all junctors serving the same line frame. 



It is kno^vn that if a conduction path through a negative resistance gas 

 tube is provided with a load impedance of proper value which is common 

 to a similar conduction path through one or more other similar gas tubes, 

 only one tube will ionize and remain ionized even if firing potentials are 

 applied to several tubes either simultaneously or in sequence. Such a 

 circuit employing two or more gas tubes with a common load impedance 

 fun(^tions as a lockout circuit. The phenomenon is due to the region of 

 negative resistance in the characteristics of the gas tube through which 

 the tube current passes in the range between the breakdown and sus- 

 taining voltages. In this region as the current through a tube increases, 

 the voltage across the tube decreases, tending to prevent other tubes 

 with the common load from firing. To reduce the possibility that two 



