266 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



CONCENTRATOR 



LINE BUSY 



CENTRAL OFFICE 



TO ALL CROSSPOINTS 

 / SERVED BY TRUNK 



+ 130 V 



VG 

 VF 



I 



L..1.. 



/ 



TO ALL CROSSPOINTS 

 FOR SAME LINE 



SELECTION 



FROM 



" CENTRAL 



OFFICE 



i-65V 



I + 100V 



Fig. 13 — Crosspoint operating circuit. 



e. Crosspoint Operating Circuit 



The crosspoint consists of a reed relay with 4 reed switches and a gas 

 diode (Fig. 1). The selection of a crosspoint is accomplished by marking 

 with a negative potential ( — 65 volts) all crosspoints associated with a 

 line, and marking with a positive potential ( + 100 volts) all crosspoints 

 associated with a trunk (Fig. 13). The line is marked through a relay 

 circuit set by signals sent over the control pair from the central office. 

 The trunk is marked b}^ a simplex circuit connected through the break 

 contacts of the hold magnet of the crossbar switch associated with the 

 trunk in the central office. Only one crosspoint at a time is exposed to 

 165 volts which is necessary and sufficient to break down the gas diode 

 to its conducting state. The reed relay operates in series with the gas 

 diode. A contact on the relay shunts out the gas diode. When the marking- 

 potentials are removed the relay remains energized in a local 30-voll 

 circuit at the concentrator. The holding current is approximately 2.5 ma. 



This circuit is designed so that ringing signals in the presence or ab- 

 sence of lino marks will not falsely fire a crosspoint diode. Furthonnoi'o, 



