SIIOllT-IIAUL CAUKIEU TELEGRAPH 



681 



Fig. 10 — loitl')! til(t\ pi'w liter subscriber set including channel terminal. 



channel terminal is powered by the teletypewriter rectifier, which fur- 

 nishes 130- volt dc and 20- volt ac power. 



The 130B1 set may be employed in private-line or TWX service. 

 In the latter, the application and removal of oscillator plate battery is 

 controlled as described above by the teletypeAwiter power switch, so 

 that the equivalent of telephone "switch-hook" super\ision is attained. 



Supervision and transmission are largely independent. The telegraph 

 receiving circuit at the central office terminal remains marking during 

 recall and disconnect signals; hence these supervisory signals do not 

 pass through the cord-circuit repeater to the TWX toll line. Since there 

 is no frequency discrimination in the supervisory receiving circuit, 

 either marking or spacing carrier from the station energizes the super- 

 visory circuit. Hence a communication break (spacing) signal from the 

 subscriber station is transmitted through the operator's cord without 

 any effect on the supervision. 



On a TWX call to the subscriber station, a series of alternate marks 

 and spaces, generated by applying 20-cycle ringing voltage to the grid 



