674 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



Supervisory Circuit 



When the channel is used in TWX service as a toll subscriber line, 

 the subscriber calls the operator to initiate a call by closing the power 

 switch on his teletypewriter. This connects power to the teletypewriter 

 motor, closes the transmission circuit to the teletypewriter and applies 

 plate battery voltage to the transmitting oscillator in the channel ter- 

 minal, resulting in the transmission of carrier current over the line. 

 At the distant (switchboard) terminal the receipt of carrier current 

 energizes a supervisory signal receiving circuit which is responsive to 

 carrier-on and carrier-off conditions in the receive band. In this circuit, 

 carrier voltage appearing at the plate of the limiter tube is rectified 

 and applied to the grid of the supervisory triode. The operation of a 

 relay in the plate circuit of this tube causes a line lamp at the switch- 

 board to light. 



A disconnect signal is sent by the subscriber at the end of a call by 

 opening the teletypewriter power switch. This removes the oscillator 

 plate voltage. At the central office, the receipt of the resulting no-carrier 

 signal de-energizes the supervisory recei\ang circuit and causes the super- 

 visory lamp in the operator's cord circuit to light steadily. To recall 

 the operator during a call the subscriber opens and recloses his power 

 switch. This causes the cord lamp at the smtchboard to flash. 



An RC circuit slows the rise of current in the supervisory receiving 

 tube to guard against false operation of the switchboard line lamp due 

 to noise impulses during the carrier-off, that is, the idle condition. 



DC Circuits 



On the dc side of the channel terminal, provision is made for optional 

 wiring arrangements to connect to the circuits of the various telegraph 

 test boards, service boards and TWX switchboards, as well as to local 

 teletypewriter loops, using telegraph voltages of either 130 or 48 volts. 

 In offices where a negative 130-volt battery is not provided, operation 

 with a single positive 130-volt battery is possible. 



The loop connections are made to an electronic circuit in the channel 

 terminal which is similar to that employed in a recently-developed 

 electronic loop repeater used in telegraph offices and which possesses 

 several interesting features. Fig. 5 compares the action of this circuit, 

 in transmitting toward the subscriber station, with that of more con- 

 ventional arrangements : 



(a) shows a conventional open-and-close circuit and the wave shapes 

 which it produces at the central office end and at the far end of a capaci- 



