676 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1952 



During open-and-close transmission by the subscriber, the high im- 

 pedance termination of the loop at the tetrode plate circuits causes the 

 current at the central office end of the loop to change very slowly — too 

 slowly for good transmission at teletypewriter speeds. However, the 

 voltage wave is very well shaped, and this is what is used to drive the 

 grid of the transmitting tube. One noteworthy fact is that the bias of 

 the signals repeated from loop to line is nearly independent of loop length; 

 consequently no inductive wave shaping is required at the subscriber 

 station, even in the longest operable loops. 



Because of the high impedance termination, loop current is insensitive 

 to circuit resistance. The loop padding rheostat is, therefore, adjusted 

 to build out the loop resistance to a standard value and the amount of 

 loop current required for proper operation of the station teletypewriter 

 is obtained by varying the screen grid potential of the tetrode tubes. 



Duplex Feature 



In half -duplex operation, one dc loop at each channel terminal serves 

 for both sending and receiving. The central office end of this loop is 

 connected to the grid of the sending triode and to the plates of the re- 

 ceiving tetrodes. If a marking signal is being received from the carrier 

 line while the teletypewriter contacts in the loop are closed, the receiving 

 tubes conduct, current flows in the loop and the teletypewriter in the 

 loop receives a marking signal. Under this condition the office end of 

 the loop is positive with respect to the cathode of the sending triode; 

 hence tliis tube passes a marking signal toward the carrier line. When a 

 spacing signal is received from the carrier line the tetrodes are cut off, 

 the loop current is reduced practically to zero, the teletypewriter re- 

 ceives a spacing signal and the voltage at the office end of the loop be- 

 comes more positive. Hence a marking signal continues to be transmitted 

 to the line during the receipt of either mark or space signals from the 

 line. 



When the subscriber opens the loop to send a spacing signal to the 

 distant terminal, the potential of the sending triode grid becomes nega- 

 tive with respect to its cathode, the tube cuts off and hence, as described 

 previously, a spacing frequency is passed to the ac line. 



In full-duplex operation, two loops are provided at each channel 

 terminal to permit sending and receiving simultaneously. The grid of 

 the sending tube is disconnected from the plates of the tetrodes and 

 transferred to a resistive connection which terminates the full-duplex 

 sending loop. The loop circuits operate in the same way as described 

 for half -duplex operation except that no break action is provided. 



