410 BELL SYSTEM TECHNICAL JOURNAL 



direction of transmission on manually operated simplex cables were so 

 time-consuming that it was impracticable to reverse direction oftener 

 than once every quarter or half hour. The delay in transmission 

 which would result from the adoption of the older methods could not 

 be permitted on the loaded cable and it therefore became necessary 

 to develop special apparatus for automatically reversing the direction 

 of transmission at comparatively short intervals in order to approxi- 

 mate simultaneous transmission in both directions and reduce traffic 

 delays to an absolute minimum. 



The design of suitable switching arrangements which would permit 

 stopping transmission on a long cable operated with multichannel 

 printing equipment and almost immediately starting transmission 

 in the opposite direction presented several difficult problems. On 

 account of the lack of uniformity in the lengths of the messages to 

 be transmitted and the number of channels employed, it rarely 

 happens that the transmitters on all channels complete the trans- 

 mission of their respective messages at exactly the same instant, 

 therefore it was necessary to arrange for making the change in direction 

 of transmission at more regular and frequent intervals even though 

 the transmitters on all channels had only partly completed the trans- 

 mission of their respective messages at the time the change was made. 

 To accomplish this without introducing any errors or other evidence 

 of the interruption into the final printed message necessitates first 

 stopping the transmitters on all channels at precisely the right instant, 

 then allowing an interval equal to the time of signal propagation over 

 the cable to elapse before cutting ofif the printers at the distant end, 

 and finally upon resumption of transmission in that direction starting 

 all of the transmitters and printers at the proper time and in the 

 correct sequence to avoid the loss, repetition, or mutilation of any 

 character. 



The last signals transmitted into the cable before changing to the 

 receiving position result in leaving the cable charged to a potential 

 which would paralyze or "block" the amplifier were it to be immedi- 

 ately connected. Part of this charge must be dissipated and the 

 current due to the residual charge and the presence of any interference 

 or earth currents must be allowed to attain its steady value in the 

 shaping network and input transformer elements of the amplifier 

 before connecting any of the actual amplifying elements to the cable. 

 The switching operations involved in applying the amplifier to the 

 cable must be effected in the proper sequence and at precisely timed 

 intervals in order to leave the amplifier in the proper condition to 

 avoid mutilation of the first signals received from the distant end. 



