468 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



with the marking path to the hue appearance on the switches operates 

 to remove the hne relay and other originating apparatus from the called 

 subscriber's line. 



Based on the result of the idle-busy-vacant test of the called line, the 

 incoming register circuit either sets the incoming trunk to provide ring- 

 ing to the called subscriber upon closure of the crosspoints and ringing 

 tone to the calling subscriber if the called line is idle, or sets the trunk 

 to return busy tone to the calling subscriber if the called line is busy 

 or vacant. In the latter case the incoming receiver is released immediately 

 without setting up a terminating connection through the switches. 



Since connections in the same reed-diode switching network are es- 

 tablished through either of two number group circuits, lockout is pro- 

 vided between the originating receiver to trunk number group connector 

 and the terminating receiver to subscriber number group connector so 

 that only one number group circuit can be in operation at a time. 



Some of the important average time intervals as measured in this 

 system are given in Table I and shown graphically in Fig. 13. 



CONCLUSION 



The electronically controlled automatic switching system described 

 in this paper was designed for large central offices and a skeletonized 

 laboratory version has been built, tested and demonstrated. Successful 

 operation at the speeds required was obtained. No failures of the gas 

 tube lockout circuits were observed under the various combinations of 

 possible simultaneous seizure. The experimental system shows that a 

 large hea\'y traffic office could be made to operate on a one-at-a-time 

 basis with advantageous reduction in the number of control and con- 

 nector circuits. Many of the necessary components employed in this 

 system for one-at-a-time operation are now available in a pre-develop- 

 ment state and will probably be used in commercial systems. However, 

 the commercial design and production of a complete office as described 

 here is not economically competitive with existing systems since the 

 subscriber subset and line circuit which are used in large numbers are 

 too complex and expensive. 



ACKNOWLEDGEMENTS 



Although acknowledgements have been made in specific cases through- 

 out this paper, we wish to point out that many others contributed to the 

 success of the project. We wish to mention G. G. Bailey and G. A. Back- 

 man who performed the physical construction and assisted in the testing. 

 In particular we wish to mention A. W. Horton, Jr., who directed the 

 project. 



