

286 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



SR operated through its own contact until the new call has been set up. 

 This prevents interference of disconnect pulses applied to the trunk 

 when a selection is being made and insures that a disconnect pulse is 

 transmitted before the trunk is reused. 



A characteristic of the No. 5 crossbar system is that the originating 

 connection to a call register including the line hold magnet is released 

 and a new connection, known as the "call back connection", is estab- 

 lished to connect the line to a trunk circuit after dialing is completed. 



With concentrator operation the concentrator trunk switch connection 

 is released but the disconnect signal is not sent to the concentrator as 

 a result of holding the SR relay as described above. However, the marker 

 does not know to which trunk the call back connection is to be estab- 

 lished. For this reason the frame circuits include an identification proc- 

 ess for determining the number of the concentrator trunk to be used 

 on call back prior to the release of the originating register connection, i 



Identification is accomplished by the marker transmitting to the 

 frame circuits the number of the link being used on the call. This in- 

 formation is already available in the No. 5 system. The link being used 

 is marked with —48 volts by a relay selecting tree^" to operate the TS 

 relay associated with the trunk to which the call back connection is to 

 be established. Relay CB (Fig. 29) is operated on this type of call in- 

 stead of relay HG. The circuits for reoperating the proper hold magnet 

 are already available on the TS relay which was operated, thereby rc- 

 selecting the trunk to which the customer is connected. The concen- 

 trator connection is not released when the hold magnet releases and 

 again the marker operates as it would on a regular line link frame call. 



9. FIELD TRIALS 



Three sets of the experimental equipment described here have been 

 constructed and placed in service in various locations. The equipment 

 for these trials is the forerunner of a design for production which will 

 incorporate device, circuit and equipment design changes based on the 

 trial experiences. Fig. 30 shows the cabinet mounted central office trial 

 equipment with the designation of appropriate parts. 



For the field trials described, the line links on a particular horizontal 

 level of existing line link frames were extended to a separate cross-bar 

 switch provided for this purpose in the trial equipment. The regular line 

 link connector circuits were modified to work with the trial control 

 circuits whenever a call was originated or terminated on this level. N(i 

 lines were terminated in the regular primary line switches for this level. 



