REMOTE CONTROLLED LINE CONCENTRATOR 273 



to determine if the line is busy or idle. These conditions are determined 

 in the same manner as described for line scanning since a service re- 

 quest condition would still prevail on the line if the call was not aban- 

 doned. If the line was busy, a line busy condition would be detected. 

 However to detect these conditions a VF pulse must be the last pulse 

 transmitted since the stepping of the VF counter generates the pulse 

 which is transmitted through an enabled line selection and passive 

 line circuit gates. Fig. 18(c) shows a typical line selection where the num- 

 ber of VF pulses is equal to or less than the number of VG pulses. In 

 all other cases there is no conflict and the sending of the last VF pulse 

 need not be delayed. On terminating calls, the line busy indication is 

 returned to the central office within 0.002 sec after the selection is com- 

 plete. During selections the central office circuits are gated to ignore 

 any extraneous service request or line busy pulses produced as a result 

 of steps of the VF counter prior to its last step. 



c. Crosspoint Operation and Check 



Associated with each concentrator transistor counter stage is a reed 

 relay. These relays are connected to the transistor collector circuits 

 through diodes of the counter stages when relay M operates. The con- 

 tacts of these reed relays are arranged in a selection circuit as shown 

 in Fig. 19 and apply the —65 volt mark potential to the crosspoint 

 relays of the selected line. 



After a selection is made as described above a "mark" pulse is sent 

 from the central office. This pulse is transmitted as a pulse of a different 

 polarity over the same control pair as the VF pulses. The received 

 pulse after amplification actuates a transistor bistable circuit w^hich has 

 the M reed relay permanently connected in its collector circuit. The 

 bi-stable circuit holds the M relay operated during the crosspoint opera- 

 tion to maintain one VF and one VG relay operated, thereby applying 

 — 65 volts to mark and operate one of the 6 crosspoint relays of the 

 selected line as described in section 6e, and shown on Fig. 13. 



The operation and locking of the crosspoint relay with the marking 

 potentials still applied enables a pulse gate associated with the holding 

 circuit of the crosspoint relays in each trunk circuit. The mark pulses 

 are sent out continuously. This does not affect the bi-stable transistor 

 circuit once it has triggered but the mark pulse is transmitted through 

 the enabled crosspoint closure check gate shown in Fig. 20 and back 

 to the central office as a line busy signal. 



With the receipt of the crosspoint closure check signal the sending 



