Patent GB-1412459-A (GB)
Title: Communication systems
IPC: H04Q3/72
Filed: 1972-12-28
Abstract: 1412459 Automatic exchange systems WESTERN ELECTRIC CO Inc 28 Dec 1972 [30 Dec 1971] 59757/72 Heading H4K In a telephone system subscriber line circuits are scanned in groups and on detection of a calling line a supervisory circuit which supplies control potentials to the line circuits selectively varies the potentials supplied to the line circuits of that group to identify that line circuit. The PABX described operates in a fashion generally similar to that described in Specification 1,108,701 but employs a group scanning technique to reduce the processing time spent on this function and thus is more suited to larger exchanges. As shown when a station such as STA 112 goes off hook the associated line circuit 112 transmits a service request to common control 1. The tens and units digits corresponding to the current setting of a scanner 40 are transmitted to a code amplifier 30 which transmits tens and units code markings to correspondingly numbered line circuits in each line circuit group. Unless by chance the setting of the scanner corresponds to the calling line no response is produced and common control 1 steps the scanner. When the scanner setting reaches the tens and units setting corresponding to the calling line, the calling line circuit responds to the combination of the off-hook signal and the markings transmitting a further signal to common control 1. Operation of the scanner is inhibited and enabling potentials on leads GB and GC to other groups are removed preventing line circuits of those groups from responding to off-hook signals. The common control can then identify the calling line from the further signal and the setting of the scanner. A connection is then established as described in the above patent between the calling line and a register 4 via a switching network 2 and the common control releases from the connection. The register returns dial tone to the caller and when it has received the dialled digits transmits a request to the common control. The common control on entering a register mode removes the enabling potentials on leads G1, GB, GC to prevent any line circuit from responding to an off-hook signal and reads the stored digits into the code amplifier 30 which used the tens and units digits to mark a corresponding line circuit in each group and transmits the hundreds digit to the supervisory circuit 20 which removes a bias on a corresponding lead FA, FB or FC to the appropriate hundreds group. If the called line is idle the common control establishes in known manner a connection from the called station to a first appearanc of a selected intercom trunk 3. The common control then restores all potentials FA-FC G1-GC and requests the register 4 to call back the calling line circuit. The register transmits a request to the code amplifier 30 and the scanner 40 advances until the calling line which is still connected to the register is scanned. When this occurs the response of the calling line circuit is detected by supervisory circuit 20, the register is disconnected and a connection is established between this identified line circuit and the second appearance of the intercom trunk.