Patent GB-978370-A (GB)
Title: Electronic switching telephone system
IPC: H04Q3/52
Filed: 1963-03-15
Abstract: 978,370. Automatic exchange systems. STANDARD TELEPHONES & CABLES Ltd. March 15, 1963 [March 22, 1962], No. 10375/63. Drawings to Specification. Heading H4K. An automatic telephone system comprises means for scanning line circuits for unanswered calling conditions, means for scanning registers, the latter being stepped once per line-scanning cycle, wherein upon simultaneous scanning of an unanswered calling line and an idle register the identity of the former is registered in the latter, which then also receives dialled digits for completion of the call. General.-Calls are set up by establishing connections from the calling and called subscribers to a link circuit by means of crosspoint switching matrices (e.g. of PNPN diodes) in which each connection is established by marking the link and the calling or called subscriber so that a plurality of self-seeking paths towards the link from the subscriber are fired, the first to reach the link being taken into use and the others discarded. Attempts are made to establish such a call through all the links in turn until one is successful, or if all are unsuccessful either due to internal blocking or because the called subscriber is busy, a final link is reached which reverts busy time. The links are scanned successively, each link being allotted consecutive odd and even pulse positions, the odd position being used for setting up the call from the calling line to the link and the even position for setting up the call from the link to the called subscriber. Scanning techniques are also used for marking the calling and called subscriber, and for allocating registers. Thus a scanner normally emits the identities of all lines in succession in a complete scanning cycle, and a register scanner steps once for each line-scanning cycle. The arrangements are such that if link scanning is taking place the line-scanning is rendered ineffective and vice versa. Normal call: connection to register.-When a subscriber goes off-hook it fires a PNPN diode in the subscriber's line circuit to enable a gate which passes the line-scanning pulse when it next arrives to an amplifier which applies a slow-rising pulse to the input to the switching matrix. At the same time the register at present being scanned if free applies marking potential to the other side of the matrix. If the switching matrix completes a call between the calling line and the register, this operates a bi-stable circuit in the calling subscriber's line circuit which places the circuit in a " lock-out " condition and prevents the circuit from responding to further line-scanning pulses. If a path through the matrix is not completed nothing happens and in the next line-scanning cycle a further attempt is made to connect to another register. Assuming connection is established then switch-through to a dial pulsereceiving and control circuit is effected, and a calling line store is enabled to register the identity of the calling line as fed to it by the scanner at that instant. If the registration is incomplete the connection is released. Switchthrough also enables a timer in the control circuit which, if not reset within a prescribed time by dial pulses, causes release of the connection. Reception of dialled digits.-A timer, a predetermined time after switch-through, moves a sequence switch to position 1 wherein dial tone is connected up and the first digit (hundreds) is received and registered, after which the timer moves the sequence switch to position 2 in which dial tone is cut off and the next digit is received. The last digit is similarly received in position 3 and the switch then goes to position 4. In case the first digit is the only relevant digit (i.e. for digits 6, 7, 8, 9 or 0) arrangements are made to skip direct to position 4. This disconnects the register from the calling line. Completion of connection.-When the register is next scanned the register sends a signal to the scanning circuit to cause it to stop scanning at the end of the line scan cycle, and then to render the link scanner effective. Each link has an odd and even input gate primed in turn by the scanner, and enabled via a common amplifier if the store called number has a hundreds digit 1, 2 or 3. The output from the odd gate causes the link (if it is not busy) to mark one side of the matrix. The calling line is marked from the called number register and the register, by virtue of the fact that the scanning pulse is " odd," marks a common " seize bus " with a 24V pulse. This pulse overrides the lock-out condition of the calling subscriber's line to cause it to mark the other side of the matrix with a slowly rising pulse. A search through the matrix then occurs for a free path to the marked link, to complete this side of the connection. On completion the link emits a first busy potential. On the subsequent marking of the even input gate if the first busy potential is present then the link is caused to mark one side of the matrix. The called number is read out to mark the called line and the " seize bus " is provided with a 12V pulse (by virtue of the fact that the scanning pulse is now " even "). If the called subscriber is free (i.e. not in the lock-out condition) the 12V pulse is sufficient to cause the line circuit to mark the other side of the matrix and a free path is found through it to the marked link. This causes the link to provide a second busy potential, which combined with the first causes the link to switch through. In the called subscriber's line circuit the flip-flop is operated by the successful completion of the path and opens a gate to pass ringing current to the calling subscriber. The same current feeds back over the connector to provide ringing tone. Closing of the called subscriber's loop inhibits the gate to shut off the ringing tone. Coincidence of the two busy potentials in the link circuit pulses a common amplifier which disables the link scanner and enables the line and register scanners. The common amplifier output is also gated into the register in question to return it to normal. Release.-When a subscriber hangs up the PNPN diode in the line circuit switches off to reset the associated bi-stable circuit, and to cause the holding current through the matrix to be interrupted. This restores the link circuit. Failures to establish call through matrix.-If for any reason, e.g. link busy or internal congestion of matrix paths, or called subscriber busy (see below), no path on one or other or both sides of the link in question can be set up, nothing happens, and when the link scanner stops on an attempt is made to establish the call through the next link and so on. If the connection remains unestablished when all links have been tried, the scanner reaches a special busy tone link to which the caller is then connected. Called subscriber busy.-As noted above, such a called subscriber's line circuit is in the lockout condition. Also when an attempt is made to connect up a called subscriber the pulse on the " seize bus " is off of the 12V variety. This is insufficient to override the lock-out condition and the call is not set up. The link scanner stops as described above until the busy tone link is reached. Executive cut-in.-Such executives have a special hundreds number. If the executive finds the wanted number busy, he makes a fresh call and dials the prefix 8. This causes the register sequence switch to skip to position 4 and also a " priority " flip-flop to operate which restores the called number register and resets the sequence switch to position 1. Dial tone is again reverted and the executive dials the wanted number. With the flip-flop operated when the register is next scanned a timer is started and a further flip-flop operated. The timer inhibits the read-out of the calling number so that as link scanning continues, no paths to them can be completed due to the inhibit on the calling line read-out. When the busy tone link (No. 40) is reached (70th time slot) the output of the further flip-flop causes read-out of the called number. At this time also an inhibit is applied through a common amplifier to the switch-through paths of all links but the fired paths are held. A 24V pulse is applied to the " seize bus " to override the lock-out condition of the called subscriber and a connection is fired through to the busy time link. Busy tone is thus fed to the called subscriber and when at the end of the 70th time slot the switch through circuits are restored it is also fed to the other party of the connection. This condition persists until the timer times out. When next a particular time slot occurs during the register scan time a pulse is passed via a common amplifier to all line circuits and the called line store is read out to apply a release pulse at the called line circuit. The call to the called line is then completed in the usual way. Calls to outgoing lines: restricted access.- Outgoing calls are obtained by dialling 9. Subscribers having restricted access have certain hundreds digits. A gate is therefore controlled in accordance with the registered called and calling members to be open or closed depending upon whether or not the call may be made. In the first case a trunk " size bus " is de-energized which enables all the outgoing trunk circuits which are scanned by the link scanner. Moreover the gate output causes the 24V pulse to be applied to the " seize bus " during both odd and even periods and also the calling line number to be read out during both these periods since otherwise the absence of read-out of a called number might be interpreted as inability to set up a call and cause reversion of busy tone. The called number does not need to be read out since all outgoing trunks are enabled over the trunk " seize bus." The path through the matrix is thus fired. If the subscriber has restricted service, read-out of the calling line identity is prevented and the trunk "seize bus" is energized. This prevents