Patent GB-1437031-A (GB)
Title: House exchange telephone system
IPC: H04Q3/62
Filed: 1973-04-04
Abstract: 1437031 Automatic exchange systems PLESSEY CO Ltd 18 March 1974 [4 April 1976] 16016/73 Heading H4K  A small private exchange has a single stage switching network on which, e.g. 16, two-ten wire extension lines and, e.g. 4, two wire exchange lines have appearances, and a common control unit which is connected to each line terminal by an individual two wire control line and which operates in a TDM mode. Each control line terminates in the control unit on an interface which is connected by a plurality of TDM buses to the other interfaces and to a central logic. Power for the extension line terminals is provided over the control lines during intercom calls but is supplied over the speech wires from the parent exchange in the case of external calls. Logic, particularly that for individual crosspoint control, is in integrated circuit format. General system, Fig. 1B.-The extension terminal 1-N are each connected to the common part of the exchange (i.e. the switch network and the control logic) via two line pairs 1a, 1b. The speech pair 1a terminate on an extension line interface SI (details not given) which appears on a respective column 141 of a folded reed-relay matrix 3. The column gives full availability connections to a set of exchange line terminals 1-M and also (after folding as at 142) to all the other extension lines. With such a network only one crosspoint needs to be operated during a connection. The control line pair 1b terminates on a control interface 2 which is given access to a set of common buses 140 during a time slot T1-TN associated uniquely with the interface. The common logic 5 decoders off-hook and dialled signals from the extensions so as to permit the control interfaces to set up appropriate connections through the switching matrix, it does not perform the usual marker-connect operations itself. Timing and common control (Figs. 10 and 15).-The output of a master clock 64 is divided so as to produce a 25À6 msec. frame period containing 16 time slots T1-T16. The Nth slot, e.g. that pertaining to extension interface 3, i.e. subscriber 3 (Fig. 10), is subdivided into eight unequal periods t1-t8 the first five periods t1-t5 serve to interrogate the particular interface that is being polled by time slot TN. The periods are repeated over the control line 1b between the interface and the extension terminal. There they interrogate a 5-bit store in which are set up codes indicating whether the terminal is on or off hook, what extension is being called (by depression of a corresponding key pad), whether an exchange line is being requested (by depression of another key), whether the terminal wishes to be connected to an incoming exchange call (another key), whether the terminal is busy on an external call or whether it does not wish to be interrupted on an internal call (another socalled privacy key), or whether the extension wishes to hold an exchange line pending a triangular, interim or transfer call action. These codes are signalled back to the interface by inverting (1-bit) or suppressing (0-bit) the incoming pulses, as shown by T1-T5 on Fig. 10 (see Figs. 12, 13, 14, not shown, for details of the extension terminal circuitry). The last three periods t6-t8 serve to inform the terminal that he is involved in an internal or external call or is in an exchange line holding or privacy mode. The pulses are appropriately width modulated and after decoding drive corresponding lamps. The codes returned from the interrogated terminal are inserted on to a common line multiplex (Fig. 15) during the same interrogating time slot and are decoded in the central logic. The shown bus outputs of the decoders are self explanatory and will be discussed subsequently. It may be noted however that in the case of a dialling code, the relevant wanted extension line interface is immediately marked over an individual wire M1-M16. The exchange lines have an additional common logic (Fig. 9, not shown) which interrogates each line for a full 16 time slot frame during which if that line is calling it is connected through to a volunteering extension or if it is free it is connected through to a calling extension. If all the exchange lines are busy an exchange line demand signal from an extension (this appears as a mark on the seize-to-call bus of Fig. 15) causes the exchange line logic to mark a busy bus (see Fig. 5 subsequently) whereby a busy signal can be returned to the calling extension. Switching matrix (Fig. 4) is made up of modules of triangular and rectangular crosspoint arrays. The former serve four extension line interfaces 15 and one exchange line interface 4. These interfaces and the crosspoints are arranged on a single printed card so as to permit easy expansion of the system in steps of 4 + 1 lines. It will be noted that the lower right crosspoint of each rectangular array is omitted so as to prevent the direct interconnection of exchange lines. Internal call.-When an extension goes offhook, a signal code to this effect is sent via the extensions interface to the central logic (Fig. 15) during the extensions time slot TN. The logic immediately marks the off-hook bus thereby setting a bi-stable 103 (Fig. 5) in the extensions interface (only a part of which is illustrated in Fig. 5). This has the effect of inhibiting a crosspoint resetting pulse which is periodically applied to a reset bus R. During a subsequent time slot TN when the calling extension has depressed the key corresponding to the wanted extension, the central logic (Fig. 15) marks the appropriate lead M1-M16. This mark extends into the wanted extensions interface over his marking lead MN (Fig. 5). If the extension is busy on an exchange call or if he is in a privacy mode a bi-stable 108 is set and this causes a mark to be applied to the busy bus. In the caller's interface, the mark is gated via 115 (it being remembered that all the actions now being described are taking place during his time slot TN) so as to enable a busy tone source to be connected to his speech wires. If the called extension is free, a toggle 112 is set. In either case, the incoming MN mark passes on to wire TN+MN. Considering now Fig. 6, we have the condition that during the caller's time slot TN, e.g. for extension 1 this is T1, there is a mark extending from his interface over a control wire (T1 + M1) into the matrix and also there is a mark MN, e.g. for extension 3 this is M3, extending from the called extensions interface over a control wire (T3+M3) into the matrix. This coincidence of marks sets bistable 20 which in turn actuates crosspoint relay 22 thereby interconnecting the two parties' speech wires SP1, SP3. In the called extension's interface, the fact that he is "free" is denoted by a reset bi-stable 108 so that this condition together with the incoming MN mark is gated via 117 on to a "marked forward" bus. In the callers interface, this mark is immediately gated by 118 to set bi-stable 119. Hence a buzz-back signal, i.e. ringing tone, is applied to his speech wires. Subsequently during the called extensions time slot, the set condition of bistable 112 is notified to the "internal calling" bus whence the central logic (Fig. 15) via gates 53-56 signals to the wanted extensions terminal circuit the fact that he is being called. A signal lamp and an audible tone generator are enabled at his extension. If he does not answer, the caller will eventually release the still depressed key and all the devices apart from his off-hook bi-stable 103 will restore. If either party presses their privacy button, the central logic marks the "privacy" bus during that party's time slot whereby a privacy mark is applied to the "held/privacy" bus. This mark is extended via the central logic back to the party as a steady lamp enabling signal. External call, outgoing.-The central logic apprized of a caller's requirement marks the "seize-to-call" bus (Fig. 5) whereby during the time frame appertaining to the first free exchange line and in the time slot of the caller, a mark is applied to the corresponding "exchange line horizontal" wire (Fig. 7). This is effective with the caller's time slot pulse TN on the matrix central wire TN + MN to set bi-stable 25 and thus actuate the corresponding crosspoint. A mark goes back on the "exchange line association" wire to an exchange time interface (Fig. 8, not shown) and the exchange line common logic (Fig. 9, not shown) to prevent any other extension from seizing the line. In addition, the exchange speech line (Fig. 21) is disconnected from a ringing detector (used for incoming calls) and connected through to the matrix. A relay H is operated momentarily to place a loop on the exchange line, this being required mainly for incoming calls to prevent main exchange ringing current being applied to a private exchange extension's line. Furthermore, the seizure of the exchange line disables a relay P which (see Fig. 21) therefore disconnects the normal microphone power feeding arrangements from the extension line and permits this function to be taken over by the main exchange battery.  External call, incoming.-The exchange line ringing detector, after a persistence check, informs the exchange line interface (Fig. 8, not shown) of the existence of an incoming call and a corresponding mark appears on the external call bus in the central logic (Fig. 15). A ringing signal is then modulated on to the t7 pulse occurring during every time slot until one of the extensions accepts the call. This ringing signal will light a lamp and, at the extension's option, may sound a bell in the extension terminal. Subsequent crosspoint actuation and removal of ringing &c. follows obviously from the preceding descriptions. Call transfer, interim and triangular calls are set up by the extension actuating his hold key. In the central logic (during the extension's time slot) a mark is applied to the "hold" bus so that in the extension's interface (F