Patent GB-761074-A (GB)
Title: Improvements relating to telephone systems
Inventor: STALLWORTHY FRANK ALLEN
IPC: H04Q1/446
Filed: 1953-10-23
Abstract: 761,074. Automatic exchange systems. SIEMENS BROS. & CO., Ltd. Oct. 22, 1954 [Oct. 23, 1953], No. 29366/53. Class 40 (4). A system in which two voice frequencies X and Y transmitted singly are used for supervisory signalling over a trunk is arranged such that (1) each frequency is transmitted as an impulse of sufficient duration to distinguish it from a transient, (2) signals other than supervisory lamp-flashing signals comprise an X frequeiicy impulse followed immediately by a Y impulse, the signal causing a locking operation which in some cases may be cancelled by a similar signal, (3) lamp flashing signals comprise pulses of X frequency which are of shorter duration than the other supervisory X frequency impulses and (4) the frequency Y by itself is transmitted as an acknowledgment or clearing signal. In the arrangement described, each end of a both-way trunk T is terminated by an exchange circuit as shown in Figs. 1 to 4. Calls incoming over the trunk: A pulse of X frequency from the distant exchange takes the trunk into use. A relay X in a voice frequency signal receiver VFR responds and pulls up relays B and CD, followed by BA, CB and CA, relay CD short-circuiting the line wires extending to selectors while CB guards the trunk from outgoing calls by earthing wire p of the trunk selector banks. When the pulse terminates relay X falls back and the trunk is connected to the selectors by operation of relays BR, Z and AX. Relay BR also pulls up an impulse repeating relay A and disables CD and CA, the trunk selector being prepared for impulsing over a loop circuit comprising both windings of a retard coil RA. Dialling impulses of X frequency over the trunk are then repeated by A, the release of which is delayed each time by a condenser C5. When the first impulse is received AX falls back to split the trunk and CD operates to short-circuit the retard coil. Slow-release relay CA also operates and holds AX back during the dialling period. When the impulse train terminates, CD and CA fall back in turn and relay AX operates, a resistor R6 being temporarily connected across the trunk by CA to avoid release of the selector impulse receiving relay when CD falls back. Further impulse trains are received and repeated in like manner. Backward supervisory signals: These are transmitted if a busy condition is encountered, impulse transmission is to be held up temporarily, or the called party answers, and are initiated by potential reversal in the line wires. Relay D(I) responds, disables an operated slowrelease signal timing relay BF, and pulls up TX and SP(I) in series. The trunk is then split and connected to an X frequency source by release of relay AX, the duration of the X reverted signal being determined by the nature of the signal controlling relay D. If a busy flash signal is received, D pulls up and releases within the release time of BF and corresponding pulses of X frequency are reverted. With steady supervisory signals the operation of D persists and BF falls back, followed by TX which terminates X frequency transmission. Frequency Y is then applied to the trunk by operation of relay TY for a time period which is terminated by the response of a slow-operating timing relay FL. Relay BF then reoperates slowly due to C6 and R4 and when up releases FL and SP with consequential completion of the trunk circuit by the response of AX. If no acknowledgment signal is required from the distant exchange a strap b, shown dotted in Fig. 3, will be in position. At the stage when FL operates, a signal registration relay AM responds and locks, followed by operation of a further registration relay AN when FL subsequently releases. When D falls back on termination of the steady signal, relays TX and SP(II) reoperate, AX falls back to split the trunk, and slow-release timing relay BF is disabled. A second reverted signal consisting of an X frequency pulse followed by a Y pulse is then transmitted as previously described. When timing relay FL responds and terminates signal transmission, the inactive second windings of registration relays AM and AN are energized in such a way that AN is held and AM disabled. Relay AM falls back and by re-operating BF causes the release of FL and AN. The receipt at the distant station of this second signal allows impulsing to be resumed or a clearing signal to be transmitted. Receipt of forward acknowledgment signals following satisfactory transmission of answering or clearing signals: In this case strap b is omitted and a short pulse of Y frequency is transmitted from the distant exchange. Relay Y in the voice frequency signal receiver responds and pulls up G and ZR as well as disabling Z which falls back slowly, the release time enabling a true signal to be distinguished from a transient. When Z releases, relay SP(III) operates and causes AX to release and split the trunk. Slowrelease relay ZR is also disabled by Z and when released pulls up registration relays AT and AM in turn. At the end of the pulse Y falls back and releases G and AT, so causing response of registration relay AN. The circuit conditions are now similar to those after transmission of an answering signal without acknowledgment facilities. Transmission of an answering signal is terminated, as previously described, by operation of timing relay FL. This relay energizes, and is subsequently disabled by, slow operating relay BF. If the acknowledgment signal is not received before FL releases, relays TX and TY will be reoperated in turn by interaction of BF and FL and a pulse of X frequency followed by a pulse of Y frequency will be transmitted, the signal being repeated until an acknowledging Y pulse is received. Forward release of trunk: A clearing signal of Y frequency which has a duration sufficient to permit relay B to release is transmitted from the distant exchange. Relay Y responds, disables slow-release relay Z and pulls up G and ZR, the latter relay operating only during the release time of Z as previously described. Subsequently, a registration relay AT is operated and relay B disabled. After the delayed release of B, relays BR and A fall back. When the forward clearing signal terminates, relay Y falls back and operates TY which transmits a Y frequency pulse as a backward release signal, this pulse being terminated by the delayed release of BA after G and AT fall back. Relay SP is also disabled and the trunk freed from busy earth on the p-wire by release of CB. Calls outgoing over the trunk: The trunk selector tests for a free trunk over wire p and resistor R8. After operation of the trunk selector test relay, GS(I) pulls up and locks to the p-wire. Relays SP(I), TX and A then operate, and a pulse of X frequency is transmitted over the trunk which is terminated by the delayed response of relay GT and the consequential relapse of TX and SP. The trunk circuit is then completed by operation of AX. When GT operates it locks and pulls up relays HB, Z, BA and CB so that the answering supervisory relay D is isolated and the trunk busied by earth on the p-wire. The circuit is then prepared for the transmission of impulses over the trunk. Within the exchange, digits are registered on a sender (not shown) which earths wire N and so operates relay XX. Relay XA is then isolated to prevent local response to v.f. impulses. The sender then transmits the digits over the trunk as trains of X frequency impulses and causes XX to release at the end of each train, thus permitting reception of an X frequency signal from the distant exchange during the inter-digital pauses. This signal is received either in the form of busy flash pulses if congestion is encountered, or as a steady signal if impulsing is to be held up or the called party answers. As a result X responds and pulls up XA when XX releases. Relay GT is then held so that a clear forward signal may be transmitted, if necessary, despite clear-down of the calling party, while supervisory wire S of the trunk selector bank is earthed to stop transmission of further impulse trains. At the same time BF operates and Z releases, the latter relay energizing a slow-operating timing relay RC as well as SP(III) which releases trunksplitting relay AX. The delayed operation of RC distinguishes true signals from transients, the relay registering the signal when it pulls up and locks. If a busy flash signal is received, XA repeats the pulses and when back causes Z to pull up. On the second operation of XA, earth on wire S causes the sender to be disconnected and a supervisory lamp at an operator's position to be flashed. When the busy signal terminates, relay BF falls back slowly and causes RC and SP to release in turn. Relay AX then pulls up to complete the trunk circuit. Impulse transmission can then proceed if necessary. The signal is identified as a steady signal if the X frequency persists and is followed by frequency Y. The circuit conditions during the receipt of frequency X are as before. When the Y frequency is received, G and AT operate and pull up registration relay AM to earth wire S and so dim a supervisory lamp. Relay Z which should reoperate when the X pulse terminates is disabled. When the Y pulse ends, Z operates and G, AT release in turn. AN then responds and disables SP, the trunk being switched through by operation of AX. Backward clear signal: This is the same as, and follows, the answering signal of X and Y pulses which causes operation of registration relays AM, AN. The circuit responds as before except that AM falls back when G and AT operate on receipt of the Y pulse. When this pulse terminates, Z operates and G, AT and AN release. The supervisory lamp is lit at the stage when AM released, and the operator clears down by removing earth from wire p so that GS falls back. Relays A and AX then release, the p-wire is earthed temporarily to busy the trunk, and SP(I) and TY operate in series over wiper M1 of a single motion switch so that a forward c