489,545. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd., WRIGHT, E. P. G., and NEILSON, C. B. V. Jan. 25, 1937, No. 2208. [Class 40 (iv)] In a toll system involving a number of exchanges, different supervisory signals have a common prefix signal of a character unlikely to occur in speech, and signal receiving equipment in response to the prefix opens the connection beyond the receiving equipment to prevent transmission of the individual portion of the signal to receiving equipment at a subsequent point in the connection or to a substation. The invention is described with reference to a system in which calls are set up over both-way junctions between three exchanges, dialled impulses and signals being sent over the junctions by alternating currents. At the tandem exchange, an incoming circuit converts the A.C. signals to D.C. signals which pass to an outgoing circuit where they are re-converted to A.C. Dialled impulses and other signals required before connection is established comprise short pulses of one or other of two frequencies, but after connection is established, the signals, e.g. answering, transferring, re-ringing, clearing, busy and release comprise a long prefix of both frequencies followed by a short pulse of one or other of the frequencies except in the case of the backward release signal which comprises a long prefix followed by a long pulse of a single frequency. The various signals after establishment, except the release signal, are repeated at intervals and, with the exception of clearing supervisory signals, require ack. nowledgment, arrangements being provided so that acknowledgment occurs in an interval in the repeated signal which is terminated by the acknowledgment. Some of the junctions involved may be equipped for 50# or D.C. signalling but when both incoming and outgoing junctions at a tandem exchange are equipped for voice-frequency signalling, signal conversion and reconversion thereat, after establishment of the connection, may be .avoided by providing a byepath for the signals, the signal receiving equipment at tandem being responsive only to the release signal. The later part of a connection may be released and the circuits prepared for setting up another connection, by sending a transfer signal and in the case of a connection provided with a byepath for the signals, the transfer signal releases the connection at the final exchange. Under operator hold conditions, the signalling equipment although responding to the clearing signals does not send the backward release signal until the operator clears. The signalling equipment comprises two switches, one recording received digits on condensers and the other following up to repeat the impulses which are converted from D.C. to A.C. and vice versa. The switches also control the transmission of the supervisory and other signals. The Provisional Specification describes the use of more than one short pulse for certain signals also refers to Specification 479,881. Incoming call. A calling signal comprising a dot of frequency X received over TL passes over transformer T], which in practice is a hybrid coil, to a receiver represented by relays RX, RY, Fig. 5, whereupon RX responds, followed by relays IC, ICA, E, and BR which lights the busy lamp BLP at a both-way operator's position and disables the jack circuit. RX operates L and BR makes a circuit for LL which however is short-circuited until RX falls back when the calling signal ceases. The other operated relays are held. Relays L and E loop the line to the incoming selector ICS, I operates in the loop, and the selector grounds wire PL to operate MH which is followed by CY whereupon BRA pulls up slowly, locks, and releases CY. Relay CY connects an acknowledge signal of frequency Y to line TL to flicker the distant operator's lamp (dialling signal). Relays RB, RD and W also pull up, the function of RD being to switch through and replace a terminating impedance LT by a low-pass filter LPF. W operates SD, SC, and disconnects these relays from the impulse regenerating switches SAM, SBM, disconnects RC so that it is not operated by RX on dialled impulses, and connects LD to a 5-second pulse lead 5PL, RD being released and made quick-to-release by LD which holds RB. Dialled impulses of frequency X are repeated bv RX to SAM and series relay C which operates LD if this has not already occurred, the consequent release of RD opening the line so that surges over the incoming line relay do not jam the impulses. When SAM is stepped from its resting position which may be any position coinciding with that of SBM, normally operated relay Z relapses and SC falls back slowly to connect up IG which pulls up if or when impulse springs IMPS open, connects pulse springs LPS directly across the selector loop and holds I. Springs IMPS step SBM to chase SAM and the counterphase springs LPS repeat impulses to the selector. The relapse of C at the end of the digit grounds wiper sam1 to charge a marking condenser so that when SBM arrives at the position corresponding to SAM, Z is operated by condenser discharge or ground on sam1 depending on whether reception of the next digit has or has not commenced. Further digits are repeated in similar manner, LD being operated by C or Z to ensure that RD is released to open the line during impulsing. The condensers are short-circuited over sam3 one step before the marking. Called subscriber replies. Battery reversal operates D followed by S, SX, SG, DFA and DF, whereupon W and Z fall back, followed by SD and SC, and the 5 second pulse lead 5PL is disconnected. When IG re-operates, SAM is stepped by springs IMPS and CXY operates to send frequencies XY back over the toll line, LD being operated at this time to release RD and open the line on the outgoing side of the A.C. connection. The wiring between the banks of SAM, SBM is such that CXY is maintained operated over sbm 3, 4 and sam 1, 2 for 3 steps of SAM relative to SBM whereby a dash of frequencies X, Y is sent back as a prefix signal. On the 4th step CXY falls back and SC pulls up to connect CX to IMPS whereby a dot of X frequency constituting the distinctive portion of the answering signal is sent back. On the 5th step of SAM, SD pulls up to change over the impulse circuit from SAM to SBM which is stopped by Z in a position corresponding to SAM, SC being also released. The operation of Z releases SD which in turn releases Z and the above cycle is repeated unless an acknowledgment signal is received during the interval provided by the stepping of SBM (SD operated). This signal comprises an XY prefix dash and a dot of Y, RX, RY responding to the dash, RB being released due to the presence of both frequencies and RC being operated due to the presence of Y. Relays RE, RF pull up and hold and on termination of the prefix RB re-operates, RC being maintained by the following dot of Y so that AK also operates on this dot and is followed by SL, relays RY, RC, RE, RF falling back at the termination of the dot. Relay SL releases SX followed by SG so that when Z re-operates on switches SAM, SBM coming into line it is held to prevent repetition of the answering signal. Z also releases LD whereupon RD re-operates to close the line again, S being now held dependent on D. Release. When the called party hangs up, D falls back followed by S to operate SG, whereupon Z falls back and the previously described cycle of operations of SAM, SBM is repeated as for the answering signal except that since SX is not operated, a dot of frequency Y applied by relay CY replaces the dot of frequency X after the prefix of frequencies X, Y. This clearing signal is repeated until S is re-operated and causes the supervisory lamp of the outgoing operator to glow. If the calling party releases first, the withdrawal of the outgoing operator causes a prefix X, Y and dot Y to be sent forward. Relays RB, RD release and RE, RF pull up to the prefix but the dot by operating RC now operates CL which releases L to open the outgoing loop and incoming selector (if not operator held) which removes ground from PL to release MH. The prefix must be long enough to release, in turn, RD, RB which have different release times, and the interval between prefix and signal must not exceed the release time of RC, also operated by the prefix, which shunts RD to hold it released to open the line. Relays D and I fall back and condenser CD charges up to operate SG on release of RC at the end of the dot. Relay SG releases Z and a cycle of operations of SAM, SBM occurs to send a prefix of XY followed by Y alone, but since MH has released. CY holds until the end of the cycle to prolong the transmission of Y. The prolonged operation of CY releases BR, which is shunted by a rectifier to make it slow, whereupon other operated relays release. SD being, however, maintained by Z until the end of the cycle to busy the outgoing selector OGS. If the incoming selector is operator held, MH holds S when D releases and the signal is transmitted when MH is released by the operator. If release had been initiated by the called party, a repeated signal comprising prefix XY and dot Y would be in course of backward transmission when the similar forward signal arrives, the arrival occurring during the interval occupied by the stepping of SBM. The operation of RF by the incoming signal releases SG so that Z when operated at the end of the backward cycle is held to prevent further backward signalling at present. The incoming Y dot operates CL with results as before including a backward transmission of a prolonged Y. It will be seen that when the circuits are receiving a repeated signal, RF is continuously operated so that SG can only be operated to send an acknowledge signal by discharge of condenser DC due to release of RC after the Y dot, that is, in the intervals of the repeated signal. Called party returns to line during supervisory release signalling. R