796,196. Electric selective signalling systems. STANDARD TELEPHONES & CABLES, Ltd. Jan. 20, 1956, No. 1989/56. Class 40 (1). In a remote supervisory system arranged to supervise groups of electrical devices in a repetitive scanning cycle, devices of a first group are connected to individual positions of a main scanning switch, and devices of a second group are connected to individual positions of a secondary scanning switch connected to an individual position of the main scanning switch. General description. In Fig. 1, the full lines represent a known system in which the indications of meters Ma, Mb, Mc, &c. are repeated on remote meters TMa, TMb, TMc, &c. An identification signal from each meter, e.g. A, B, &c. is converted into teleprinter code in respective coding units Ca, Cb, &c., and a cyclic sending switch S tests in turn over the outlets from the coding units and transmits the code signals to line. At the receiver the signals are received and decoded at D and passed to corresponding meters TMa &c. Each meter M has a continuously rotating disc the speed of rotation of which increases as the quantity measured by the meter increases, the meter emitting a pulse signal for each revolution. The coding units respond to these pulse signals and mark the corresponding terminals of the switch S to register a meter revolution., and when the switch S reaches a marked terminal it stops and causes the coding unit to transmit the identification signal of the corresponding meter. The coding unit then returns to normal and switch S steps on. The switch S only stops on the marked terminals so that the speed of rotation of switch S depends on the number of coding units waiting to transmit signals, and in some cases the switch S may complete a number of cycles without being required to transmit a signal. To utilize this idle line time a uniselector U1 is connected to a coding unit C which is connected to one position of switch S so that the outlet from the unit C is tested once per cycle. Supervisory contacts of, for example, circuit-breakers V1, V2, V3, &c. are connected to individual bank contacts of uniselector U1 and the coding unit C converts an open condition into one teleprinter code, e.g. X, and a closed condition into another, e.g. Y. When a signal is to be transmitted, the unit C marks the outlet to switch S so that at the next test the switch S dwells on the outlet and transmits the signal to line. After the signal has been transmitted U1 steps to the next contact, At the receiving end leads from the decoding equipment D are taken to the wipers of a uniselector U2 which steps in conformity with U1 and distributes the received signals to indicators 11, 12, 13, &c., corresponding to the various circuit-breakers. Circuit details; first switching unit. The first switching unit, Fig. 2, connects supervisory contacts of circuit-breakers V1, V2, &c. to the coding unit C. In the open condition of a circuit-breaker the supervisory contact So is closed, and in the closed condition the contact Sc is closed. Normally the uniselector stands on home contact O, no signal is passed to the coding unit C, and the switch S does not dwell on the coding unit outlet. In Fig. 2, circuit-breakers are assumed to be connected to contacts 1, 2, 3 and 4, and when the uniselector steps to contact 5, relay Z operates to drive the uniselector home and originate a check signal for checking the operation of the second switching unit U2 at the receiving end. If either contact So or Sc of a circuit-breaker is closed, the relay A corrresponding to that circuit-breaker is energized and a1 makes before a2 breaks to energize relay SL. ST operates, releases SL, prepares a circuit for relay AR on receipt of positive potential on lead 3 from the coding unit, connects the wipers W1, W2, to leads 1, 2, of the coding unit, energizes relay SR and energizes the uniselector driving magnet M, but the uniselector does not step until this circuit is released. Positive potential is passed over wiper W and lead 1 to the relay P at the coding unit, Fig. 4, which transmits a preparatory indication to the receiving station, and is fed back over lead 3 when transmission of the teleprinter code signal has started. AR is energized, disconnects wipers W1, W2 from the coding unit, and opens the circuit of magnet M causing the uniselector to step to contact 1. When the potential on lead 3 is cut off AR releases, and if the contact Sc associated with uniselector contact 1 is closed, positive potential is applied to lead 2, or if So is closed lead 1 is energized. Positive potential received on lead 3 energizes AR and the cycle is repeated. When the signal relating to the last of the four circuit-breakers has been passed to the coding unit the uniselector steps to contact 5 and relay Z operates and sends a check signal over lead 4 to the coding unit, and after the check signal has been received AR again operates and is held over contacts ar3, z1. Relay ST releases, followed by SR, which is made slow-to-release so that the re-operation. of relay ST is delayed to allow the stepping of the second switching unit, at the receiver, to overtake a possible time lag. Release of ST releases Z which releases AR. The uniselector then drives to the home contact O If the condition of a circuit-breaker changes when the uniselector is not on the home contact O or the first contact 1, relay A operates and energizes relay SL, which is held over sl1 and wiper W3. After the check signal has been passed forward, ST is released as before, and when the uniselector steps from contact 6 to contact O, SL is held over sl1 and st6, ST is operated and the whole process is repeated. Coding unit. As shown, Figs. 3, 4 and 5, the coding unit comprises four testing tubes T1 to T4, three character trains of tubes, one character train only, T8-T13, being shown, and an entry tube T5, for the character train T8-T13, and entry tubes T6, T7, for the character trains CT2, CT3, not shown. There is a character train of tubes for each signal to be converted into a teleprinter code character for transmission, i.e. circuit-breaker open, circuitbreaker closed, and the check signal. All the tubes T1 to T13 are cold-cathode tubes and the tubes T1 to T4 and the tubes in each of the character trains are connected to a common anode load resistor (not shown) such that only one tube can conduct at any time. The operation of the coding unit is controlled by two series of pulses il, i2, occurring alternately. It is stated that when the switch S stands on the coding unit outlet, the coding unit receives positive potential over one of the leads 12 or 13. One of the testing tubes T1, T2, will fire on receipt of an i1 or i2 pulse and if no code character is to be transmitted the testing tubes strike in turn, after which switch S is released. If a character is to be transmitted, the testing sequence is interrupted at the appropriate testing tube. In the normal condition tubes T5, T6, T7 are conducting and rectifiers W1, W2, W3 are blocked. With the switch S on the outlet of the coding unit positive potential is applied to say lead 12 and on receipt of an i2 pulse T1 fires, a mark signal is passed to the switch S over a common mark lead M, and tube T2 is primed by potential from T1 directed by W7 to the tube' trigger. The next i1 pulse fires T2 and T1 is extinguished and a further mark signal is passed to the sending switch. Alternatively T2 is fired by the coincidence of potential on lead 13 and an i1 pulse, and W4 is blocked. Since W1 is already blocked, tube T3 is primed by potential from source A1 and is fired by the next i2 pulse, T2 being extinguished and passing forward a mark signal over common lead M. T8 is also primed by the firing of T2. T4 is primed by potential from source A2 and the following i1 pulse fires T4 and extinguishes T3, passing on a mark pulse, positive potential from A3 being directed to lead 14 by rectifier W9. Positive potential on either of leads 14, 15 releases switch S to step to its next position. Each of the leads 1, 2, 4, from the first switching unit is connected to a relay P, Q or R; in the coding unit and energization of one of these relays controls a break-contact in the anode circuit of an entry tube. For the open condition of a circuit-breaker relay P is operated and breaks the anode circuit of T5 allowing Wl to conduct and prevent T3 from being primed. When positive potential is applied to one of the leads 12, 13, and tube T2 is fired by an il pulse, the tube T8 is fired by the ensuing i1 pulse and passes forward a space signal to the sending- switch over the common space lead S which is the start element of a teleprinter code character. When T8 is fired, T2 is extinguished, and T9 is primed. Each of the tubes T9 to T13 corresponds to an element of a teleprinter code and each of their outlets X9 to X13 is connected to the common mark lead or common space lead in accordance with the character to be transmitted. The tubes T9 to T13 are fired in turn by successive i1 pulses and the code elements are passed forward in turn. The firing of T13 primes T4 over lead 8 and when. T4 is fired by the next i1 pulse the resulting mark signal serves as the stop element. When any of the tubes T9, T10, T11 is fired a tube T14 conducts and relay SO is operated and contact SO1 connects positive potential to lead 3 to initiate the stepping of the first switching unit. This causes relay P to release and restore the anode circuit of T5 and when T12 fires potential is applied via lead 6 to cause T5 to resume its normal conducting condition. When T13 is extinguished and T4 has been fired, the testing cycle is resumed. The closed condition of a circuit-breaker causes operation of relay Q, tube T6 is extinguished, and the second character train CT2 operates. When the last tube of train CT2 fires, potential is passed over lead 10 and rectifier W9 to lead 14 to cause switch S to ste