Patent GB-830543-A (GB)
Title: Electronic code communication system
IPC: B61L7/08
Filed: 1956-12-21
Abstract: 830,543. Electric selective signalling systems. GENERAL RAILWAY SIGNAL CO. Dec. 21, 1956 [Dec. 30, 1955], No. 39010/56. Class 40(1). In the signalling system described each of a number of field stations has associated, with it a particular period-called a station period-within a main scanning cycle, during which periodit transmits data to a central station. Thus data received by the central station can be ascribed to a particular field station by virtue of its time of receipt within the main scanning cycle. Timing is controlled by the central station which transmits, during each scanning cycle, a continuous stream of basic timing pulses at a p.r.f. of 120 c/s. on a carrier frequency of 6.75 kc/s., with the exception that the carrier frequency of every twelfth pulse is changed to 6.25 kc/s., Fig. 3, and these latter pulses-referred to as station pulses-are used to demarcate the station periods. At the end of every scanning cycle, that is after forty station periods in the embodiment described, there is a period of Silence-referred to as a sync. period-after which a further scanning cycle is commenced automatically. Data is transmitted by each field station (during its station period) to the central station by means of pulses on one or other of two carrier frequencies. Fig. 3, a pulse on a carrier frequency of 11.25 kc/s. being referred to as a space pulse and one on a carrier frequency of 10.75 kc/s. being referred to as a mark pulse. These pulses are produced at the field station under the control of a multivibrator locked to the basic timing pulses transmitted by the central station, and. although each station period comprises twelve pulse periods only ten are actually used for the transmission of data. Data received by the central station from a field station is stored in banks of cold-cathode gas tubes, and in doing this the central station takes into account delays caused by line repeaters. Error detecting means are provided which at each field station, prevent the field station from transmitting anything at all for the following 30 seconds if the field station does not satisfactorily receive ten basic timing pulses during its station period and which, at the central station, fail to energize a neon indication lamp if ten pulses (each of which may be a mark or a space) are not received from a field station during its station period. After the 30-second period the field station is given a further opportunity to function correctly in case the detected fault was of a transient nature. If the data to be transmitted by one field station cannot be accommodated within one station period it is possible to cause one field station to transmit data in more than one station period of a scanning cycle. That is, it is possible to associate more than one station period with each field station. When this is done the field station functions as more than one field station but since they are located at the same physical place some of the equipment (e.g. various counters) can be shared. Central station.-The central station, Figs. 4A-4F, comprises principally a 120 c/s. multivibrator 15 which causes a carrier transmitter 17 to transmit basic timing pulses, Fig. 3, of which every twelfth pulse has its carrier frequency changed under control of a signal on a lead 72 from a scale-of-twelve divider 19, a station counter having units and tens denominations. 104 and 105, respectively, which counts station pulses (these pulses coming normally from the zero cathode of the scale-of-twelve divider, via cathode follower 118 and amplifier 122 of a delay unit No. 1 via a lead 125 to a units station counter driver 130 which drives the units denomination 104 via a lead 147, and via a tens station counter driver 148 which drives the tens denomination 105 via a lead 149) and a store comprising one bank of ten cold cathode tubes for each field station. Data received by the central station from the field station is routed to the store by the station counter together with a step counter 52. Each station counter advance pulse, from lead 147, turns on the first stage, No. 0, of the step counter 52, which is then advanced by pulses on a lead 196 from a step counter driver 51 which produces one advance pulse in response to every signal (either mark or space) received from a field station. The " on " stage of the step counter primes a similarly numbered stage of the particular storage bank selected by the station counter via a storage bank gating circuit so that if a mark is received from the field station at that time it causes that stage of the selected storage bank to fire (lead 219 from a mark amplifier 54 and the selected storage bank gating circuit) whereas the reception of a space signal has no effect (except to advance the step counter). When the station counter selects a storage bank gating circuit, it also causes the associated storage clear-out tube to reset the entire bank so that at the beginning of a station period the associated storage bank is cleared. This clearing can be inhibited, if desired, by depressing a re-check push-button. The transmission line 10 from the central station to the field stations includes line repeater amplifiers which introduce transmission delays. These are taken into account in the central station by a number of delay units which control the time of advancement of the station counter. The system is arranged so that field stations are numbered serially in order of increasing distance from the central station. Thus in a scanning cycle the first field stations to transmit data are those near the central station. For these field stations no significant transmission delay is involved and the station counter is advanced by station pulses received from the No. 0 cathode of the scale-of-twelve divider 19 via delay unit No. 1, tube 90 thereof being conductive having been fired when the station counter returned to zero. In the particular embodiment described this continues until station period 22. When the station counter reaches 22 it fires tube 91 of delay unit No. 2, thereby extinguishing tube 90 of delay unit No. 1 so that subsequent advance pulses for the station counter come from cathode No. 1 of the scale-of-twelve divider via leads 128 and 125. Thus as far as the station counter is concerned station period 22 contains one extra pulse period and subsequent station periods are one pulse period behind those as demarcated by the transmitted signal pulses which occur when the scale-of-twelve counter is on its No. 0 cathode. In a similar manner delay unit No. 3 can be made effective at some latter time in the scanning cycle to cause the station counter to be advanced by pulses from cathode No. 2 of the scale-of-twelve divider. When the station counter reaches its highest count (39 as shown, which occurs when the scale-of-twelve divider is on its No. 3 cathode, the station advance pulse therefrom passing via delay unit No. 4), a sync. driver circuit 165 is fired to apply a negative pulse to a mono-stable flip-flop synchronizing circuit 27 causing the cut-off of its right-hand tube 70 which raises the cathode potential of the left-hand multi-vibrator tube 68 which therefore stops oscillating. This quiescent state (the period of " silence " or sync. period) continues until the mono-stable synchronizing circuit reverts to its stable condition whereupon the multi-vibrator recommences oscillation and the scale-of-twelve counter advances from its No. 3 cathode. When it next reaches its No. 0 cathode it causes, as usual, a station pulse to be transmitted, which station pulse is station pulse No. 0 (i.e. the first in a scanning cycle) but station period 0 is not utilized, and there is no field station No. 0, because as the station counter is being advanced via delay No. 4 it is not advanced to zero until the scale-of-twelve divider reaches its No. 3 cathode, and when this occurs tube 90 of delay unit No. 1 is fired (automatically disenabling delay unit No. 4) so that as far as the station counter is concerned station period No. 0 is shortened by three pulse periods and this period cannot therefore be used by a field station. Field station.-Each field station, Figs. 8A, 8B, comprises a station counter having units and tens denominations 289 and 302 respectively, which counts station pulses transmitted by the central station, a step counter 35 whose stage No. 0 is fired when the station counter reaches a count corresponding to the number of the field station, and a mono-stable flip-flop step pulse former 31 which is normally blocked by a conducting station cut-off tube 262. When stage No. 0 of the step counter 35 is fired, station cutoff tube 262 is extinguished and subsequent timing pulses received from the central station are shaped by the step pulse former 31 and advance the step counter. After ten timing pulses have been received the station cut-off tube is again turned on, so that further timing pulses received from the central station are blocked at the step pulse former 31. As the step counter advances its cathodes are successively raised in potential to sample the condition of a number of contacts and each time a contact is in a certain position a signal is applied via a lead 340 to a carrier transmitter 33 to cause a mark pulse to be transmitted. When a mark pulse is not transmitted a space pulse is under control of a signal on a lead 339 from the step pulse former. At the end of a scanning cycle, during the sync. period, no timing pulses are received at the field station from the central station. This causes a capacitor 308 in a sync. period detector 44 to be no longer regularly discharged by timing pulses on a lead 255 so that the charge on the capacitor 308 rises, causing a tube 310 to fire, which in turn resets the station counter to zero. Error preventing arrangements.-At each field station the output tubes of the carrier transmitter receive their H.T. supply via a no